Green algae fix over a billion tons of carbon a year
Chlorophytes range from single swimming cells to seaweeds on rocky shores, and as phytoplankton they lock away more than a billion tons of carbon annually. One species colours alpine snowfields in summer; others team up with fungi to make lichens. A couple of rogue relatives even cause disease in people and animals.
Chlorophyta is one of the two great divisions of green algae. Its sister is Charophyta, the lineage that gave rise to land plants, and together with a proposed basal group, Prasinodermophyta, they make up Viridiplantae, the green plants. Most chlorophytes are single cells, including the earliest-branching prasinophytes, but the classes Chlorophyceae and Ulvophyceae trend toward elaborate colonies and true multicellularity.
Their chloroplasts carry chlorophylls a and b and the same carotenoids found in land-plant leaves, plus special pigments such as astaxanthin that pile up under nitrogen shortage, strong light or high salt. Swimming cells usually bear two or four equal flagella anchored by a cross-shaped root system with a distinctive X-2-X-2 pattern of microtubules found only in this group. Chemistry separates them from land plants too: chlorophytes process glycolate during photorespiration with an enzyme in their mitochondria, whereas streptophytes use peroxisomes.
Reproduction is inventive. Cells may simply divide, fragment, bud or release spores, and colonial forms can build daughter colonies with exactly the parent's cell count and layout. Sexual forms range from identical gametes to distinct sperm and eggs, with evolution leaning toward the latter, and most have a life cycle in which only the zygote is diploid. Early prasinophytes form tough cysts up to 230 micrometres wide, boosting their buoyancy with lipids before bursting to release many new cells.
Roughly 90 percent of known species live in fresh water. Chlamydomonas nivalis lives on summer snow, and Trentepohlia clings to rocks and bark. Partners include fungi, ciliates, forams, cnidarians and molluscs, while the heterotrophic Prototheca causes protothecosis. Monostroma kuroshiense is the costliest edible green alga. Heinrich Reichenbach coined the name in 1828 for a hodgepodge including mosses and ferns; Adolf Pascher narrowed it to green algae in 1914.
Source: Chlorophyta
