ASSESSMENT OF ENVIRONMENTAL-CONDITIONS THAT FAVOR HEPATOTOXIC AND NEUROTOXIC ANABAENA SPP. STRAINS CULTURED UNDER LIGHT LIMITATION AT DIFFERENT TEMPERATURES

Toxic cyanobacterial mass occurrences have caused animal poisonings wo rldwide and may pose a health hazard for humans. Strains of the genus Anabaena are either non-toxic or produce hepatotoxins, microcystins (M CYST), or neurotoxins (such as anatoxin-a). In order to study which gr owth conditions favor hepatotoxic vs neurotoxic strains and how produc tion of toxins varies, we compared the responses of two microcystin- a nd two anatoxin-a-producing Anabaena strains in continuous turbidostat cultures, at different temperatures, under growth-limiting light leve ls. Growth rates consistently remained <0.8 divisions per 24 h. Differ ences were strain-specific and not associated with hepatotoxicity or n eurotoxicity. Thus, differential adaptation of strains to temperature and to growth-limiting light levels cannot explain why, in some cyanob acterial water blooms, hepatotoxic strains, and in others, neurotoxic ones become dominant. A statistical analysis of field data showed that the most significant discriminating factors between different types o f blooms were the concentrations of dissolved PO4-phosphorus and NO3-n itrogen. Anabaena blooms with unknown neurotoxicity associated with lo w PO4-phosphorus and high NO3-nitrogen concentrations. Among other Ana baena blooms, the hepatotoxic ones associated with the lowest, and mos t of the non-toxic ones with higher concentrations of PO4-phosphorus. Anabaena blooms that contained anatoxin-a and hepatotoxic Microcystis blooms showed tendencies towards the highest concentrations of PO4-pho sphorus. Non-toxic blooms dominated by genera other than Anabaena occu rred over a wide range of growth conditions. In turbidostat cultures, maximal production of microcystins correlated with maximal growth rate s. Light regulated the production of MCYST-LR variants, and temperatur e affected the production of MCYST-RR variants. Anatoxin-a seemed to b e produced most under temperatures and light levels slightly suboptima l for growth. Under low light, considerable amounts of extracellular a natoxin-a were detected while microcystins consistently remained intra cellular.