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1:34 AM
Algae
Algae
Algae comprise a diverse group of (with few exceptions) photosynthetic oxygen-producing organisms, ranging in size from microscopic single cells to gigantic seaweeds.

The study of algae is known as phycology (in Greek, phycos means "algae"). Currently, most authors place eukaryotic algae in the kingdom Protista (domain Eukarya) and prokaryotic algae in the domain Bacteria.

In the past algae were considered to be lower plants because some forms look like plants. As in plants, the primary photosynthetic pigment in algae is chlorophyll a, and oxygen is produced during photosynthesis.

What Are Algae?

Algae can be found nearly everywhere on earth: oceans, rivers, lakes, in the snow of mountaintops, on forest and desert soils, on rocks, on plants and animals (such as within the hollow hair of the polar bear), or even on other algae. They are involved in diverse interactions with other organisms, including symbiosis, parasitism, and epiphytism.


Lichens are symbiotic associations between algae (blue-green algae, or cyanobacteria) and fungi. Atmospheric nitrogen-fixing cyanobacteria occur in symbiotic associations with plants such as bryophytes, water ferns, gymnosperms (such as cycads), and the angiosperms.

The aquatic fern Azolla, commonly used as a biofertilizer in rice fields in Asian countries, harbors the symbiotic cyanobacterium Anabaena azollae. Gunnera, the only flowering plant to house symbiotic cyanobacterium Nostoc, is widely distributed in the tropics.

Symbiotic dinoflagellates known as zooxanthellae livewithin the tissues of corals. Corals get their colors and obtain energy from their photosynthetic symbionts. About 15 percent of red algae occur as parasites of other red algae. Parasitic algae may even transfer nuclei into host cells and transform them.

After transformation, the reproductive cells of the host algae carry the parasite’s genes. Various algae live on the surfaces of plants and other algae as epiphytes. Sometimes algae can be found in strange places—the pink color of flamingos originates, for example, comes from a pigment in the algae consumed by these birds.

Algal Structure and Properties

Algal cells are bounded by a cell wall. Algal cells are either prokaryotic or eukaryotic. All prokaryotic algae belong to Cyanophyta (cyanobacteria) and lack both a nucleus and complex membrane-bound organelles, such as chloroplasts and mitochondria.

Photosynthesis occurs in cyanobacteria in thylakoid membranes similar to those of plants. However, there is no double membrane surrounding the thylakoids of cyanobacteria.

All other algal groups are eukaryotic. Eukaryotic algae differ from cyanobacteria in that they possess chloroplasts and flagella with associated structures and in their cellwall composition. According to the endosymbiont hypothesis, some eukaryotic algae (red and green algae) obtained their chloroplasts by acquiring symbiotic prokaryotic cyanobacteria. This is known as primary endosymbiosis.

types of alga
types of alga

Other eukaryotic algae probably obtained their chloroplasts by taking up eukaryotic endosymbiotic algae, a process known as secondary endosymbiosis. The existence of secondary endosymbiosis is indicated by the occurrence of more than two membranes around the chloroplasts of some algae, such as haptophytes, euglenophytes, dinoflagellates, and cryptomonads.

Pigments found in algae include chlorophylls, phycobilins, and carotenoids. All algae contain chlorophyll a. Accessory pigments vary among different algal groups.

Photoautotrophy is the principal mode of nutrition in algae; in other words, they are "self-feeders", using light energy and a photosynthetic apparatus to produce their own food (organic carbon) from carbon dioxide and water. The majority of algal groups contain heterotrophic species, which obtain their organic food molecules by consuming other organisms.

Numerous algae are mixotrophs; that is, they use different modes of nutrition (such as autotrophy and heterotrophy), depending on the availability of resources. The molecules used as food reserves differ among and are characteristic for algal groups. Food reserve molecules are polymers of glucose with different links between monomers.

Many algae are capable of movement. Movement is accomplished by means of flagellar action and by extrusion of mucilage. There are also peristaltic and amoeba-like algal movement. Within algal cells, movement of the cytoplasm, plastids, and nucleus also occurs.

Advantages conferred by mobility include achieving optimal light conditions for photosynthesis, avoiding damage caused by excess light, and obtaining inorganic nutrients.

Algal Reproduction and Life Cycles

Algae may reproduce either asexually or sexually. Asexual reproduction among algae includes production of unicellular spores that germinate without fusing with other cells, fragmentation of filamentous forms, and cell division by splitting.

In sexual reproduction, parent cells release gametes, which then fuse to form a zygote. Zygotes may either develop into new filaments or produce haploid spores by meiotic division.

Algae exhibit different types of life cycles. Some algal life cycles are characterized by an alteration of generations similar to that of plants. Two phases occur: sporophyte (usually diploid) and gametophyte (usually haploid).

The sporophyte produces haploid spores through meiosis, and the haploid gametophyte produces male or female gametes by mitosis. Gametophyte and sporophyte may be structurally identical or dissimilar, depending on the algal group.

Roles of Algae

Algae have played significant roles in the earth’s ecosystems since the origin of cyanobacteria (also known as blue-green algae)more than three billion years ago. Early cyanobacteria were responsible for the development of significant amounts of free oxygen in the atmosphere, which then made aerobic respiration possible.

More than 70 percent of all photosynthetic activity on earth is carried out by phytoplankton—floating microscopic algae—rather than plants. Phytoplankton recharge the atmosphere with oxygen and simultaneously absorb carbon dioxide, helping to support the complex web of aquatic biota.

Algae are also very important in the global cycling of other elements, such as carbon, nitrogen, phosphorus, and silicon. Several algal groups—such as cyanobacteria, green algae, red algae, and the haptophyte algae—are able to generate calcium carbonate.

Sedimented algae are the major contributors to deep-sea carbonate deposits (sand), which cover about half of the world’s ocean floor. Calcified coralline red algae contribute to coral reefs in tropical waters. Silica sediments in oceans (sand) are based on abundant growth of another algal group, the diatoms, which contain silica in their cell walls.

