Journal of Plankton Research Vol.22 no.11 pp.2137-2153, 2000
© Oxford University Press 2000
Cell surface elemental composition of Microcystis aeruginosa: high-Si and low-Si subpopulations within the water column of a eutrophic lake
School of Biological Sciences, 3.614 Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT, UK
Energy-dispersive X-ray microanalysis (10 kV microprobe) was carried out to determine the surface elemental composition of single cells of Microcystis aeruginosa within mixed phytoplankton preparations, sampled at different depths (0–8 m) within a stratified eutrophic lake. Mean elemental concentrations (mmol kg–1 dry weight) throughout the sampled water column were: magnesium, 125; silicon (Si), 1864; phosphorus, 341; sulphur, 122; chlorine, 88; potassium, 282; calcium, 63. Although some significant differences in elemental composition occurred with depth, the underlying pattern was one of relative uniformity within the top 8 m of the water column. At each depth, the frequency distribution of Si was bimodal, indicating two distinct subpopulations of cells with cell surface Si concentrations ranging from 0 to 1000 mmol kg–1 (overall mean 112, low-Si cells) and from 1000 to 6800 mmol kg–1 (overall mean 3649, high-Si cells). The two subpopulations occurred mixed together within individual colonies, and both included dividing and non-dividing cells. Both cell types had a wide range of sizes, but high-Si cells reached a higher maximum size in both dividing (diameter greater by 0.90 µm) and non-dividing (difference of 0.44 µm) cells. This difference in size is consistent with Si being present as a surface layer (up to 0.2–0.4 µm in thickness) in high-Si cells. Support for the presence of a surface layer of Si [with aluminium (Al)] is also provided by correlation analysis (Si and Al are significantly negatively correlated with other cell elements), principal component analysis (Si and Al occur as a distinct subgroup) and lower mean concentrations of elements (other than Al) in high-Si cells (due to reduced X-ray contribution from the cell interior). The proportion of high-Si cells in the water column was locally high in the 2 m depth sample, and had an overall value of 40.5% for the whole data set. The biological significance of high- and low-Si cells is not known.