Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of California, Berkeley 1976.

Description: A total of 34 strains of cyanobacteria (blue-green algae) representative of the orders Chamaesiphonales and Pieurocapsales were isolated in axenic culture from marine and fresh water sources. Two strains isolated from fresh water, both assignable to the genus Chamaesiphon, are unicellular cyanobacteria which reproduce by budding; the buds being formed in succession at one pole of the oval cell. These buds are the structures currently termed "exospores" by algologists. The other 32 strains, some marine and some fresh water, reproduce by the formation of small spherical spores produced through the multiple fission of a vegetative cell. In these strains the peptidoglycan and lipopolysaccharide-containing cell wall layers common to all cyanobacteria are enclosed by a third, fibrous layer which increases in thickness during vegetative cell enlargement: spores are liberated by rupture of this wall layer of the parental cell. In some strains, the fibrous wall layer is synthesized during multiple fission, and the spores, each enclosed by this layer, are immotile after release. In others, the synthesis of the fibrous wall layer is arrested during multiple fission, and the spores, initially lacking this layer, show transient gliding motility after release. Among the spore-forming strains, six major strain clusters could be distinguished in terms of their developmental patterns and each strain cluster could be identified with an existing genus. These included strictly unicellular forms (Dermocarpa, Xenococcus), forms which undergo only one or two vegetative divisions (Dermocarpella), and forms where extensive vegetative growth normally precedes spore formation (Myxosarcina, Chroococcidiopsis, Pleurocapsa). Revised definitions of the orders Chamaesiphonales and Pleurocapsales and of some of their constituent genera are proposed in the light of these findings. The potential taxonomic utility of certain physiological and chemical properties was also examined. These included: ionic requirements including sodium, chloride, calcium and magnesium; vitamin requirements; nitrogen sources; temperature relationships; heterotrophy; pigment composition; and mean DNA base composition. There were no clear-cut correlations between genera defined in terms of developmental patterns and groups defined in terms of any of the physiological and chemical properties examined. However, many of the latter properties will be useful ·in distinguishing species within each genus. Strains of marine origin can be distinguished from fresh water strains in culture by their elevated requirements for sodium, halide, calcium and magnesium. This work has revealed the intrinsically unsatisfactory nature of the existing classification of cyanobacteria based almost exclusively on the description of field materials. Many of the characters previously used to differentiate species (and even genera) are not valid. It is therefore proposed that all future taxonomic descriptions should be based on the detailed characterisation of axenic cultures, and that cultures should replace herbarium specimens as reference materials.