Since 1993, the 22nd of March has been designated as World Water Day in recognition of the critical role that freshwater plays in life.

Prokaryotes known as cyanobacteria, or blue-green algae, are single-celled organisms that can be found in fresh, brackish, and marine waters and some soils. About 2,000 to 8,000 species have been identified worldwide. During photosynthesis, cyanobacteria produce oxygen, which helped to create the oxygen-rich atmosphere we see today. The ocean’s primary nitrogen fixers are cyanobacteria. Some cyanobacteria-produced secondary metabolites may improve agricultural crop productivity.

Most people are familiar with cyanobacteria because of the harm they do to waterways and human health. Cyanobacteria can quickly multiply and produce blooms known as cyanoblooms, harmful algal blooms (HABs), or cyanoHABs under certain conditions. During the summer months, this is due to the high temperatures and heavy rain, that causes flooding. Phosphorus and nitrogen are washed into rivers and lakes during flooding. Nitrogen and phosphorus are essential nutrients for cyanobacteria. There is a risk that their presence in the water will cause the water to have an unpleasant odour or taste.

External sources of nitrogen and phosphorus are:

• Agricultural fertilisers; and
• household products; and
• sewage effluent; and
• stormwater runoff.

Cyanotoxins, on the other hand, are a cause for greater alarm. Cyanotoxins come in a wide variety of forms, but some pose a greater threat to public health than others. Saxitoxin, nodularin, cylindrospermopin, and microcystins are a few of the more than a hundred varieties of these toxins. Human health is at risk from cyanotoxins, which target a variety of organ systems, such as the liver (hepatotoxins), kidneys (nephrotoxin), central nervous system (neurotoxin), and skin (dermatoxin). Intense vomiting, diarrhoea, numbness of lips and mouth, paralysis of the respiratory and skeletal systems, and, in rare cases, death are all symptoms of acute exposure.

Tumor-promoting activity can be induced by long-term exposure to microcystins and nodularin, which can lead to liver tumours in some people. Group 2B, possibly carcinogenic to humans, has been assigned to the most common microcystin variant, MC-LR (MC-LR). Fish and shellfish that have been raised in contaminated water can make you sick. You can also get cyanotoxins by swimming in or showering in contaminated water, inhaling vapour and aerosolised droplets from contaminated water, and eating food crops that have been watered with water from contaminated water.

Crops irrigated with cyanobacteria-contaminated water can accumulate cyanotoxins, which can lead to toxic build-up in the crops. Toxin bioaccumulation can be affected by a wide range of factors, including the plant stage at which it is exposed to cyanotoxins, the soil bacteria composition, the concentration of cyanotoxins in the water, the amount of water used for irrigation, and the length of exposure via irrigation. But the fact that bioaccumulation can happen has raised the question of whether eating food crops grown in this way is bad for people.

Cyanobacterial blooms can be controlled

To prevent cyanobacterial blooms, reduce the amount of nutrients entering waterways:

• Proper stormwater, agricultural, industrial, and sewage effluent treatment and disposal; and
• keeping riparian vegetation in good condition; and
• blue-green algae blooms can be prevented by conserving soil and adjusting flow; and
• a cycle of responses to cyanobacteria management.

Because of the wide range of local conditions and circumstances, the most effective way to control algal blooms is at the community level.

If there is an outbreak, local governments and state water authorities are the best people to look into it and tell the public about it.

References

Miller, A. and Russell, C. (2017). Food crops irrigated with cyanobacteria-contaminated water: an emerging public health issue in Canada. Environmental Health Review. Volume 60, Issue 3, pages 58-63. Available at: https://doi.org/10.5864/d2017-021

Mokoena, M.M., and Mukhola, M.S. (2019). Current Effects of Cyanobacteria Toxin in Water Sources and Containers in the Hartbeespoort Dam Area, South Africa. International journal of environmental research and public health, Volume 16, Issue 22, 4468. Available at: https://doi.org/10.3390/ijerph16224468