Algae are a diverse group of simple organisms that live in aquatic environments, including the ocean, freshwater, ponds and moist land. Algae contain chlorophyll and are able to go through the process of photosynthesis, like plants; however, they lack the organs and kinds of cells found in plants. The structure of algae may be unicellular, colonial or multicellular. Studies have shown that green and red algae are common ancestors to modern plants. Algae are classified based on the composition of their cell walls, the way in which food reserves are stored and the type of photosynthetic pigments present.
Bacillariophyta is a phylum of algae comprising the diatoms. Bacillariophytes are unicellular photosynthetic eukaryotes, whose cell shapes are either round (centric) or elongated (pennate). They’re abundant in marine and freshwater environments and form the base of many aquatic food chains. Bacillariophyta have cell walls composed of pectin mixed with silica and comprising two halves, one overlapping the other. The fucoxanthin pigment gives them their characteristic golden brown color. They also contain chlorophylls a, c and carotenoids. They store food as starch. The deposition of diatoms has resulted in diatomaceous earths. Diatomaceous earth is sometimes used as pesticide in agriculture, and in the manufacturing business.
Euglenophyta is a phylum of photosynthetic unicellular organisms that are found in freshwater bodies, although some species may also be found in estuaries and the intertidal zone. In the absence of light, some species of euglenids may become heterotrophic, making them mixotrophic. They contain the chlorophylls a, b and carotenoids. They store food as a carbohydrate known as paramylon. The cell wall is covered with a pellicle, which may be flexible, allowing the euglenid to contort while moving. Euglenids are the only autotrophs, belonging to the super group excavates. A red photoreceptor known as the eyespot is a distinctive feature of the euglenids.
Dinophyta is a phylum of unicellular organisms characterized by two flagella, one protruding from the cell and the other situated in a groove surrounding the cell. Most dinophytae are mixotrophic, though autotrophic and heterotrophic species exist. They store their food as starch outside of the chloroplast. They contain the chlorophylls a and c, as well as the pigment fucoxanthin. Some species are found as photosynthetic endosymbionts in marine organisms, including corals and molluscs. Dinoflagellates are some of the most common sources of bioluminescence in the ocean.
Dinoflagellata: Large group of flagellate protistis. Some species are heterotrophic, but many are photosynthetic organisms containing chlorophyll. Various other pigments may mask the green of these chlorophylls. Other species are endosymbionts of marine animals and protozoa, and play an important part in the biology of coral reefs. Other dinoflagellates are colorless predators on other protozoa, and a few forms are parasitic. Reproduction for most dinoflagellates is asexual, through simple division of cells following mitosis. The dinoflagellates are important constituents of plankton, and as such are primary food sources in warmer oceans. Many forms are phosphorescent; they are largely responsible for the phosphorescence visible at night in tropical seas.
Chrysophyta (golden algae) are a large group of eukaryotes found in freshwater bodies. The cell walls of chrysophytes are composed of cellulose, along with large quantities of silica. They contain chlorophylls a and b. Chrysophytes reproduce sexually and asexually by cell division. Certain species of chrypsophytes are colorless; however, the majority are photosynthetic. The fossils of chrysophytes are used as indicators to reconstruct ancient environments. They form an important source of food in the freshwater ecosystem.
Chlorophyta (green algae) comprise a large phylum of algae which contain chlorophylls a and b, but may also contain carotenoids as accessory pigments. They have cell walls made of cellulose and store their food reserves as starch. Chlorophyta are very diverse in form, with unicellular forms often occurring with flagella for movement. Unicellular forms may sometimes congregate in colonies, while multicellular forms may occur as filamentous structures, such as spirogyra, or plate-like, such as ulva. Because of their similarities in structure to plants, chlorophytes are often placed, along with plants, in a super group known as archaeplastida.
Phaeophyta (brown algae) are a phylum of algae usually found in the cold waters of the marine environment. Phaeophytes are multicellular organisms, some of which include seaweed and kelp, often reaching many meters in length. They usually form chains of cells known as filaments. Some phaeophyta possess structures resembling roots (haptera), stems (stipes) and leaves (blades), like a plant. Haptera, a holdfast structure, maintains the algae attached to the substrate, and air bladders maintain the algae afloat, allowing them to carry on photosynthesis. Phaeophyta contain the chlorophylls a and c. They also contain carotenoids and fucoxanthin, which give them their brownish coloration. They store food as a carbohydrate called laminarin.
Rhodophyta (red algae) are a phylum of algae often found along the coasts of tropical and temperate seas; however, some species of rhodophytes may be found in fresh waters. Rhodophytes are multicellular, forming branched flattened thalli or filaments. They can also be unicellular in a few rare exceptions. Rhodophytes contain chlorophyll a. They also contain phycoerrythrin and phycocyanin as accessory pigments that help them trap sunlight for photosynthesis. They store food as starch and their cell walls are made of a substance known as carrageenan. Rhodophytes are classified as belonging to the group of archaeplastida.
Cyanobacteria were considered as algae in the past; however, recent studies have discontinued them due to the large differences in types of organelles from those found in eukaryotes. Cyanobacteria carry on photosynthesis in cytoplasmic membranes rather than in chloroplasts. Nearly all algae derive their photosynthetic structure from cyanobacteria. According to algaebase.org, the identification of algae requires specialised examination and expertise. Some types of algae require culturing and extraction of DNA for sequencing.