Molecular characterization of cyanobacterial diversity and yearly fluctuations of Microcystin loads in a suburban Mediterranean Lake (Lake Pamvotis, Greece)

Abstract

Cyanobacterial blooms are a frequent phenomenon in eutrophic freshwaters worldwide, and are considered as potential hazards to ecosystems and human health, while it has been shown that on average 60% of these cyanobacterial blooms are toxic. Hepatotoxic blooms are more common than neurotoxic ones and microcystins have been found to be the most prevalent cyanobacterial hepatotoxins. Lake Pamvotis is an ancient (having been in continual existence throughout the Plio-Pleistocene period) suburban Mediterranean Lake used for recreation, fishing and irrigation purposes which has suffered eutrophication for the last three decades. We investigated cyanobacterial species composition and microcystin loads in this lake over a 16-month period. The highest microcystin concentrations were recorded in autumn, one to two months after the midsummer severe bloom. With the exception of the winter months, microcystin concentrations exceed the WHO upper limits for drinking water but not for recreational waters. Seasonal changes of microcystin bioaccumulation in edible species were also investigated. Microcystin concentrations never exceed the WHO upper limits in those species with the exception of bivalves. For a detailed characterization of the cyanobacterial species composition of the lake, we used polymerase chain reaction (PCR) amplification of the internal transcribed spacer (ITS) between 16S and 23S rRNA genes, in combination with denaturing gradient gel electrophoresis (DGGE). ITS sequences from Lake Pamvotis revealed that the cyanobacterial community of this lake is made of two major populations. A population well defined both microscopically and molecularly as Microcystis sp. dominated during autumn, and another population of filamentous cyanobacteria microscopically characterized as Anabaena sp./Aphanizomenon sp. dominated during midsummer blooms. Sequences of filamentous cyanobacteria from Lake Pamvotis revealed that this cyanobacterial population is homogeneous, although divergent from other populations worldwide. Finally, by using a combination of general and genus specific primer sets against the mcyE gene, we identified Microcystis as the only genus responsible for microcystin production in Lake Pamvotis.