Response of cultured Microcystis aeruginosa from the Swan River, Australia, to elevated salt concentration and consequences for bloom and toxin management in estuaries

Abstract

A mixed bloom of Microcystis aeruginosa forma aeruginosa and forma flos-aquae from the Swan River, Western Australia, was confirmed toxic by HPLC analysis. At least four, and possibly 11, microcystins were detected in cell-free extracts. Live bloom material was cultured at salt concentrations up to 21.2 g L–1 (total salts). The cultures were salt tolerant up to 9.8 g L–1. Reduction in the total cell concentration in the first 23 h was only observed in the highest salt treatment and first-order rate constants for cell lysis were higher than the rates for reduction of the intracellular microcystin pool size for that treatment. This suggests preferential lysis of genotypes with lower salinity tolerance and toxigenicity. This increased the toxicity of the mixed bloom population and the apparent microcystin cell quota without any change to the intracellular microcystin pool size. Therefore, the toxicity of bloom material may change through preferential lysis of cells with lower tolerances to changing environmental conditions, including salinity. Managers should be aware that the World Health Organization alert levels of 105 cells mL–1 for human contact exposure to cyanobacteria may not be a suitable prima facie test during these periods.