Summary
The morphological diversity of naturally occurring populations of the chroococcoid cyanobacterial genus Synechococcus were investigated by electron microscopy. Analysis of samples from a coastal Tasmanian site (38° S 148° E) and from south of the Sub Tropical Convergence (54°S 146°E) revealed substantial diversity in Synechococcus ultrastructure. A classification system based on variations in cell wall structure and the arrangement of the thylakoid membranes was developed. Preliminary characterisation of the two natural populations into this system suggested subtle differences in the distribution of the Synechococcus forms. Two cultured strains, CS205 originally isolated from Menai Strait UK, and CS95 originally isolated Berkley California, were also examined to investigate if the observed variations in Synechococcus are a result of physiological response to different growth conditions. Morphological similarities within the cultures, but significant intra-strain differences, indicate that the observed morphological diversity is due to phenotypic expression. Synechococcus are an important component of marine food webs, as they are a major contributor to the primary production in temperate and tropical ecosystems. The different morphological forms of Synechococcus may influence the microbial loop by affecting the flux of organic material, through such mechanisms as selective grazing pressure and differential susceptibility to viral infection. This knowledge will allow more accurate global carbon flux and thus climate models to be developed.