Dynamics of cyanobacteria and cyanotoxins in Kranji Reservoir
Te, Shu Harn
Date of Issue2012
School of Civil and Environmental Engineering
Mass occurrences of toxic cyanobacterial blooms have been reported world-wide in eutrophic waters. Due to rapid urbanization, the reservoirs in Singapore will be exposed to pollution. Therefore, it is important to assess toxic cyanobacteria species in local reservoirs to ensure a safe drinking water supply. While publications on the occurrences and characterizations of toxigenic cyanobacteria are rapidly increasing, most of the studies were done in temperate or cold climate regions. To date, there are only limited reports on cyanobacterial blooms in South East Asian region. With high and uniform temperature throughout the year, cyanobacterial blooms in the tropics could have distinct behaviors compared to those in the temperate regions as responses to local climatic condition. The temporal variations in cyanobacteria and microcystin production in a tropical reservoir, Kranji Reservoir in Singapore, were studied using molecular techniques and enzyme-linked immunosorbent assay (ELISA) respectively. Water samples were collected monthly from February 2008 to August 2009. Results from PCR and qPCR showed that Microcystis was the major microcystin producer. Anabaena spp. were found to coexist with the Microcystis spp. with Spearman’s rho coefficient, rs = 0.498 (P < 0.001). The average concentrations of Microcystis and Anabaena (equivalent to 16S rRNA) were 4.16×106 gene copies/mL and 4.47×104 gene copies/mL respectively. The average percentage of toxigenic Microcystis spp. was 55.92 % (SD 21.1 %), whereas no Anabaena-specific microcystin producing gene was detected. Microcystin-producing genes mcyB and mcyD were detected in all samples, together with the detections of mcyA and mcyE genes in 98 % and 96 % of the samples, respectively. However, a low detection rate was observed for mcyC. Due to the combined effects of high temperature, light and nutrient conditions in the tropics, the Microcystis cell concentrations were higher and more uniform compared to other Microcystis blooms reported in subtropical and temperate regions. Furthermore, the proportion of toxigenic Microcystis was considerably high and more stable throughout the year. Principle component analysis and multiple linear regression analysis showed that total nitrogen and total phosphorus were positively correlated with the abundances of total Microcystis and toxigenic Microcystis. In addition, total nitrogen, pH and dissolved oxygen were positively correlated with the microcystin concentration. The total microcystin concentration measured using an ELISA kit ranged from below the detection limit to14.4 µg/L with a mean concentration of 2.66±2.80 µg/L, which is higher than those reported by local water agency, PUB, using LC-MSMS detection on 4 microcystin variants. This suggests a more thorough survey of natural toxin variants in local waters is needed.