Acetabularia, an unusual unicellular green algae
Acetabularia is a member of a very diverse group, the green algae. Other members of the same group that we will consider are Oedogonium, Chlamydomonas and Cladophora, all of which are quite different in form and structure.
Taxonomy and Phylogeny
The green algae are generally put in their own phylum, Chlorophyta, but this phylum is placed variously depending upon the perceptions of the observer. Some workers still consider them to be one of the protist phyla. Workers who reject the protist kingdom often put green algae in the ‘Archaeplastid supergroup’. However, since some of the green algae are closely linked to plants, some workers combine plants and some or all of the green algae into a ‘Viriplantae’ group. (See the reading on Chlamydomonas for more details on their classification.)
Acetabularia has an unusual structure by being large, unicellular and possessing features that might be considered organs—‘roots, stems and leaves’. The single cell is attached to the substrate by root-like cellular extensions. These extensions connect to an elongate stalk which ends in an umbrella-like cap that is often 1 cm or more across. The single nucleus of this remarkable organism is found at the base of the stem.
Sexual reproduction in Acetabularia is initiated when the single (diploid) nucleus goes through multiple mitotic divisions; these nuclei subsequently undergo meiosis and migrate to the cap where they are released in cysts that break open to release mobile gametes. These gametes find each other and fuse to form a zygote which attaches to a substrate and grows into the mature form. Asexual reproduction is also possible if mobile (diploid) zoospores are released and behave like zygotes, attaching to a substrate and developing into the mature form.
Matter and energy
Acetabularia is a typical photoautotroph, using the energy of sunlight to synthesize carbohydrates from carbon dioxide and then using the carbohydrates as an energy source in cellular respiration and as building materials to synthesize a variety of biomolecules.
Acetabularia is generally found in warm, marine waters. Since it attaches to substrates and requires light for photosynthesis it is generally found in shallow waters, to depths that depend on water clarity. Acetabularia is eaten by sea urchins and fish.