Microcystins in natural blooms and laboratory cultured Microcystis aeruginosa from Laguna de Bay, Philippines.

Summary Laguna de Bay, the largest freshwater lake in the Philippines, experiences periodic blooms of the cyanobacteria Microcystis aeruginosa. Blooms of these cyanobacteria in 1996, 1998 and 1999 were sampled. HPLC and MALDI-TOF mass spectrometry were used to analyze for microcystins. A total of 16 structural variants of the toxin were isolated from the samples with microcystin LR(MC-LR) as the most abundant variant in the samples from 1996 and 1999 making up 77 to 85% of the total, respectively. MC-RR was the dominant variant in the 1998 bloom making up 38%. The samples from 1996 had the highest total toxin concentration (4049 μg g –1 ) followed by those from 1998 (1577 μg g –1 ) and 1999 (649 μg g –1 ). A strain of M. aeruginosa previously isolated from the lake was also cultured in the laboratory under different nitrogen concentrations (1, 3 and 6 mg L –1 ) and elevated phosphorus concentration (0.5 mg L –1 ) to determine the influence of these factors on toxin production. A total of 9 different structural variants of microcystin were isolated from the laboratory cultures with MC-LR consisting more than 75% of the total in all treatments. No significant differences in the total toxin concentration as well as the % distribution of the different variants among treatments were observed. However, the strain of M. aeruginosa cultured in the laboratory had from 3 to 20 times higher total microcystin than those harvested from the lake.