Lakes, natural and man-made, provide water, food and habitats for wildlife, as well as supporting local economies. Around the world, though, there’s a growing threat to lakes: toxic bacteria which turn the water green.
This is the same green as you see on stagnant ponds. It’s caused by tiny organisms called cyanobacteria and can be deadly .
Cyanobacteria thrive in warm, sunny lakes and ponds that contain excess nitrogen and phosphorus nutrients derived from fertiliser, manure and sewage. When conditions are right, cyanobacteria multiply rapidly and form smelly green scums on the water’s surface.
Known to science as cyanoHABs (cyanobacterial harmful algal blooms), the scums are harmful to livestock, wildlife, pets, people and aquatic organisms like fish. Toxins make untreated water unsafe to drink, swim in, or even touch. Sometimes they can become suspended in air and be inhaled. The cyanoHABs also harm ecosystems by depleting oxygen, killing off whatever lives in the water, and disrupting food webs and fisheries.
CyanoHABs are a global threat and receive considerable scientific attention in North America and Europe . Blooms are becoming more widespread worldwide because rising temperatures promote cyanobacterial growth and more intense rainfall delivers nutrients from the landscape. Only effective management of nutrients can reverse this trend.
The problem is understudied in Africa’s main lakes, including its largest – Lake Victoria. Past research on cyanoHABs has mostly used microscopy to study the kinds found there, but microscopy cannot differentiate between toxic and non-toxic cyanobacterial cells.
Lake Victoria now receives large influxes of nutrients because of growing lakeside populations and land-use changes. Nutrients from agriculture, industry and urbanisation fuel the growth of cyanoHABs.
CyanoHABs occur in many basins in Lake Victoria but are highly concentrated in Kenya’s shallow Winam/Nyanza Gulf. Changing nutrient and temperature conditions can also alter which types of cyanobacteria dominate the gulf and the types and levels of toxins in the water. Lakeside communities that rely on the gulf for drinking water and domestic tasks are at risk of exposure to cyanoHAB toxins.
CyanoHAB in the Winam Gulf.
Photo credit: Pool
Past research on cyanoHABs has mostly used the oldest of microbiological techniques — microscopy — to classify the types of cyanobacteria in the gulf. This cannot differentiate between toxic and non-toxic cyanobacterial cells.
Modern genome sequencing technologies can identify genes encoding the production of known and novel toxins and other molecules of interest, such as those with medicinal properties. Genomic data from African Great Lakes is scarce, so the chemical capabilities of bacteria in this region are largely unexplored. But this is beginning to change.
Our latest study adds to a growing number of recent studies our team has carried out in and around Lake Victoria. In this study, our research vessel stopped at over 31 sites to collect scientific samples and data. The samples were later analysed for DNA, the biological “instruction manual” inside every living thing. DNA tells an organism how to grow, function, reproduce, and – in the case of cyanobacteria – make deadly toxins. This analysis produced near-complete genome sequences – that is, the set of all genes in the DNA – for organisms at each sampling site.
Past reports identified Microcystis as the dominant cyanobacteria in the Winam Gulf. Our research, however, found Dolichospermum was the most abundant type in major cyanoHAB events there. This finding might be due to recent environmental changes in the region.
But we linked Microcystis to microcystin. This is a liver-damaging toxin that can kill livestock, wildlife and humans, especially those whose immune system isn’t working well. In Winam Gulf, it’s often more abundant than the health limits set by the WHO.
Our study also found that Microcystis occurs mainly in murkier river mouths where green scums are not visible, making scientific monitoring and public alerts even more important.
Local authorities can now monitor for these cyanobacteria and warn residents to stay away when blooms are present.
The findings also mean that authorities know which cyanobacteria to target in prevention efforts like reducing the amount of phosphorus and other nutrients entering the gulf.
Lastly, our genomic study uncovered over 300 uncharacterised genes that may produce novel cyanobacterial molecules. These molecules could have toxic or therapeutic effects, and provide an opportunity for future investigators to explore.
A model for what is to come
Rapid human population growth and settlement around lakes and their watersheds is leading to high levels nutrients in lakes around the world. This results in excessive growth of algae and aquatic plants. This danger is likely to increase with global warming because warm temperatures promote algal blooms.
Lauren Hart, environmental chemical biologist
George S Bullerjahn, Research Professor and Director, Great Lakes Center for Fresh Waters and Human Health,
Gregory J. Dick, Professor, University of Michigan
Kefa M. Otiso, Professor of Geography, Bowling Green State University