Some algae (cyanobacteria) are able to fix atmospheric nitrogen and convert it to ammonia. Ammonia, in turn, can be a nitrogen source for plants and animals. On the other hand, high levels of nitrogen and phosphorus in rivers and lakes owing to pollution can cause the rapid and uncontrollable growth of algae, known as algal blooms.

A bloom of algae is a threat to human and marine health, both directly and indirectly. It clogs fishes’ gills, interferes with water filters, and ruins recreation sites. More than 50 percent of algal blooms produce toxins.

Cases of human respiratory, skin, and gastrointestinal disorders associated with algal toxins have been reported. Certain blooms of algae are called red tides. The water appears to be red or brown because of the color of algal bodies, mainly dinoflagellates that contain the pigment xanthophyll.

Technological Applications

Algae can be grown and turned into biofuel - an ethanol that can power homes and cars.
Technological Applications

Algae have been used as food, medicine, and fertilizer for centuries. The earliest known reference to the use of algae as food occurs in Chinese poetic literature dated about 600 b.c.e. More recently, algae have begun to play important roles in certain biotechnological processes.

Several algae, including reds, browns, greens, and cyanobacteria, are used for food in Pacific and Asian countries, especially Japan. The annual harvest of the red alga Porphyra worldwide is worth several billion dollars. Porphyra (Japanese nori, Chinese zicai) is used as a wrapper for sushi or may be eaten alone. Another edible alga with a high iodine content is the brown alga Laminaria (Japanese kombu). The cyanobacterium Spirulina, with a protein level of 50 to 70 percent, was cultivated for centuries by indigenous Central Americans at Lake Texcoco near modern-day Mexico City for use as human food.

Several gelling agents are produced from red and brown algae. Agar from red algae is used as a medium for culturing microorganisms including algae, as a food gel, and in pharmaceutical capsules. Red algal carrageenan is used in toothpaste, cosmetics, and food such as ice creamand chocolate milk.

Alginates from brown algae have extensive applications in the cosmetics, soap, and detergent industries. Sources of alginates are Laminaria, some Fucus species, and the giant kelp Macrocystis,which can grow to more than 60 meters long. Algae are also used as feed in the culture of commercially important fish and shrimp.

Mass cultivation of algae (microalgae)—in open ponds and photobioreactors for production of fuels (such as biomass) and biochemicals (such as carotenoids, amino acids, and carbohydrates) and for water purification—is a rapidly developing area based on the use of solar energy as energy source. The green alga Dunaliella is used in the industrial production of carotene. In wastewater treatment plants, algae are used to remove nutrients and heavy metals and to add oxygen to the water.

Algae are used worldwide as indicators (biomonitors) of water quality, helping to detect the presence of toxic compounds in water samples. Several fast-growing algae are used, including the green alga Selenastrum capricornutum.

Many algae are widely employed as research tools because they are easy to culture and manipulate. Danish biologist Joachim Hammerling’s experiments with the green alga Acetabularia identified the nucleus as the likely storage site of hereditary information.

Diversity

Taxonomists believe that there are between thirty-six thousand and tenmillion species of algae. Molecular comparisons using nucleotide sequences in ribosomal RNA (ribonucleic acid) suggest that algae do not fall within a single group linked by a common ancestor but that they evolved independently.

The algae are divided into ninemajor phyla, which differ in their photosynthetic pigments, food reserves, cell structure, and reproduction. These groups include euglenoids, cryptomonads, dinoflagellates, haptophytes, and red algae.

Phylum Euglenophyta contains mostly unicellular formswith one or two flagella. Only one-third of this group possess chlorophyll-containing chloroplasts. Other euglenoids are strictly heterotrophic.

The phylum contains more than nine hundred, mostly freshwater, species. The food reserve is the carbohydrate paramylon, a polymer of glucose. Euglenophytes have chlorophyll a and b as well as carotenoids as their photosynthetic pigments. There is no cell wall.

Cells have several small chloroplasts; each is surrounded by three membranes. A close relative of euglenophytes is the protozoan Trypanosoma, which causes the human disease African sleeping sickness. Reproduction in the euglenophytes occurs by division of cells. Sexual reproduction is unknown.

Phylum Cryptophyta includes unicellular biflagellates. In addition to chlorophyll a, chloroplasts can contain chlorophyll c, carotenoids, and phycobilins. The carotenoid pigment alloxanthin is unique to Cryptophyta. Four membranes surround each chloroplast.

Chloroplast endoplasmic reticulum borders the chloroplasts. The principal food reserve is starch. Instead of a typical cell wall, a periplast composed of protein plates occurs beneath the cell membrane. There are about two hundred freshwater and marine species. Reproduction is primarily asexual.

Members of the phylum Dinophyta, or dinoflagellates, have unicellular forms with two different flagella. There are between two thousand and four thousand marine species and about two hundred freshwater forms. Many have chlorophylls a and c as well as the unique carotenoid peridinin. Some members of Dinophyta have fucoxanthin.

Chloroplasts have three closely associated membranes. The primary food reserve is starch, but lipids are also important storage molecules. A dinoflagellate cell is not surrounded by a cell wall but has a theca (a sort of armor) made of cellulose. Dinoflagellates can reproduce asexually and sexually.

Phylum Haptophyta includes primarily marine unicellular biflagellated algae. A haptophyte cell also has a flagellum-like haptonema, used to capture food. There are about three hundred species. The photosynthetic pigments include chlorophyll a and accessory pigments chlorophyll c and the carotenoid fucoxanthin.

Each chloroplast has four membranes. The food reserve is chrysolaminarin, which is a polymer of glucose. Several layers of scales, or coccoliths, composed primarily of calcium carbonate may cover the haptophyte cell. Asexual and sexual reproduction is widespread.

Phylum Rhodophyta, or the red algae, has between four thousand and six thousand species. Red algae lack any flagellated stages. The photosynthetic pigments include chlorophyll a as well as accessory phycobilins and carotenoids. Two membranes surround each chloroplast.

