Every tree and flower on land is technically a branch of green algae
Land plants did not evolve alongside green algae; they grew out of them. Genetic evidence places the first land plants deep inside one algal group, as sisters of a family of pond-dwelling freshwater algae. So oaks, roses and grasses are, in family-tree terms, a highly successful offshoot of pond scum.
Green algae are photosynthetic organisms containing chlorophyll a and b, which give them their vivid colour, plus orange beta carotene and yellow xanthophylls. They usually build cellulose cell walls and store energy as starch, just as land plants do. About 22,000 species range from single swimming cells, most with two whip-like flagella, through colonies and filaments to large seaweeds. Together with land plants they form the clade Viridiplantae. Their ancestor was a single-celled flagellate, and their chloroplasts trace back to an ancient event in which a cell swallowed a cyanobacterium-like microbe and kept it as a permanent organelle.
They are generous partners. Chloroplasts in some euglenids and dinoflagellates were stolen from engulfed green algae, and the algae live inside Paramecium, the green hydra and certain flatworms. In lichens, algae such as Trebouxia team up with fungi; typically the fungus cannot survive alone, while the alga often can. In Antarctica, Prasiola crispa spreads carpets over damp soil, especially near bird colonies.
Their sex lives are varied. Some species fuse identical cells; others pair a big immobile egg with a small swimming sperm. Filamentous Spirogyra builds bridges between cells to merge them, leaving empty walls visible under a microscope. The colonial alga Volvox carteri, living in summer pools, switches to sex just before its pool dries out, producing tough dormant zygotes. The trigger is a glycoprotein pheromone that works at concentrations as low as 10 to the power of minus 16 molar, making it one of the most potent biological signalling molecules known. Heat shock may be what sets it off in nature.
Because their cells are large and simple, green algae, including the characean algae, have long been lab models for studying how membranes handle water and ions, how cells regulate pressure and salt, and even how electrical action potentials arise. The Closterium complex is the closest single-celled relative of land plants, making it a window on how plants began.
Source: Green algae
