Biological features of algae

The simplest phototroph imaginable is a single cell floating in a liquid medium, synthesizing its own sugar, and reproducing at intervals by binary fission. Such organisms do in fact exist in both fresh and salt waters. Examples are provided by the cyanophyte Synechococcus (p. 28) and the minute marine Micromonas (Fig. 1.6).

These organisms are examples of algae, the group of plants showing the greatest diversity of any major division of the plant kingdom. They range from minute, free-floating, unicellular forms (represented by both prokaryotes and eukaryotes) to large plants, exclusively marine, several meters in length. Many of the smaller algae form a component of plankton, the communities of minute plants and animals which float at or near the surface of fresh waters and oceans. Algae are responsible for a large part of the photosynthesis in the biosphere, the productivity of some coastal communities in the surf of warm seas exceeding that of the tropical rain forest. Much of the carbon so fixed enters the food chain of the aquatic heterotrophs.

Despite the enormous range in size, the algae remain comparatively simple in organization. In the smaller multicellular species (e.g., Merismopedia, Fig. 2.6; Pediastrum, Fig. 3.8) the cells resemble each other in appearance and function, and they can be regarded as forming little more than an aggregate of independent units.