The food reserve is a floridean starch. A red algal cell is encircled by a wall composed of cellulose. Asexual and sexual re production, as well as alteration of generations, are widespread among Rhodophyta. A triphasic life cycle is unique for this group of algae.
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8:17 PM
Bioluminesence
Bioluminesence
Bioluminesence is the production of light by living organisms, including algae and phytoplankton in the oceans and fungi on land.

Bioluminescence is a specific form of chemiluminescence in which the chemical energy that is produced in a chemical reaction is converted into radiant energy. In bioluminescence the reaction originates in a wide variety of living organisms, including a small number of plants. It should not be confused with fluorescence or phosphorescence, both of which do not involve a chemical reaction.

In either of the former cases the energy from a source of light, not from a chemical reaction, is basically absorbed and then re-emitted in some form of another photon. The chemical reactions that lead to bioluminescence release energy in the form of light.

Unlike the light bulb, in which electrical energy is converted into light, with some of this energy lost in the form of heat, a bioluminescent reaction is 100 percent efficient and converts all the emitted energy into light. Because there is no heat released, bioluminescence is also known as "cold light."

Species and Habitats

Bioluminescence is primarily marine in nature and is the only source of light in the deep ocean, which is the largest habitable biome of the earth. The phenomenon rarely occurs in any source of fresh water. Bioluminescent organisms include ctenophores, annelid worms, mollusks, insects, and fish.

Species and Habitats
Species and Habitats
The most common manifestation of this phenomenon on land is seen as a glowing fungus on wood or in the few families of luminous insects. This property can be used as ameans of species recognition in the darkness as well as for courtship, preying, and mating.

There are several bioluminescent fungi that are not marine in nature, occurring primarily in the tropics. These fungi appear in different colors. The most common is Panellus stiptucus,which is a small decay fungus that is mostly restricted to North America. The jack-o’-lantern mushroom (Omphalotus olearius) glows brightly, especially when fresh. A few Armillaria species are also reported to glow mildly. No luminous tree or plant is known, however.

Mechanisms of Bioluminescence

Bioluminescence occurs only when two different species are in contact and, almost exclusively, when oxygen is present. The two species are luciferin, which produces the light, and luciferase, a protein that triggers and catalyzes the reaction. The mechanism involves the loss of two electrons, also known as oxidation, by luciferin, a process achieved only through the intervention of luciferase to yield oxyluciferin.

Occasionally luciferin, luciferase, and a cofactor such as oxygen are bound together in a single moiety called photoprotein, which leads to light formation upon contact with a positively charged species, such as the calcium cation. The mechanism appears to involve a peroxide decomposition with free radical intervention.

Dinoflagellates

Dinoflagellates known as Pyrrhophyta, or fire plants, are the most common sources of bioluminescence at the surface of the ocean. They are a group of marine algae that produce light upon mechanical, chemical, or temperature changes. The phenomenon was first observed in the genus Noctiluca in the nineteenth century and has since been observed to occur within other species.

Generally, three types of stimuli can cause bioluminescence in dinoflagellates: mechanical, chemical, and temperature stimulation.

Dinoflagellates
Dinoflagellates
Mechanical forms of stimulation, such as the stirring of water from a moving boat, a swimming fish, or a breaking wave, are prevalent in many Pyrrhophyta. The light appears to serve as a "burglar alarm" against grazing predators, which are then being seen through the flash by a larger second predator.

For example, as a copepod approaches the dinoflagellate, agitation of the seawater stimulates light flashes which a small fish, the secondary predator, uses to pinpoint the position of the copepod and eventually consume it. It appears that the mechanical stimulation deforms the cell membrane to create a short flash as little as one one-hundredth of a second.

Dinoflagellate luciferin is thought to derive from the similarly structured chlorophyll, which is found in most plants. The molecule is protected from luciferase at slightly basic medium by a luciferin-binding protein. However, once the acidity increases, the free luciferin reacts, and light is emitted.

The light produced by a single dinoflagellate is only six to eight photons in energy, and the flashing may last only one-tenth of a second. Larger organisms, such as jellyfish, provide flashes that may last up to tens of seconds. Temperature lowering in some dinoflagellate species also creates bioluminescence.

Purpose and Applications

The disappearance of the flash, once oxygen is consumed, has suggested that the bioluminescent reaction was originally used to remove toxic oxygen from primitive types of bacteria that developed at a time when oxygen was not available.

Bioluminescence has also played a crucial role in the direct studies of several cellular and biochemical processes, such as in the formation of ultimate carcinogens from benzoapyrene. The phenomenon has served scientists in many ways.

Calcium levels are monitored via the jellyfish biochemical system, adenosine triphosphate (ATP) measurements are achieved through the firefly, and the gene activity of organisms can be detected by splicing known bioluminescent proteins.
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10:14 PM
Seaweeds that are brown to olive-green in color belong to the phylum Phaeophyta, or brown algae, which includes between fifteen hundred and two thousand species.

Brown algae (phylum Phaeophyta) are familiar to most people as brown or dark green seaweeds. Some brown algae are microscopic in size, but many are relatively large: One giant kelp measured 710 feet in length. All brown algae are multicellular.

Appearance and Distribution

Brown algae have a body, called a thallus, which is a fairly simple, undifferentiated structure. Some thalli consist of simple branched filaments. Some brown algae have more complex structures called pseudoparenchyma because they superficially resemble the more complex tissues of higher plants.

Giant kelp have a thallus that is differentiated into a holdfast, a stipe, and one or more flattened, leaflike blades. The holdfast functions as the name implies, and holds the rest of the organism to the substrate.


It is a tough, sinewy structure resembling a mass of intertwined roots. The stalk that constitutes the stipe is often hollow, with a meristem (a zone of growing tissue) either at its base or at the blade junctions. Because the meristem produces new tissue at the base, the oldest parts of the blades are at the tips.

The blades, which, like most of the rest of the giant kelp body, are photosynthetic, may have gasfilled floats called bladders toward their bases, which may contain carbon monoxide gas. The function of this particular gas has not yet been determined.

