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Cyanobacteria
Cyanobacteria, also called "bluegreen algae," are members of Kingdom Monera. They are prokaryotes, which are single-celled organisms lacking structures called organelles and nucleus-contained DNA. Blue-green algae contain chlorophyll and conduct oxygen-generating photosynthesis. For more than 3 billion years they have flourished in the oceans, even altering seawater's overall chemical makeup. Kingdoms Protista, Fungi, Plantae and Animalia all apparently evolved from cyanobacteria. Today more than 200 species of cyanobacteria still live, 75 percent of them marine.
Chlorophyte Plants
Chlorophytes, green algae, may be ancestors to all other plants. There are over 7,000 chlorophyte species, many of them single-celled members of phytoplankton communities. More chlorophytes live in freshwater than saltwater habitats. Like other plants, these single-celled green algae are eukaryotes. Eukaryotic cells contain organelles and DNA enclosed in a membrane-bound nucleus. The most characteristic organelles in chlorophytes and other plants are chloroplasts, the chlorophyll-bearing sites of photosynthesis.
Chrysophyte Protists
Kingdom Protista, which encompasses an amazing range of single-celled eukaryotes, is very well represented in phytoplankton. Chrysophyte protists generally engage in photosynthesis. Chrysophytes encompass golden-brown algae, coccolithophores, silicoflagellates and diatoms. Golden-brown algae is predominantly found in fresh water. The other chryosphytes are mostly ocean species, with often beautiful internal skeletons or cell walls built up of silica or calcium carbonate. Diatomaceous earth, an ingredient in toothpaste, is comprised of fossilized shells from diatoms, the most numerous kind of chrysophyte.
Dinophyte Protists
Most, but not all dinophytes, conduct photosynthesis. They are found most abundantly in warm seas. Of the more than 110 dinophyte species, 93 percent live in salt water. Some dinophyte species, such as Noctiluca, are famous for their ability to glow in the dark. When they become overpopulated, possibly in response to pollution or weather conditions, dinophytes such as Ptychodiscus create toxic blooms called "red tides." These harm humans and animals, especially fish and marine mammals.
Euglenoid Protists
The 800 species of euglenoids live exclusively in fresh water. Some make their own food through chlorophyll-based photosynthesis. Others are heterotrophs, meaning that they ingest organic substances or other microbes for nourishment. Still others can nourish themselves either way. Many science students are familiar with Euglena gracilis, a pond euglenoid with a red eyespot.