Until recently, all photosynthetic eukaryotes were classified as members of the kingdom Plantae. The brown and golden algae, however, are now reassigned to the protist supergroup Chromalveolata. This is because apart from their ability to capture light energy and fix CO2, they lack many structural and biochemical traits that are characteristic of plants. The plants are now classified, along with the red and green algae, in the protist supergroup Archaeplastida. Green algae contain the same carotenoids and chlorophyll a and b as land plants, whereas other algae have different accessory pigments and types of chlorophyll molecules in addition to chlorophyll a. Both green algae and land plants also store carbohydrates as starch. Their cells contain chloroplasts that display a dizzying variety of shapes, and their cell walls contain cellulose, as do land plants. Which of the green algae to include among the plants has not been phylogenetically resolved.
Green algae fall into two major groups, the chlorophytes and the charophytes. The chlorophytes include the genera Chlorella, Chlamydomonas, the sea lettuce Ulva, and the colonial alga Volvox. The charophytes include desmids, as well as the genera Spirogyra, Coleochaete, and Chara. There are familiar green algae in both groups. Some green algae are single cells, such as Chlamydomonas and desmids, which adds to the ambiguity of green algae classification, because plants are multicellular. Other green algae, like Volvox, form colonies, and some, like Ulva are multicellular (Figure). Spirogyra is a long filament of colonial cells. Most members of this genus live in fresh water, brackish water, seawater, or even in snow patches. A few green algae can survive on soil, provided it is covered by a thin film of moisture within which they can live. Periodic dry spells provide a selective advantage to algae that can survive water stress.
The chlorophytes and the charophytes differ in a few respects that, in addition to molecular analysis, place the land plants as a sister group of the charophytes. First, cells in charophytes and the land plants divide along cell plates called phragmoplasts, in which microtubules parallel to the spindle serve as guides for the vesicles of the forming cell plate. In the chlorophytes, the cell plate is organized by a phycoplast, in which the microtubules are perpendicular to the spindle. Second, only the charophytes and the land plants have plasmodesmata, or intercellular channels that allow the transfer of materials from cell to cell. In the chlorophytes, intercellular connections do not persist in mature multicellular forms. Finally, both charophytes and the land plants show apical growth—growth from the tips of the plant rather than throughout the plant body. Consequently, land plants and the charophytes are now part of a new monophyletic group called Streptophyta.