The vast majority of species are marine, living in cold, shallow ocean waters, and may be the dominant plant life on rocky coastlines. The giant kelp can be found in waters around 100 feet deep. Only 4 of the 260 identified genera occur in fresh water. Brown algae of the order Fucales are commonly called rockweeds; kelp belong to the order Laminariales.

Brown algae are less common in tropical and subtropical areas. However, in the Caribbean region, sargassum (large masses of brown algae having a branching thallus with lateral outgrowths differentiated into leafy segments, air bladders, or spore-bearing structures) make up large floating mats; they gave their name to the Sargasso Sea.

Pigments and Food Reserves

The color of the brown algae can vary from light yellow-brown to almost black. Its color reflects the presence of varying amounts of the brown xanthophyll pigment fucoxanthin, a carotenoid pigment, in addition to chlorophylls a and c. The main food reserve is a carbohydrate called laminarin, although giant kelp can also translocate mannitol. Algin (alginic acid) can be found in or on the cell walls and may comprise as much as 40 percent of the dry weight of some kelps.

Reproduction

Reproductive cells of brown algae are unusual in that their two flagella are located laterally, instead of at the ends. The only motile cells in the brown algae are the gametes or reproductive cells. In the common genus Fucus, separate male and female thalli are produced. Fertile areas called receptacles develop at the tips of the lobes of the thallus. Each receptacle has pores on the surface.

These pores open into special spherical, hollow chambers called conceptacles, in which the gametes are formed. Eight eggs are produced in the female structure, while sixty-four sperm cells are produced in the male structure. Eventually, both eggs and sperm are released into the water, where fertilization takes place and the resulting zygotes develop into mature thalli.

Economic Uses

Brown algae have several uses and applications for humans. Giant kelp is eaten, and one species found in the Pacific Ocean has been used, in chopped-up form, as a poultice applied to cuts.

Algin, a colloidal substance produced by brown algae, is used as a thickener or stabilizer in commercially produced ice cream, salad dressing, beer, jelly beans, latex paint, penicillin suspensions, paper, textiles, toothpastes, and floor polish. Brown algae, with its high concentration of the element iodine, has been used to treat goiter, an iodine-deficiency disease. Kelp, also high in nitrogen and potassium, has been used as fertilizer and as livestock feed.

Some types of brown algae, such as Fucus, contain either phenols or terpenes. Botanists believe these substances may discourage herbivory. These substances also have been shown to possess microbe- and cancer-fighting properties. Brown algae is the subject of continuing research in these areas of medicine.
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7:31 AM
Charophyceae
Charophyceae
It is almost impossible not to see Spirogyra floating on the surface of a pond on a hot summer day, but most people dismiss it as pond scum. Few realize that what they are looking at is a member of the Charophyceae, a class in the phylum Chlorophyta, or green algae, and a cousin of the ancestor of the Embryophyta, or bryophytes and vascular plants.

Most Charophyceae, like Spirogyra, live in freshwater habitats, but some also occur in moist soil in terrestrial habitats. Charophyceae can live as single cells, colonies, or branched and unbranched filaments and come in a variety of shapes.

The characteristics that unite members of the class—and which link them with the embryophytes—include flagellated cells (similar to sperm cells in vascular plants), a nuclear envelope that breaks down during mitosis, mitotic spindles that persist as phragmoplasts (a type of cytoskeletal scaffolding) through cell division either by furrowing or by forming a cell plate, the presence of chlorophylls a and b and phytochrome, and the storage of starch inside plastids.


Charophytes possess decay-resistant cell walls made of phenolic compounds as well as lignins or ligninlike compounds. Cell walls made of similar compounds are found in bryophytes and vascular plants as well.

Likewise, all three groups of plants also contain sporopollenin, the substance in the walls of spores and pollen grains that makes them virtually indestructible.

Communication channels between cells are similar, too. Plasmodesmata similar to that seen in embryophyte cells allow between-cell communications in charophytes and embryophytes.

Life Cycle

Life Cycle
Charophytes have a two-stage life cycle involving a dominant haploid stage, upon which develops the sex organs; antheridia, which produce sperm cells; and oogonia, which produce egg cells. Typically, individual charophytes produce both antheridia and oogonia, but in some species an individual will produce only one or the other.

Fertilization—which in one group, the Zygnematales, takes place via conjugation—produces a diploid zygote, which quickly undergoes meiosis. If the environment is unfavorable, the zygote will go dormant and remain so for a long period of time. Dormancy ends when the environment improves.

Classification

Genetic analysis supports the recognition of six orders within the Charophyceae: the Mesostigmatales, Chlorokybales, Klebsormidiales, Zygnematales, Coleochaetales, and Charales.

Of these, the most abundant group is the Zygnematales, a large order which consists of more than three thousand species, including Spirogyra and the desmids, a group of mostly single-celled organisms with a constriction across the middle which nearly divides the cells in two.

Zygnematales live primarily in freshwater habitats as phytoplankton, as benthic dwellers, or attached to other aquatic plants. Some species live on snow and ice.

The Charales, commonly called stoneworts or brittleworts, are a large group of filamentous charophytes that feature complex branching patterns. Some can reach lengths of more than a meter. The branching pattern—branches reach out from nodes along the filament—is similar to that of higher plants.

Charales reside primarily in freshwater habitats, but some can be found in brackish water as well as on land. The stems of stoneworts and brittleworts can be encrusted with calcium and magnesium carbonates.

As a result, their hard bodies are well known from the fossil record. The lineage extends back to more than 400 million years ago. Two current genera, Chara and Nitella, date back about 200 million years.

The Coleochaetales are a small group of complex, microscopic filamentous algae that can be found only in freshwater habitats. Klebsormidiales are a small group of unbranched, filamentous charophytes that occur in both freshwater and terrestrial environments.

Mesostigmatales and Chlorokybales are two groups of rare algae. The Coleochaetales and Charales are more closely related to bryophytes and vascular plants than the other groups.

Evolutionary Significance

For decades, structural similarities led plant biologists to suspect that the Embryophyta evolved from charophytes. Recently, cladistic analyses of chemical, structural, and genetic characteristics have opened up research on the topic.

