T he cyanobacteria (blue-green algae) are a morphologically diverse group of Gram-negative prokaryotes within the domain Bacteria (Giavannoni et al. 1988). This group includes unicellular forms, filamentous forms that contain only vegetative cells, and filamentous forms that differentiate specialized cells (Castenholz and Waterbury 1989). Cyanobacteria are photoautotrophs with an oxygenevolving photosynthetic mechanism that is identical to that of chloroplasts of algae and higher plants. Indeed, molecular genetic evidence strongly supports the idea that a recent ancestor(s) in the cyanobacterial evolutionary line gave rise to chloroplasts (Douglas 1994). Many cyanobacteria fix atmospheric nitrogen, which is an oxygen-sensitive process; they fix nitrogen either under low-oxygen growth conditions or, when in air, in specialized cells called heterocysts (Fay 1992). As a group, oxygenic photoautotrophic cyanobacteria, especially those species that fix nitrogen, are the most nutritionally independent organisms in the biosphere. Because of this nutritional independence, as well as their tolerance to physicochemical extremes,