Elemental composition of Microcystis aeruginosa under conditions of lake nutrient depletion

Electron probe X-ray microanalysis (XRMA) was used to determine the elemental composition of the blue-green alga, Microcystis aeruginosa, in a stratified freshwater lake during the late summer. Colonies of this alga were initially observed in mid-July, at a time when phosphorus concentration in the lake water had decreased to minimal levels (total P 0.04 mg l(-1)). The P quota of these cells was high (mean concentration 132 mmol kg(-1) dry weight) with a cell P to lake water P concentration ratio of 10(5). The elemental concentrations of Microcystis remained relatively stable throughout the sampling period (July-September), with mean cell concentrations of Mg, P, S and Ca showing no significant changes. Mean elemental ratios and the ratio of monovalent/divalent cations were also relatively constant (SE < 10% mean). The pattern of cell elemental associations, determined by Factor and Pearson correlation analysis, was consistent throughout - with Mg, P, K and S forming a core tetrad of inter-correlated elements. The relative constancy of cell composition seen in Microcystis would be expected of an alga with a K-selection strategy. The continued high P quota over a period of nutrient depletion in lake water is consistent with the ability of this alga to sink to nutrient-rich lower regions of the water column.