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

Abstract

Saxitoxins have been found in Australian populations of the cyanobacterium Anabaena circinalis. The C-toxins (C1 and C2) and gonyautoxins (GTX2 and GTX3) are dominant components, while saxitoxin (STX), GTX5 and decarbamoyl gonyautoxins (dcSTX, dcGTX2 and dcGTX3) are minor constituents. Variation in the concentration and composition of saxitoxins has been observed in natural populations and cultured strains of A. circinalis and may reflect environmental conditions. Laboratory experiments were conducted with a single strain of A. circinalis to examine the effect of different nitrogen sources (dissolved atmospheric nitrogen, nitrate or ammonium) and varying concentrations of nitrate (0.0028, 0.28 and 28 mg N 1−1) on growth and saxitoxin levels. Growth was determined by cell enumeration and saxitoxin concentrations were analysed by high-performance liquid chromatography. All experiments consistently 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 1−1) and by high levels of nitrate (28 mg N 1−1), and these treatments were associated with an increased toxin release. The concentration of extracellular saxitoxins increased with the age of cultures. The composition of intracellular and extracellular toxin profiles was usually similar; however, the relative abundance of the different toxins was not always the same. Extracellular toxin profiles generally comprised a higher proportion of STX and GTX2 and less C-toxins. A strong correlation between toxin quota (saxitoxin concentration per cell) and logarithmic growth rate was found in three of four experiments. Saxitoxin concentrations in A. circinalis appear to be indirectly affected by the source and concentration of nitrogen through growth.