In cladistics, characteristics among a number of organisms are analyzed statistically in the hopes of developing a classification system for the group which will reveal evolutionary relationships. Cladistic analyses support the notion that embryophytes are monophyletic; in other words, bryophytes and vascular plants descend from a common ancestor.


Furthermore, several recent analyses support the notion that a member of the Charophyceae gave rise to embryophytes. Cladistic analyses were somewhat unclear, however, about the relationships of the charophyte orders with one another and with other green algae (Chlorophyta), bryophytes, and land plants.

One of the latest analyses of mitochondrial, chloroplast, and nuclear genes helps resolve some of the confusion. The research indicates that the Mesostigmatales were probably the most ancient group of charophytes, followed by the Chlorokybales, Klebsormidiales, Zygnematales, Coleochaetales, and Charales.

The work also supports earlier suggestions that the Charales are the closest living relatives to extant embryophytes and that the charophytes descended from other green algae.
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9:02 PM
Chlorophyceae
Chlorophyceae
Chlorophyceae (from the Greek word chloros, meaning “green”) make up an extremely large and important class of green algae. Members may be unicellular, colonial, or filamentous. Cells of unicellular and colonial chlorophyceans may have two or more flagella.

There are about 2,650 living species of chlorophyceans. The main features of the class (and most plants) are the use of starch as the principal food reserve and the green chloroplasts with chlorophylls a and b. In spite of plant characteristics, this algal group is not directly related to early land plants.

Chlorophyceans are almost entirely restricted to freshwater and terrestrial habitats. Some members of this class have adapted to life on snow as snow algae. Snow algae cause snow to appear red-burgundy or orange in color because of high levels of unusual carotenoid pigments within the algal cells.


There are a variety of asexual and sexual reproduction modes among members of this class. Sexual reproduction is characterized by the formation of a zygote produced by gametic fusion.

Chlorophyceans show differences during cell division compared to other green algal groups. For example, they produce a set of microtubules, the phycoplast, that is parallel to the plane of cell division.

Diversity

Diversity
Diversity
The Chlorophyceae include some familiar green algae. Perhaps the most famous chlorophyceans are Chlamydomonas (from the Greek word chlamys, meaning “cloth”) and Volvox (from the Latin volvo, meaning “to roll”).

Both are important research models in laboratories. Chlorophyceans fall into several orders, including Volvocales, Chlorococcales, Chaetophorales, and Oedogoniales.

Volvocales

Members of the order Volvocales include both unicellular organisms, such as those in the genus Chlamydomonas with their two equal flagella, and colonial forms. The Chlamydomonas are a large genus of chlorophyceans.

More than six hundred species have been described worldwide. The Chlamydomonas probably represent the most primitive structure among chlorophyceans. Nevertheless, their basic cell features may be found among other representatives of this order.

A cell wall made of glycoproteins, rather than cellulose, surrounds each Chlamydomonas cell. Inside the cell, there is a single large chloroplast and a pyrenoid, which forms starch.

Other cytoplasmic structures include the contractile vacuole rather than a central vacuole. The contractile vacuole is responsible for the removal of water from the cell. Cells of Chlamydomonas are capable of phototaxis: They swim toward moderate light but away from high-intensity light.

Rhodopsin-like pigment is their primary lightsensing photoreceptor. Under dry conditions, Chlamydomonas form a palmelloid stage, in which nonflagellate cells are held together by common mucilage.

Chlamydomonas reproduce asexually via cell division. Also, cells of this alga can become gametes. In most species of Chlamydomonas, the male and female gametes appear the same; they are designated (+) and (–).

Colonial flagellates of the order Volvocales range from simple colonies of Gonium to visible-without-magnification spheres of Volvox with up to several thousands of cells and some sort of cellular specialization.

Volvox are one of the most structurally advanced colonial forms of green algae.Only specialized cells participate in reproduction. During asexual reproduction, some cells of Volvox divide and bulge inward, forming new daughter colonies, which are held for some time within the parent colony. Volvox are also capable of sexual reproduction. They produce gametes that differentiate into sperm and eggs.

Chlorococcales

Members of the order Chlorococcales include nonmotile unicellular and colonial algae. Typical representatives of the unicellular nonmotile form are found in Chlorococcum. They occur as spherical single cells or cell aggregates and produce flagellated zoospores.

Examples of colonial representatives of Chlorococcales are Hydrodictyon, commonly known as the “water net”; Pediastrum, famous for their distinctive, starlike shape; and Scenedesmus, wide-spread inhabitants of the freshwater phytoplankton.

The order Chlorococcales has now been divided on the basis of small subunit ribosomal ribonucleic acid (RNA) sequence data into several groups, including the Sphaeropleales, Tetracystis clade, and Dunaliella clade.

Chaetophorales and Oedogoniales

Chaetophorales and Oedogoniales
Chaetophorales and Oedogoniales
The most complex of the class Chlorophyceae are the filamentous members in orders Chaetophorales and Oedogoniales, some of which exhibit features that are observed primarily in plants. The chaetophoralean green algae have plantlike bodies with a system of primary and secondary branches.

The Draparnaldia (named for Jacques Phillipe Raymond Draparnaud, a French naturalist) from order Chaetophorales have a main filamentous axis with relatively large cells, primary branches with smaller cells, and secondary branches with even smaller cells.

One representative of Oedogoniales, the green alga Oedogonium (from the Greek oidos, meaning “swelling”), has been a subject of intense study for its unusual cell division technique.

The entire contents of an Oedogonium cell may be used in the for mation of one large zoospore with multiple flagella. Members of Bulbochaete (from the Greek bolbos, meaning “bulb”) resemble Oedogonium in cell division but differ in being branched and having a distinctive hair cell at the end of each branch.

Technological Uses

A few chlorophycean green algae have commercial value. These algae are good candidates for the industrial production of hydrogen gas because they are able to release the gas from water using solar energy. Hydrogen gas is an environmentally desirable fuel because the burning of hydrogen produces water, and it can be converted effectively to electricity.

