The influence of nitrogen and growth phase on the toxicity of the cyanobacterial strain Microcystis PCC 7806

Abstract

The worldwide occurrence of toxic cyanobacterial blooms and their numerous harmful effects have instigated extensive research into the environmental conditions promoting such events. Among the environmental factors which have been suggested to influence the increase in cyanobacterial proliferation, nutrient levels have been identified as one of the most prominent, affecting the growth and toxic metabolite production of cyano?bacteria in freshwater ecosystems. In the present study, toxicity of the cyanobacterial strain Microcystis PCC 7806 was evaluated after growth in media with three different nitrogen concentrations. The toxicity of intracellular extracts was analyzed during different growth phases (after 7, 21, and 35 days of cultivation) by observing mortality rates in the Artemia salina bioassay after 24h and 48h of exposure. The results have not shown significantly higher mortality levels between the test organisms exposed to extracts obtained from the cultures grown in the presence of higher nitrogen content (1.5 g/l and 0.8 g/l) and those grown in a nitrogen-free medium. A dose dependent effect, however, can be observed in most cases, with the most substantial changes observed in the high-dose groups. Also, the toxic effects and larval mortality increased during the exposure, suggesting the time-dependent toxicity. Extracts obtained after longer periods of cultivation (21 and 35 days) had stronger effects on the test organisms, which indicates that the toxicity of the tested cyanobacterial strain depends on the specific growth phase.