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Bacillariophyta are unicellular organisms that are important components of phytoplankton as primary sources of food for zooplankton in both marine and freshwater habitats. Most diatoms are planktonic, but some are bottom dwellers or grow on other algae or plants.
Except for their male gametes, diatoms lack flagella. Instead many diatoms achieve locomotion from controlled secretions in response to outside physical and chemical stimuli. Diatoms have unique shells, which serve as their cell wall. The overlapping shells, or frustules that surround the diatom protoplasm are made of polymerized, opaline silica. Identification of diatom species is based on the delicate markings on their frustules, comprising a large number of tiny, intricately-shaped depressions, pores and passageways that bring the diatom’s cell membrane in contact with the environment. Diatom frustules have accumulated over millions of years to form the fine, crumbly substance known as diatomaceous earth, which has a variety of uses (e.g. for filtration and insulation). Diatom remains in both marine and freshwater sediments are also important as indicators of paleo-environmental conditions at the time the sediments were formed.
Bacillariophytes have brownish plastids containing chlorophylls a and c and fucoxanthin. The primary means of reproduction is asexual, by cell division. Most diatoms are autotrophic, but a few are obligate heterotrophs (they must absorb organic carbon) because they lack chlorophyll altogether. Some diatoms even lack their distinctive frustules and live symbiotically in large marine protozoa, providing organic carbon for their hosts.
Charophyta are freshwater plants and generally grow anchored to the substratum by rhizoids with a shoot extending upward. The shoot then divides and forms nodes from which a whorl of side filaments projects. Charophyta reproductive structures develop at these nodes and are, along with the biflagellate sperm produced in the male gametangium, quite similar to those of mosses. These similarities have led some scientists to identify the charophytes as ancestors of the mosses. Their green color comes from chlorphylls a and b.
Most chlorophytes are aquatic, but some green algae can live on the surface of snow, on tree trunks, in soils, or symbiotically with protozoans, hydras or lichen-forming fungi. Numbering about 8,000 species, the chlorophytes range in size from microscopic to quite large. The typical color of plants in the Chlorophyta, resulting from the dominant chlorophyll pigments, is some shade of apple or grass green, although certain species may appear yellow-green or blackish-green due to the presence of carotenoid pigments or high concentrations of chlorophyll. Chlorophytes appear more than a billion years ago in the fossil record.
Calcified green algae, particularly Halimeda spp., are especially important as major contributors of marine sediments. The sparkling white sand beaches of the Caribbean and many other areas in the world are largely the sun-bleached and eroded calcium-carbonate remains of green algae. The deepest occurring, fleshy, erect alga Johnson-sea-linkia profunda, (Littler et al., 1985)was found attached to bedrock at a depth of 157 meters off the Bahamas and is a member of this group.
Green algae have chlorophylls a and b and store starch as a food reserve inside their plastids. Most green algae have firm cell walls composed of cellulose along with other polysaccharides and proteins.
Chrysophytes are photosynthetic, unicellular organisms that are abundant in freshwater and marine environments. Chrysophytes contain chlorophylls a and c, which are masked by the accessory pigment fucoxanthin, a carotenoid. In many ways, golden algae are, biochemically and structurally similar to brown algae. Both golden algae and brown algae store food outside of the chloroplast in the form of polysaccharide laminarin, or chrysolaminarin. In both groups, motile cells have unequal flagella of similar structure.
Even though the Cyanobacteria are classified as bacteria (lacking a membrane-bounded nucleus) they are photosynthetic and are included among our algal collections. Cyanobacteria played a decisive role in elevating the level of free oxygen in the atmosphere of the early Earth. Cyanobacteria can change remarkably in appearance, depending on the environmental conditions. Blue-green algae are common in soil, in both salt and fresh water, and can grow over a wide range of temperatures. They have been found to form mats in Antarctic lakes under several meters of ice and are responsible for the beautiful colors of the hot springs at Yellowstone and elsewhere. Cyanobacteria can also occur as symbionts of protozoans, diatoms and lichen-forming fungi, and vascular plants. Some blue-greens can fix nitrogen as well as photosynthesize, allowing them to grow with only light, water, a few minerals, and the nitrogen and carbon dioxide in the atmosphere.
Cyanobacteria are different in many important ways from other photosynthetic prokaryotes. Instead of the bacteriochlorophylls found in purple and green bacteria, blue-greens contain chlorophyll a, as in eucaryotic phototrophs, and, produce free oxygen as a byproduct of photosynthesis. Cyanobacteria, however, lack the organized chloroplasts of eukaryotes and have their photosynthetic apparatus distributed peripherally in the cytoplasm.
The variety of striking colors exhibited by Cyanobacteria are a result of their major light-gathering pigments, the phycobilins, that are bound to protein granules, (phycobilisomes), that are attached to the photosynthetic membranes.
Large blooms of freshwater Cyanobacteria may produce toxins that can kill livestock. Other forms (Spirulina) are grown commercially and marketed as a high-protein dietary supplement.
[Dinoflagellata - Pyrrophyta - Pyrrhophyta] * (Current names in use by various authorities)
The division Pyrrophyta (from the Greek "pyrrhos" meaning flame-colored) comprises a large number of unusual algal species of many shapes and sizes. There are about 130 genera in this group of unicellular microorganims, with about 2000 living and 2000 fossil species described so far.
