Saxitoxins associated with the growth of the cyanobacterium Anabaena circinalis (Nostocales, Cyanophyta) under varying sources and concentrations of nitrogen

Saxitoxins have been found in Australian populations of the cyanobacterium Anabaena circinalis. The C-toxins (Cl and C2) and gonyautoxins (GTX2 and GT X3) are dominant components, while saxitoxin (STX), GTX5 and decarbamoyl go nyautoxins (dcSTX, dcGTX2 and dcGTX3) are minor constituents. Variation in the concentration and composition of saxitoxins has been observed in natura l populations and cultured strains of A. circinalis and may reflect environ mental conditions. Laboratory experiments were conducted with a single stra in of A. circinalis to examine the effect of different nitrogen sources (di ssolved atmospheric nitrogen, nitrate or ammonium) and varying concentratio ns of nitrate (0.0028, 0.28 and 28 mg N l(-1)) on growth and saxitoxin leve ls. Growth was determined by cell enumeration and saxitoxin concentrations were analysed by high-performance liquid chromatography. All experiments co nsistently showed a linear relationship between cell density and saxitoxin concentration (intracellular + extracellular). Growth of A, circinalis was depressed by addition of ammonium (0.04 mg N l(-1)) and by high levels of n itrate (28 mg N l(-1)), and these treatments were associated with an increa sed toxin release. The concentration of extracellular saxitoxins increased with the age of cultures. The composition of intracellular and extracellula r toxin profiles was usually similar; however, the relative abundance of th e different toxins was not always the same. Extracellular toxin profiles ge nerally comprised a higher proportion of STX and GTX2 and less C-toxins. A strong correlation between toxin quota (saxitoxin concentration per cell) a nd logarithmic growth rate was found in three of four experiments. Saxitoxi n concentrations in A. circinalis appear to be indirectly affected by the s ource and concentration of nitrogen through growth.