Strong effects of amoebae grazing on the biomass and genetic structure of a Microcystis bloom (Cyanobacteria).

Despite its importance for bloom toxicity, the factors determining the population structure of cyanobacterial blooms are poorly understood. Here, we report the results of a two-year field survey of the population dynamics of Microcystis blooms in a small hypertrophic urban pond. Microscopic enumeration of Microcystis and its predators and parasites was combined with pigment and microcystin analysis and denaturing gradient gel electrophoresis of the ITS rDNA region to assess population dynamics and structure. Two main Microcystis morpho- and ITS types were revealed, corresponding to M. aeruginosa and M. viridis. In both years, high population densities of naked amoebae grazing on Microcystis coincided with rapid decreases in Microcystis biomass. In one year, there was a shift from heavily infested M. aeruginosa to the less-infested M. viridis, allowing the bloom to rapidly recover. The preference of amoebae for M. aeruginosa was confirmed by grazing experiments, in which several amoeba strains were capable of grazing down a strain of M. aeruginosa, but not of M. viridis. Zooplankton and chytrid parasites appeared to be of minor importance for these strong and fast reductions in Microcystis biomass. These findings demonstrate a strong impact of small protozoan grazers on the biomass and genetic structure of Microcystis blooms.

A method for acetonitrile-free microcystin analysis and purification by high-performance liquid chromatography, using methanol as mobile phase.

Cyanobacteria produce a wide range of potent toxins, including hepatotoxic microcystins. HPLC methods for microcystin analysis and purification almost invariably include acetonitrile in the elution gradient mobile phase. The recent, acute, global acetonitrile shortage requires that adequate methods are available for microcystin analysis and purification without the need for acetonitrile. Here we present a convenient methanol-based method for effective HPLC analysis and purification of the toxins, with full separation of a range of microcystin variants.