Another “commercial” organismis Dunaliella salina, a saltwater alga that accumulates massive amounts of beta-carotene, a vital antioxidant also used in food coloring and in pharmaceuticals.

Selenastrumcapricornutum are the most widely used algal biomonitors in the detection of water pollution. Chlorophyceans are used in freshwater aquaculture systems as food for fish.One alga with possible potential for salmon feeds is Haematococcus.

Algae contain large amounts of the pigment astaxanthin, which is responsible for the red coloration typical of salmon flesh. Chlorella (formerly classified in the order Chlorococcales) are famous both as the experimental systems in the discovery of the photosynthetic Calvin cycle and as health food in Asia.
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6:06 AM
The Chrysophyceae, classified within the kingdom Chromista, are mostly unicellular or colonial organisms found in fresh and salt water throughout the world.

The Chrysophyceae (in some systems corresponding to the phylum Chrysophyta) are related to heterokont algae and include more than eight hundred described species that are classified in approximately one hundred genera.

They aremost closely related to the Synurophyceae and other pigmented heterokont algae, including the Bacillariophyceae (diatoms), Eustigmatophyceae, Phaeophyceae (brown algae), and Xanthophyceae (yellow-green algae), among others.

The classification of chrysophycean species remains in a state of flux. In one system of classification primary importance is placed upon the number of flagella (zero, one, or two) that are present in the motile cell stage.


A second classification organizes species based upon the predominant vegetative state of the organism. For example, in this classification amoeboid, coccoid, palmelloid, and flagellate species are assigned to separate orders.

Ecology and Diversity

Chrysophytes are predominantly found in fresh-water environments, although some are marine, and a few are reported from soil or snow.

Members of the group are widely distributed but are most common in cold-temperate lakes, ponds, bogs, and ditches. Some species are common members of the phytoplankton,whereas others are epibionts or are neustonic (attached to the surface film of quiet water). Other species are only rarely observed.

Most chrysophytes are free-swimming unicellular or colonial flagellates. Others are coccoid (that is, immobile, walled unicells), amoeboid, or palmelloid (with cells enveloped in a gelatinous matrix). A few species are parenchymatous.

Cell Walls

Most chrysophytes lack a cell wall, but others produce species-specific outer coverings of scales or loricae. For example, complex siliceous scales or spines that are produced in silica deposition vesicles cover the cells of Paraphysomonas. The scales of Chrysolepidomonas are organic and of two types: those that are dendritic (tree-shaped) and those that are canistrate (cylindrical).

The cells of other species may be enclosed within an organic vaselike or flasklike lorica composed of cellulose and proteins or chitin (for example, Dinobryon, Pseudokephyrion, Poteriochromonas, Lagynion, and Stenocalyx).

In such species the lorica is typically composed of fine, interwoven fibrils. In Dinobryon these fibrils are helically arranged and secreted as the cell rotates about its longitudinal axis.

In contrast, the loricae of Epipyxis species are composed of imbricate, overlapping scales. The posterior pole of the cell is typically positioned at the base of the lorica and may be attached by a fine cytoplasmic extension; the flagella protrude externally through the lorica opening.

Flagella

Chrysophytes are heterokont, biflagellate organisms that swim with at least one flagellum forwardly directed. The two flagella ofmotile cells are anteriorly inserted in an apical or subapical position and are unequal in length.

The flagella differ morphologically and are heterodynamic. In most species, the basal bodies from which the flagella arise are either oriented at an acute angle to one another or are perpendicular to one another.

In Hydrurus, Chromphyton, and Lagynion, the basal bodies form an obtuse (oblique) angle with respect to one another. The long (immature) flagellum is anteriorly directed and is ornamented with two rows ofmastigonemes and finer lateral filaments. Eachmastigoneme is composed of a base, a tubular shaft, and one to three terminal filaments; these are known as tripartite tubular hairs.

Mastigonemes are produced in the perinuclear space between the two outer membranes of the chloroplast and the two surrounding membranes of the chloroplast endoplasmic reticulum. The long flagellum beats in an undulatory, sine-wave-like motions that are initiated at the base of the flagellum.

The relatively stiff short (mature) flagellum is directed laterally or posteriorly, lacks mastigonemes, and rotates helically. A distinct swelling associated with the eyespot is typically present at the proximal base of the smooth flagellum.

In some taxa (such as Chromulina, Chrysococcus, and Sphaleromantis), the short flagellum is highly reduced and may be nonemergent; it is therefore undetectable by light microscopy. In a handful of species the short flagellum is entirely absent, although the mature basal body may persist within the cell.

Naked motile cells bearing two visible flagella are often referred to as Ochromonas-like (or ochromonadalean), whereas those with one visible flagellum are typically assigned to the genus Chromulina.

The transitional region between the basal body and flagellum contains an electron-dense transitional plate, above which lies a coiled, apparently springlike transitional helix. The functions of the transitional plate and helix, which are also found in other flagellates, are uncertain.

In heterotrophic and mixotrophic species, the flagella play a role in prey capture. Particles actively captured by the flagella that are recognized as food are pushed into a feeding basket; those not recognized as food are released.

The feeding basket is formed and closed by movements of underlying microtubules. Water currents produced by the undulation of the long flagellum may passively bring food particles in contact with the cells that, in some species, are collected by pseudopodia.

Cell Organization

Cells possess a single pear-shaped nucleus that is positioned at the anterior end of the cell. The narrow end of the nucleus typically lies close to the basal bodies. A prominent Golgi apparatus with distended cisternae lies against the nucleus. Contractile vacuoles (absent in some marine forms) are also found at the anterior end of the cell.

One or more mitochondria with tubular cristae are present in the cell. Because the mitochondria are usually long and coiled, the actual number of mitochondria present is difficult to discern. Fibrous bands, sometimes referred to as connecting fibers, connect the basal bodies to one another.

A cross-striated band of fibers known as the rhizoplast extends from the basal apparatus and forms a connection to the nucleus. Typically four microtubular roots (R1, R2, R3, and R4) originate near the basal bodies, take characteristic paths through the cell, and proliferate beneath the plasmalemma.

