Determination of Cyanobacterial toxins in Lake Naivasha, Kenya

Abstract

Lake Naivasha is a freshwater lake located about 80 km northwest of Nairobi and lies about 1890 metres above sea level. The freshwater system is used for drinking, washing and livestock watering by more than 600,000 people. However, declining water quality due to environmental perturbations has raised public concerns worldwide. The occurrence of blooms of cyanobacteria has become a serious problem as they produce a wide spectrum of toxins including microcystin which cause various adverse effects on organisms. Microcystins are a class of cyanobacterial toxins largely found in water and are often responsible for poisoning of animals as well as humans. The current work presents empirical data on the first identification and characterization of hepatotoxic microcystins in water samples of Lake Naivasha. Samples from the lake were stored in polythene bottles and analyzed over a period of six months. The colour of the water samples was found to be (520 ± 91) ptco, while the conductivity was 234 ± 0.8 μs/cm and the total dissolved solids were (1035 ± 12) mg/L. Due to the high turbidity (59.0 ± 24 ntu), phytoplankton biomass was low, ranging between 1.5 and 8.2 mg L-1. UV-Vis spectrum of sample A (for the month of April) had a peak with absorbance of 0.68 and a maximum wavelength (λmax) of 247 nm. Sample A (for the month of January) had a peak with absorbance of 0.99 and a maximum wavelength (λmax) of 240.94 nm. This peak was at slightly higher λmax than that of MC-LR which absorbs at the wavelength of 238 nm. This peak did not match with any microcystin of our interest. This was most likely due to the emergence of a new microcystin or a product formed from the degradation of microcystin. Using HPLC, chromatograms were obtained at different retention times. Three conspicuous peaks with different retention times were obtained for the samples collected in January. The toxin with a retention time of 11.65 minutes was identified as MCYR. This peak was shorter as compared to the rest implying that its concentration is much lower in existence. The second peak was observed at retention time 12.87 minutes and it was more protruding. This means that it is more concentrated in this sample. Retention time of 12.87 minutes corresponds to MC-LR. Finally, a sharp peak at retention time 7.27 minutes that did not correspond to any microcystin of interest was observed. The peak was thought to result from a microcystin degradation product. HyperChem computational package was used to estimate the toxicity index of microcystin-RR based on the octanol-water partition coefficient and found to be 230 times more soluble in water than in octanol. Thus, MC-RR is more soluble in biological tissues causing oxidative stress, and ultimately cancer. The concentrations of the microcystins in the different samples were observed to vary.