Blue-green Algae -Cyanobacteria
Author: Dr Paul J Oberholster - CSIR
( Article Type: Explanation )
Cyanobacteria (blue-green algae) are a group of diverse gram-negative prokaryotes and are one of the earth’s most ancient life-forms. Evidence of their existence on earth, derived from fossil records, encompasses a period of some 3.5 billion years. Cyanobacterial blooms have become an increasing problem in South African freshwater bodies. The massive proliferation of these organisms is largely caused by over enrichment (eutrophication) of dams and rivers, which is due to progressive increase in human-derived pollution. Cyanobacteria produce some of the most potent toxins known and have no known antidotes.
|These biotoxins fall into three categories, namely neurotoxins, hepatoxins and lipopolysaccharides. The biotoxins in the first two groups can produce severe reactions in animals and humans, while the third group appears to be less virulent. However, the latter have been less intensively studied. Any release of these biotoxins into surrounding water can present a significant hazard to humans and the ecosystem. The existence of gastrointestinal disorders linked to the ingestion of cyanobacterial biotoxins, as well as the chronic risks posed by hepatoxins make these toxins a serious threat to human health when they are present in drinking water supplies. When nutrient-rich effluent enters impoundments or slow retention-time rivers, it stimulates rapid growth of phytoplankton species and aquatic macrophytes. In extreme cases, leads to the development of mono-specific blooms of cyanobacteria which can change the whole aquatic ecosystem. Harmful cyanobacterial blooms in South African impoundments are typically characterized by heavy biomass accumulations of cyanobacteria that often consist of a single or a few species, usually members of the genera Microcystis and Anabaena. These species can persist for long periods in eutrophic impoundments.
The majority (80%) of rural communities in South Africa rely on surface water as the main source of domestic water. In South Africa and other parts of the world, cyanobacterial biotoxins (microcystins) are a major concern to drinking water providers from a health and economic perspective. The microcystins have been linked to liver damage that prompted the World Health Organization (WHO) to adopt a provisional guideline value for microcystins-LR (L for leucine and R for arginine) of 1.0 μg l-1 in drinking water in 1998. Earlier, in 1996 Ueno proposed a more stringent guideline value of 0.01 μg l-1 based on a possible correlation of primary liver cancer in certain locations in China.
Consumers in these locations used potable water contaminated with microcystins. In Australia, the potable water standard for microcystins was set at 1.3 μg ℓ-1. A 2000 study conducted by Humpage and co-workers indicated that microcystins from a cyanobacterial extract provided in drinking water to rats increased the area of aberrant crypt foci in the colon, suggesting that microcystins promote preneoplastic colonic lesions. More importantly, microcystins are potent tumour promoters and there is an indication that they also act as tumour initiators.
Epidemiological studies have suggested that microcystins are an important risk factor for the high incidence of primary liver cancer in certain areas of China, where people have consumed pond-ditch and river water contaminated with low levels (within the range of 0.09-0.46 μg/l) of microcystins. Furthermore, bathing and showering in water containing cyanobacterial cells can result in allergic reactions resembling hay fever, asthma and skin, eye and ear irritation. A 2009 study by Oberholster reported an increase in cyanobacterial bloom duration over the last 60 years in selected dams from 4 months to 9 months. This implies that in the future, eutrophic lakes in South Africa will stratify earlier in spring and destratify later in autumn. The consequences of this scenario will induce much higher costs for water treatment plants as well as increases in incidence of livestock intoxification by cyanobacteria.