For example, in most species roots R3 and R4 often form a loop beneath the short flagellum. Other microtubules are nucleated from the four major roots that provide the cytoskeletal elements needed to maintain cell shape.

Muciferous bodies or discobolocysts are present in some species. Muciferous bodies are capable of extruding long threads, whereas discobolocysts forcefully eject discoid projectiles. These functions of these organelles have been little studied but may be involvedin prey capture or predator avoidance.

Nutrition

The Chrysophyceae employ a variety of means to obtain energy. Most chrysophytes are photosynthetic but require an exogenous source of vitamins (such as vitamin B12, biotin, and thiamin) for growth.

It is probable that all chrysophytes are opportunistically or facultatively osmotrophic; that is, they are capable of directly absorbing small inorganic or organic molecules (such as sugars and amino acids) from the surrounding medium. Several species, particularly those with leucoplasts, are obligate heterotrophs that are bactivorous or consume small organic particles.

Mixotrophic species are also well represented among the chrysophytes. This category includes photosynthetic species that, routinely or under unfavorable conditions, supplement their nutrition via phagotrophy.

Chloroplasts, Photosynthetic Pigments, and Storage Products

The chloroplasts of chrysophytes are typically golden-brown or yellow-green in color, and there are usually one to two chloroplasts per cell.

Chloroplasts are peripherally located, and pyrenoids may be present or absent. Four unit membranes surround each chloroplast; the outer two are derived from the endoplasmic reticulum and are typically continuous with the nuclear envelope.

Chloroplast lamellae are typically composed of three adpressed thylakoid membranes, and a girdle lamella, which completely encircles the chloroplast, is usually present. The chloroplast deoxyribonucleic acid (DNA) is ring-shapedandlies just beneath the girdle lamella.

The light-harvesting complex of chrysophytes contains chlorophylls a and c, beta-carotene, and the xanthophylls fucoxanthin, neoxanthin, violaxanthin, and zeaxanthin. Among these, fucoxanthin is dominant and is therefore responsible for the golden-brown color observed in most chrysophytes.

The major product of photosynthesis is a water-soluble ο-1,3-linked glucan (known as chrysolaminarin or leucosin) that is stored in cytoplasmic vacuoles in the posterior region of the cell. Lipids may also be produced and are also stored in the cytoplasm.

Eyespots (or stigmata) are present in many, but not all, species. The eyespot takes the form of a single layer of orange or reddish colored, lipidlike droplets that are located just beneath the chloroplast membrane. These droplets lie near a swelling located at the base of the smooth (short) flagellum; together the eyespot and flagellar swelling form a photoreceptor apparatus.

Several chrysophyte genera are known that contain a vestigial chloroplast (leucoplast) that lacks pigments (including Anthophysa, Monas, Oikomonas, Paraphysomonas, and Spumella).

Reproduction

Asexual reproduction in amoeboid and flagellate species occurs by longitudinal division of the cell; fragmentation is common among colonial, palmelloid, and parenchymatous species. In coccoid species reproduction may proceed via cell division or the formation of autospores that rupture and exit the parent cell wall.

Some taxa, such as the parenchymatous genera Phaeodermatium and Hydrurus or members of the palmelloid family Chrysocapsaceae, reproduce by means of flagellated swarmers (zoospores).

Under certain environmental conditions, silicified resting cysts, or statospores, are produced by many species. Statospores are formed endogenously, are roughly spherical or ellipsoidal, and have walls that may be smooth or ornamented.

The stomatocyst opening (porus) may be simple, possess a thickened collar, or take the form of a narrow neck. The cyst wall is formed by the deposition of silicate on an internal membrane, and the porus is preformed or produced by resorption of a portion of the cyst wall.

Depending on the species, cytoplasm located outside the cyst wall may or may not be absorbed through the porus, which at maturity is occluded by a pectic plug. During excystment the plug is lost, and one or more amoeboid or free-swimming flagellate cells emerge.

Sexual reproduction is known only in a handful of species. In those cases observed, vegetative cells behave as gametes and fuse apically. The resulting quadri flagellate cell (planozygote) will encyst forming sexually derived binucleate hypnozygotes or stomatocysts.

It is presumed that karyogamy (nuclear fusion) and meiosis occur within the cyst, but these processes have yet to be studied. Depending upon the species, sexual stomatocysts may give rise to one, two, or four vegetative cells.
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10:04 AM
Cryptomonads
Cryptomonads
The phylum Cryptophyta describes tiny, motile, unicellular organisms with two slightly unequal flagella bearing lateral hairs. Cryptomonads live mainly in marine and freshwater environments.

Some cryptomonads are alga-like, with bluegreen, red, and olive-brown photosynthetic pigments including chlorophylls a, c2, alpha-carotene, xanthophylls (alloxanthin, crocoxanthin, zeaxanthin, and monadoxanthin), and phycobiliproteins (phycoerythrin and phycocyanin).

Cryptomonads are found in a variety of moist places, such as algal blooms in the ocean or in fresh water, and on beaches. Some members are intestinal parasites in animals.

Classification

Historically, botanists and zoologists alike have adopted cryptomonads. Botanically, cryptomonads would be included in kingdom Plantae, phylum Cryptophyta, class Cryptophyceae, order Cryptomonadales, and family Cryptomonadaceae.


Zoologists would place the cryptomonads in kingdom Animalia, phylum Sarcomastigophora, class Phytomastigophora, order Cryptomonadida, and family Cryptomonadidae.

Also, the cryptomonads are currently included in a separate kingdom, Protoctista (also known as Protista), with phylum, class, order, and family taxa being the same as that for the botanical taxa above.

Synonyms for Cryptophyta are Cryptomonadales, Cryptophyceae, and Chromophyta, and they were once placed with the algae. Genera for the cryptomonads include the following: Chilomonas, Chroomonas, Cryptomonas, Cyathomonas, Falcomonas, Geminigera, Goniomonas, Guillardia, Hemiselmis, Komma, Plagiomonas, Pyrenomonas, Rhodomonas, Storeatula, and Teleaulax.

