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 105. 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