The name "dinoflagellate" refers to the forward- spiraling swimming motion of these organisms. They are free-swimming protists (unicellular eukaryotic microorganisms) with two flagella, a nucleus with condensed chromosomes, chloroplasts, mitochondria, and Golgi bodies. Biochemically, photosynthetic species possess green pigments, chlorophylls a and c, and golden brown pigments, including peridinin. Dinoflagellates primarily exhibit asexual cell division, some species reproduce sexually, while others have unusual life cycles. Their nutrition varies from autotrophy (photosynthesis; in-nearly 50% of the known species) to heterotrophy (absorption of organic matter) to mixotrophy (autotrophic cells engulf other organisms, including other dinoflagellates).
Free-living dinoflagellates are an ancient and successful group of aquatic organisms. They have adapted to pelagic (free-floating) and benthic (attached) habitats from arctic to tropical seas, and to salinities ranging from freshwater, to estuaries, to hypersaline waters. Many species are found in numerous habitats, living in the plankton or attached to sediments, sand, corals, or to macroalgal surfaces or to other aquatic plants. Some species are present as parasites in marine invertebrates and fish. Some even serve as symbionts, known as zooxanthellae, providing organic carbon to their hosts: reef-building corals, sponges, clams, jellyfish, anemones and squid.
Dinoflagellates exhibit a wide variety in morphology and size (from 0.01 to 2.0 mm). They commonly have a cell covering structure (theca) that differentiates them from other algal groups. Cells are either armored or unarmored. Armored species have thecae divided into plates composed of cellulose or polysaccharides which are key features used in their identification. The cell covering of unarmored species is comprised of a membrane complex. The theca can be smooth and simple or laced with spines, pores and/or grooves and can be highly ornamented.
In systematics, dinoflagellates have been claimed by both botanists and zoologists. Dinoflagellates share features common to both plants and animals: they can swim, many have cell walls, and both photosynthetic and nonphotosynthetic species are known. Botanists have grouped them with the "microalgae" and zoologists have grouped them with the protozoa, and both have produced classification schemes for this diverse and confusing group.
Dinoflagellates have attracted a lot of negative attention from the general public in recent times. For example, blooms (population explosions) of dinoflagellates can cause the water to turn a reddish-brown color known as "red tide". Red tides can have harmful effects on the surrounding sea-life and their consumers. Additionally, certain species of dinoflagellates produce neurotoxins. These toxins are carried up the food chain, ultimately to humans and can, sometimes result in permanent neurological damage or even death. Yet dinoflagellates are important members of the phytoplankton in marine and freshwater ecosystems.
The Phaeophyta are almost entirely marine, frequently dominating rocky shores in cold and temperate waters throughout the world. The giant kelp, Macrocystis pyrifera , forms expansive seaweed forests off the west coast of North America and provides habitat and shelter for many other organisms. Tropical waters have fewer species of brown algae, although genera such as Sargassum and Turbinaria can dominate in some areas to form small-scale forests. Sargassum is also unique among macroalgal genera in that it contains totally free-floating species with no requirement for attachment to the bottom, as in the Sargasso Sea.
The colors of brown algae (predominantly due to the brown accessory pigment fucoxanthin) cover a spectrum from pale beige to yellow-brown to almost black. In tropical seas, they range in size from microscopic filaments to several meters in length.
The large kelps are harvested for use as an emulsion stabilizer, in products such as ice cream. They are also used as fertilizer and as a vitamin rich food source.
Of the approximately 6000 species, most red algae are marine; only a few occur in freshwater. Rhodophytes are usually multicellular and grow attached to rocks or other algae, but there are some unicellular or colonial forms. They do not have flagellated cells, are structurally complex, and have complex life cycles divided into three phases. Many red algae feature pit connections between the cells, and their cell walls include a rigid component composed of microfibrils and a mucilaginous matrix. Agar and carrageenin are two red algal mucilages that are widely used for gelling and thickening purposes in the food and pharmaceutical industries.
Rhodophytes contain chlorophyll a which is masked by phycobilin pigments bound to proteins. The chloroplasts in red algae resemble Cyanobacteria both biochemically and structurally. Food reserves are stored outside of the chloroplasts as Floridean starch.
The coralline red algae deposit calcium carbonate in their cell walls, making them particularly tough and stony. They are often abundant, ecologically important, and widespread from the arctic regions to the tropics and play an important role in building tropical reef communities. Coralline red algae can form an algal ridge that absorbs wave energy and thereby protects the more delicate organisms that inhabit the sheltered lagoons and back-reef habitats.
Research scientist and curator Dr. Walter H. Adey has extensively collected the crustose corallines throughout the world since the 1960's, most notably in the North Atlantic, Caribbean and Hawaii. The specimens from the northwest Atlantic, north Norway and Iceland have been curated and inventoried during 2000-2001 with the assistance of Sue Lutz (Research and Curatorial Assistant). The specimens from Japanese regions, as well as the remaining European collections, are currently being curated and inventoried.
Some red algae are of economic importance, either as food (Porphyra) or as producers of secondary products (Gelidium, Gracilaria, Chondrus, etc.) used in the food and drug industries.