Elemental composition of phytoplankton in a subtropical lake: X-ray microanalytical studies on the dominant algae Spirulina platensis (Cyanophyta) and Cyclotella meneghiniana (Bacillariophyceae)

Studies were carried out on the phytoplankton of a subtropical polluted lake (Lake Maryut, Egypt) over a 1 year sampling period. The elemental composition of two major algal constituents-Spirulina platensis (Cyanophyta) and Cyclotella meneghiniana (Bacillariophyceae) – was determined from mixed plankton samples using electron-probe X-ray microanalysis. X-ray emission spectra obtained from S. platensis revealed a wide range of detectable elements. Clear positive (e.g. K-P, Na-Cl) and negative (e.g. Na-S, P-Cl) statistical correlations were interpreted in terms of elemental associations and compartmentation within the cell. C. meneghiniana had a broadly similar range of elements, but differed in the additional presence of Al and frequent peaks of Fe and Cu. The high concentration of Si reduced the detectability of other elements, causing a general reduction in mass fraction levels. Comparison of correlation coefficients derived from four sets of data revealed a clear pattern of elemental occurrence in the cell. There were two major groupings (involving divalent and monovalent cations) which may relate respectively to the frustule and protoplasmic compartments of the cell. Uptake of metal pollutants and P was investigated by comparison of cellular and environmental levels. Although metal pollution in the lake was high, no significant uptake was detected in S. platensis and only limited uptake (of Fe and Cu) occurred in C. meneghiniana. In this alga, cell loading with Fe and Cu did not relate to concentrations in contemporary water samples, suggesting that metal accumulation may not be in synchrony with environmental changes. Similar considerations apply to P, since neither S. platensis nor C. meneghiniana showed clear or consistent correlations between cellular and environmental concentrations.