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 MCLR,
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 DoHchospermum circinate 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 pg 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 //g'L1 of MC-LR for
exposure via drinking water. However, the WTPs efficiently removed the cyanobacteria cells,
intracellular and dissolved MC to < 1 /zg 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 pg/g FW of MC-LR) with MC detection
in the liver (up to 0.48 pg/g of MC-LR FW) and muscle (up to 0.3 pg/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.