Submitted to: International Journal of Systematic and Evolutionary Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/30/1995
Publication Date: N/A
Citation: Interpretive Summary: Although most blue-green or cyanobacteria are free-living in fresh water, marine or terrestrial habitats, there is a group that lives in close dependency, or symbiosis, with the water fern Azolla. They are called cyanobionts. Azolla cyanobionts are poorly described in the scientific literature. Their cellular structures appear to vary depending on which of the 7 species of Azolla they are associated with. Scientists haven't decided whether Azolla cyanobacteria belong to one or more species, and how these species relate to those of the host plant. Analysis of the cellular fatty acids of bacteria is a useful method for their classification since fatty acid composition is highly conserved in evolution. The fatty acids of cyanobacteria from each of the 7 species of Azolla were analyzed and it was found that the cyanobacteria segregated into two major groups. The groups correspond to the two subgenera of Azolla. That is, cyanobacteria from the 5 Azolla species in the subgenus Azolla all belong to one biochemical group, as do the cyanobacteria from the two Azolla species in the subgenus Rhizosperma. This suggests that there are two species of Azolla cyanobacteria, and that they co-evolved with their plant hosts.
Technical Abstract: Total cellular fatty acids of 40 freshly-isolated cyanobionts from 7 different species of Azolla host plants were determined by gas-liquid chromatography-mass spectroscopy. Sixty-three different fatty acids were identified, belonging to 7 distinct chemical classes. Composition varied among cyanobacteria depending on their host species. Parameters that differed significantly (at the 99% level of probability) were percent concentrations of the 16:0 and 18:3 fatty acids, total poly-unsaturated acids, total 16-carbon and 18-carbon fatty acids, the ratios of unsaturated to saturated fatty acids, and percent total of straight chain even-carbon fatty acids, of unsaturated fatty acids and of branch chain unsaturated fatty acids. Analysis of variance suggested statistical regression for percent totals of these fatty acids and chemical classes on the following linear alignment of cyanobacteria by host: A. filiculoides, A. microphylla, A. caroliniana, A. mexicana, A. rubra, A. nilotica and A. pinnata (including subspecies pinnata and pinnata imbricata). The 7 groups segregated into 2 distinct clusters based on dendogram analysis of Euclidian distances. Cyanobionts from A. filiculoides, A. microphylla, A. mexicana and A. caroliniana constituted one cluster; those from A. rubra, A. nilotica and A. pinnata a second. A minor dichotomy separate A. filiculoides from other members of the first group. The clustering of Azolla cyanobionts based on fatty acid analysis agrees remarkably with the taxonomic groupings of the neotropical Azolla species. This correlation suggests that the Azolla cyanobacteria co-evolved into distinct genetic groups with their hosts.