Guillardia theta, formerly known as Cryptomonas phi, has been studied most extensively, and the complete chloroplast genome is known (Genbank accession number AF041468).

Cryptomonads
Cryptomonads

Ecology

The cryptomonads are part of the nanoplankton (typically phytoplankton between 2 to 20 micrometers in diameter) and are a relatively small but ecologically and evolutionarily important taxon. Both freshwater and marine representatives are known.

Many photosynthetic species likely retain their capacity to eat prey (mixotrophy). Fluctuations in their numbers are correlated with increases in levels of nitrogen in the water in which they live. Some species of cryptomonads form gelatinous colonies.

A few cryptomonads have reached the palmelloid or sessile stage of organization, but most are free-living flagellates common in nutrient-rich water. A weakly filamentous member of the cryptomonads is Bjornbergiella. Well-known examples of cryptomonads are Cryptomonas ovata, C. similis, and Chilomonas paramecium.

The cryptomonads can form major blooms in Arctic and Antarctic waters as well as in North America’s Chesapeake Bay. They can be very important food sources for smaller heterotrophic or mixotrophic plankton, including ciliates and dinoflagellates. Cryptomonads are found in freshwater lakes, ponds, and ditches—especially in colderwaters.

They are dominant species in many Antarctic lakes, and they are also found in interstitial water on sandy beaches. Reproduction in the cryptomonads is generally asexual in culture, but sexual reproduction has been documented in the Cryptophyta.

Characteristics

cryptomonads characteristics
cryptomonads characteristics

Cryptomonads are tiny flagellates, 5 to 30 microns (most around 10 to 20microns). They are flattened dorsiventrally in shape and are asymmetrical, with a periplast (a proteinaceous structure that lies inside the plasmalemma and is attached to it). Cryptomonads are mostly algal forms, with an anterior (ventral) groove or pocket and a gullet, which has refractile ejectosomes or trichocysts.

The unequal flagella are inserted nearly parallel to the pocket, not inside the gullet as in the euglenophytes. Mitochondria have flat cristae, while the plastids are secondary with a highly reduced eukaryotic nucleus, the nucleomorph.

Although there are a few colorless forms, such as Chilomonas, most cryptomonads have a chloroplast. The chloroplast is not contained directly, however, because there is a reduced eukaryote symbiotic within the cell containing a normal prokaryote chloroplast. Usually, there are two chloroplasts, which are secondary plastids.

The chloroplast is bound by four membranes (two being chloroplast endoplasmic reticulum, or CER, continuous with the nuclear envelope and homologous to a food vacuole) with a tiny nucleus (nucleomorph) between the middle two membranes.

Much like the dinoflagellates (with which the cryptomonads were formerly grouped), it has chlorophylls a and c; chlorophyll b is never present. Thylakoids are paired, and phycobilin pigments are present in the spaces between the thylakoids but not in phycobilisomes, such as in the cyanobacteria and Rhodophyta.

Food reserves are starch like, accumulating in the periplastidal space stored between the starch envelope and the chloroplast reticulum. If there is an eyespot, it is inside a plastid not associated with the flagella.

There is a large nucleus at the posterior end. Mitosis is open, and centrioles are not associated with mitosis. Cell division is by furrowing. No histones are associated with the chromosomes.

The unique nucleomorph has deoxyribonucleic acid (DNA), is contained within a double membrane, and also has a nucleolus-like region.Molecular data suggest and strongly support the idea that the nucleomorph is a vestigial nucleus from the original endosymbiont, which became the cryptophyte chloroplast.

Three chromosomes are associated with the nucleomorph: 240 kilobase pairs (kb), 225 kb, and 195 kb. The bulk of chromosome II (175 kb) is now sequenced with a preponderance of “housekeeping genes” apparently existing for the service of just a few genes encoding plastid proteins.

There are parallels between the nucleomorphs of cryptomonads and chlorarachniophytes. The cryptomonad nucleomorph is depauperate in introns, with ribosomal ribonucleic acid (rRNA) genes at the chromosome ends just within the telomeres.

The longer flagellum has two rows of mastigonemes (lateral hairs), while the shorter flagellum has a single row. Mastigonemes are two-parted bristles or hairs made up of a rigid, tubular base and, usually, two terminal hairs.

These bristles are formed within the endoplasmic reticulum (or nuclear envelope) and are thus transported to the exterior of the cell. The flagella are covered with scales, too.

Trichocysts or ejectosomes are in the oral groove, and they are scattered around the cell surface. There is a tightly spooled protein in the trichocysts, which can undergo a very rapid, irreversible conformational change in which it pops out of the cell, pushing the cell backward as a result. The trichocysts are considered a defense mechanism or are perhaps involved in predation.

Evolution

Evolution and phylogeny of cryptomonads have not been well documented using molecular techniques. An 18S ribosomal RNA phylogeny of cryptomonads has been made, however.

The nucleus of the endosymbiont (the nucleomorph) does not seem to have a complete complement of genes for photosynthesis, these being relocated to the host nucleus now. Plastid targeting mechanisms of cryptomonads for light-harvesting complex proteins have been studied, though.

The primary plastid is of unknown origin and remains under scrutiny but is probably from a red algal lineage, as the presence of phycobilins is suggestive of the Rhodophyta (red algae). Chlorophyllc is unknown among the red algae, however.

Molecular phylogenetic studies place the nucleomorph close to red algae, and the chloroplast genomemap has characteristics suggesting a reduction series from red algae to cryptomonads to heterokonts. This does not imply any descendant relationship among extant groups but rather retention of ancestral character states from common ancestors.

Lateral gene transfer from an ancestral cryptomonad to a dinoflagellate is postulated from sequence analysis of two nuclear-encoded glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes isolated from the dinoflagellate Gonyaulax polyhedra.

The plastid sequence forms amonophyletic group with the plastid isoforms of cryptomonads, distinct from all other plastid GAPDHs. This provides the first example of genetic exchange accompanying symbiotic associations between cryptomonads and dinoflagellates, which are common in present-day cells.
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