Green Algae (Chlorophyceae)

As the name indicates, the algae of this class are green in color. The pigments of the chloroplasts include the two types of chlorophyll, a and b, and the various carotinoids. The yellow and orange of the latter pigments are masked by the abundance of the green chlorophyll. In contrast to the chitinous cell wall of the blue-greens, these plants produce walls that are largely cellulose—a carbohydrate as opposed to the nitrogenous product, chitin. Some green algae of the sea—for example, Halimeda of the Siphonales—become incrusted with calcium carbonate, and thus may contribute materially in some places to the formation of lime deposits in warmer seas. The joints of the plant remain uncalcified, and thus allow flexibility in the moving water.

There is great diversity in the morphological features of this class. Common forms are filamentous with septa (Urospora) or without septa (Codium), tubular (Enteromorpha), and sheet-like (Ulva, or sea lettuce) (fig. 68d).

Methods of Reproduction. Common methods of reproduction may be illustrated by the habit of the cosmopolitan Ulva. In sexual reproduction the contents of any of the ordinary cells of the flat two-layered plant may form biciliated bodies called gametes which, upon escaping into the water, unite in pairs and by cellular division grow to form the new plant, known as the sporophyte, but usually passing first through a filamentous stage. Reproduction may also be asexual, in which case any of the common cells of the sporophyte plant may form microscopic quadriciliate zoospores (spores are simple reproductive cells which differ from seeds mainly in that they do not contain any ready-made embryo plant). These zoospores, upon being discharged, grow directly into gametophytes, the plants that produce the gametes.

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This process is known as alternation of generations, and associated with it are important cytological changes. In the case of Ulva, however, the macroscopic features (the sporophyte and gametophyte plants) are indistinguishable—that is, they are isomorphic or homologous. Asexual reproduction may also occur by fragmentation, thus forming unattached plants.

During the period of reproduction, large swarms of gametes and zoospores may be released, leaving the parent plant colorless and forming a green “bloom” on the waters of quiet bays. For many filter-feeding animals, the floating microscopic reproductive products of these and other algae form a source of food that must not be overlooked in a study of food of littoral animals. In bays, also, these swimming stages of algae, as well as algal slime, contribute to primary film formation that leads to an eventual fouling growth on ships and other submerged structures.

Distribution of Green Algae in the Sea. The green algae are found mainly in the upper littoral zone, especially in the lower half of the tidal zone, and in the immediate subtidal region down to a depth of 10m or more, and therefore in a relatively well-lighted habitat. It is with the green algae that the fresh-water algae are most closely related.

In geographic distribution, green algae are found most abundantly in the warmer seas. Algologists have remarked on the relative scarcity and dwarfed development of the Chlorophyceae in the Arctic Sea.