The dynamics of cyanobacteria and potential threats of cyanotoxins in Uganda: a case of Murchison Bay and Napoleon Gulf, northern Lake Victoria

Abstract

Lake Victoria has diverse cyanobacteria associated with varying types and concentrations of cyanotoxins. Over 31 variants of microcystins (MC) have been identified in the lake with MC-LR, MC-RR and MC-YR as the commonest. Therefore, this study determined the factors associated with cyanobacteria and cyanotoxins dynamics, and potential health risk of cyanotoxin exposure routes in Lake Victoria. For a comparative assessment, data were collected from the lake, water treatment plants (WTPs) and mesocosm experiments in Napoleon Gulf (NG) and Murchison Bay (MB) between November 2016 and October 2019. Cyanobacterial were identified based on morphological features while cyanotoxins, was based on enzyme-linked immunosorbent assay (ELISA), and Liquid Chromatography-Mass Spectrometry (LC-MS). Although the phytoplankton community was similar, their dynamics were driven by the dominant cyanobacteria, Microcystis flos-aquae and M. aeruginosa in MB and Dolichospermum circinale and Planktolyngbya circumcreta in NG. Thus, closed embayments, such as MB more susceptible to toxigenic cyanobacteria with potential cyanotoxin production. Thus, two classes of cyanotoxins were detected: MC and homoanatoxin (HTX) an analogue of anatoxin-a (ATX). Considering MC, significantly higher concentration (5-10 µg MC-LR equiv. L-1) was detected in MB compared to NG. This frequently exceeded the recommended sanitary threshold of >100,000 cells·mL−1 and 1 µg·L−1 of MC-LR for exposure via drinking water. However, the WTPs efficiently removed the cyanobacteria cells, intracellular and dissolved MC to < 1 µg·L−1 of MC-LR. The observed MC have been related to the biovolume of Microcystis, influenced by solar radiation, mean wind speed (N-S direction) and turbidity. From the in-situ mesocosm experiments, continuous eutrophication will enhance the growth of Microcystis and consequently increase MC production and concentrations. As Nile tilapia consumed cyanobacteria, such as Microcystis sp. there was transfer of MC into the intestines of fish (up to 27.5 µg/g FW of MC-LR) with MC detection in the liver (up to 0.48 µg/g of MC-LR FW) and muscle (up to 0.3 µg/g FW of MC-LR) in MB. This could increase human exposure through fish consumption. Therefore, due to the dominance of toxigenic Microcystis there is need for regular sensitization during bloom events and integration of Microcystis biovolume estimation in WTPs are recommended. Locals should desist from using contaminated lake water for domestic purpose but rather use treated water and reduce recreational activities during bloom events. From this perspective, there should be dedicated water quality and fish monitoring from water bodies with toxigenic cyanobacteria and evaluate the potential exposure risks to humans.