Fatty acid composition as an indicator of physiological condition of the cyanobacterium Microcystis aeruginosa

The present study examined the fatty acid composition of Microcystis aeruginosa grown in a batch culture and that of Microcystis-dominated plankton collected in an experimental enclosure in a shallow, eutrophic embayment of Lake Biwa (Akanoi Bay). In pure culture, we detected 16 : 0, 18 : 2 omega 6, 18 : 3 omega 3, 18 : 3 omega 6, and 18 : 4 omega 3 acids as major fatty acids of M. aeruginosa, with trace amounts of C sub(20) polyunsaturated fatty acids. In both pure culture and the field enclosure, the ratio of total fatty acid weight to dry weight decreased with decreasing availability of dissolved inorganic nitrogen. The omega 3/ omega 6 ratios of C sub(18) polyunsaturated fatty acids [(18 : 3 omega 3 + 18 : 4 omega 3)/(18 : 2 omega 6 + 18 : 3 omega 6)] varied greatly (range, 2-5) in response to the changes in physical and chemical conditions for Microcystis growth. Most notably, the omega 3/ omega 6 fatty acid ratios were significantly positively correlated with the growth rate of cells in a batch culture. We suggest that the fatty acid composition is a useful indicator of the physiological state of Microcystis in freshwater lakes.