Comparative morphology of five species of symbiotic and non-symbiotic coccoid cyanobacteria

The morphology of five unicellular cyanobacterial species from two genera was compared using transmission electron microscopy (TEM) and statistical analyses. Cyanobacteria symbiotic with marine sponges from Australia and the Mediterranean were investigated, together with cyanobacteria from the water column in Western Australia. The aim was to establish whether phenotypic characteristics are useful for distinguishing cyanobacterial species, and to investigate the possible relationships of host species and geographic location on cyanobacterial morphology. Analysis of variance ( ANOVA) demonstrated that cell size of the symbionts, Synechococcus spongiarum and Aphanocapsa feldmannii, is directly correlated with the number of turns of the thylakoid, and this must be accounted for in comparative morphological analyses. S. spongiarum cells were approximately one third smaller than those of A. feldmannii, but did not vary significantly in size in Chondrilla australiensis collected from different localities, or in different hosts, including C. nucula and Ircinia variabilis. Other symbionts, including Oscillatoria sp. from Cymbastela marshae, and Aphanocapsa raspaigellae from I. variabilis, were readily distinguished in the sponges examined, both by size and ultrastructural features. While morphological differences appear to be useful in differentiating A. feldmannii from S. spongiarum, morphological similarities are not a reliable indication that coccoid cyanobacteria are the same species.