Plantlike Protists - Algae

The phyla grouped here as eukaryotic algae consist mainly of photosynthetic organisms, although some phyla include heterotrophic or mixotrophic members. The term alga refers to relatively simple aquatic organisms that are photoautotrophs. Except for the prokaryotic cyanobacteria (formerly called blue-green algae), the organisms generically called algae are eukaryotes that most systematists classified in kingdom Protista in the five-kingdom system.

Algae are extremely important ecologically, accounting for about half the photosynthetic production of organic material on a global scale. As freshwater and marine phytoplankton and intertidal seaweeds, algae are the bases of aquatic food webs, supporting an enormous abundance and diversity of animals.

All algae have chlorophyll a, the same “primary” pigment found in plants. But algae differ considerably in their accessory pigments, pigments that trap wavelengths of light to which chlorophyll a is not sensitive. These pigments include other forms of chlorophyll (greenish), carotenoids (yellow-orange), xanthophylls (brownish), and phycobilins (red and blue varieties). The mixture of pigments in the chloroplasts lends characteristic colors to related algae. Many of the scientific and common names of algal phyla are based on these colors (the Chlorophyta, or green algae, for example). Study of these pigment mixes has helped establish taxonomic affinities among the algae. Additional clues have come from chloroplast structure; the chemistry of cell walls (a few algal phyla lack walls); the number, type, and position of flagella; and the form of food stored by the cells.
 
 
Dinoflagellata (Dinoflagellates)

Dinoflagellates are abundant components of the vast aquatic pastures of phytoplankton, microscopic algae floating near the surface of the sea that provide the foundation of most marine food chains.

Dinoflagellate blooms, episodes of explosive population growth, cause the red tides that occur occasionally in warm coastal waters. These blooms are brownish-red because of predominant xanthophylls in the chloroplasts. When suspension-feeding shellfish such as oysters feed on these blooms, they concentrate the algae along with toxic compounds released by the dinoflagellate cells. These toxins are extremely dangerous to humans, who collect and eat the invertebrates. As a consequence, collecting shellfish during red tides is often regulated to prevent the widespread occurrence of “paralytic shellfish poisoning.”


 
Bacillariophyta (Diatoms)

The members of the phylum Bacillariophyta, or diatoms, are yellow or brown in color. They are closely related to two other algal phyla with brown plastids, the golden algae (Chrysophyta) and brown algae (Phaeophyta).

Diatom cells have unique glasslike walls consisting of hydrated silica embedded in an organic matrix. Each wall is in two parts that overlap like a shoe box and lid. Many diaotoms are capable of a gliding movement caused by chemicals secreted out of slits in their cell walls.

Chrysophyta (Golden Algae)

Golden algae are named for their color, which results from yellow and brown carotenoid and xanthophylls accessory pigments. Their cells are typically biflagellated, with both flagella attached near one end of the cell. Golden algae live among freshwater plankton. Most species are colonial. In ponds and lakes that freeze in winter or dry up in summer, golden algae survive by forming resistant cysts, from which active cells emerge when conditions are favorable. Microfossils resembling the ruptured cysts of chrysophytes and other algae are among the acritarchs found in Precambrian rocks.

Phaeophyta (Brown Algae)

The largest and most complex algae are members of the phylum Phaeophyta, or brown algae. All are multicellular and most are marine, including the largest seaweeds. Brown algae are especially common along temperate coasts, where the water is cool. They owe their characteristic brown or olive color to accessory pigments in the chloroplasts. The chloroplast structure and pigment composition of brown algae are homologous to the photosynthetic equipment of golden algae and diatoms.

Rhodophyta (Red Algae)

The majority of members of Rhodophyta, or red algae, live in the ocean, but there are also some freshwater and soil species. Rhodophytes are commonly reddish because of an accessory pigment called phycoerythrin. It belongs to a family of pigments known was phycobilins, found only in red algae and cyanobacteria.

Red algae are most abundant in the warm coastal waters of the tropics. The phycobilins and other accessory pigments allow some species to absorb filtered wavelengths (blues and greens) in deep water. A species of red alga has recently been discovered living near the Bahamas at a depth of more than 260 m.

Chlorophyta (Green algae)

This phylum is named for its members’ grass-green chloroplasts, which are much like those of plants in ultrastructure and pigment composition. In fact, most botanists believe that the ancestors of the plant kingdom were related to a group of green algae.

More than 7000 species of green algae have been identified. Most live in fresh water, but there are also many marine species. Various species of unicellular green algae live as plankton, inhabit damp soil or snow, or occupy the cells or body cavities of protozoa and invertebrates as photosynthetic symbionts that contribute to the food supply of the hosts. Chlorophytes are also among the algae that live symbiotically with fungi in the mutualistic collectives known as lichens.