Microcystis aeruginosa is a species of freshwater cyanobacteria which can form harmful algal blooms of economic and ecological importance. They are the most common toxic cyanobacterial bloom in eutrophic fresh water. Cyanobacteria produce neurotoxins and peptide hepatotoxins, such as microcystin and cyanopeptolin.
Cells usually are organized into colonies (large colonies of which may be viewed with the naked eye) that begin in a spherical shape, but lose their coherence to become perforated or irregularly shaped over time.
The protoplast is a light blue-green color, appearing dark or brown due to optical effects of gas-filled vesicles; this can be useful as a distinguishing characteristic when using light microscopy. These vesicles provide the buoyancy necessary for M. aeruginosa to stay at a level within the water column at which they can obtain optimum light and carbon dioxide levels for rapid growth.
M. aeruginosa is favored by warm temperatures, but toxicity and maximal growth rates are not totally coupled, as the cyanobacterium has highest laboratory growth rates at 32 °C, while toxicity is highest at 20 °C, lowering in toxicity as a function of increasing temperatures in excess of 28 °C. Growth has been found to be limited below 15 °C.
Because of M. aeruginosa´s microcystin toxin production under the right environmental conditions, it can be a source of drinking water pollution. water quality mitigation measures in the form of water filtration facilities can lead to increased economic costs as well as damage to local tourism caused by lake or other waterway closures.
In 2009, unprecedented mammal mortality in the southern part of the Kruger National Park led to an investigation which implicated M. aeruginosa. The dead animals included grazers and browsers which preferred drinking from the leeward side of two dams, a natural point of accumulation for drifting Microcystis blooms. Mammals such as elephants and buffalo which usually wade into water before drinking, were unaffected, as were the resident crocodiles. The source of nutrients which supported the Microcystis growth was narrowed down to the dung and urine voided in the water by a large resident hippo population, unaffected by the bloom. The immediate problem was solved by breaching of the dam walls and draining of the water. M. aeruginosa is the most abundant cyanobacterial genus in South Africa, may be a toxic or a harmless strains. Some South African water bodies are now highly contaminated, mostly from return flows out of dysfunctional waste water treatment works that discharge over 4 billion liters of untreated, or at best partially treated sewage into receiving rivers ever day, with Hartebeestpoort Dam being among the worst.
Microcystin has been linked to the death of sea otters in 2010, a threatened species in the US. The poisoning probably resulted from eating contaminated bivalves often consumed by sea otters and humans. The researchers noted that such bivalves in the area exhibited significant biomagnification (to 107 times ambient water levels) of microcystin.
Algal blooms of cyanobacteria thrive in the large phosphorus content of agricultural runoff. Besides consuming phosphorus, M. aeruginosa thrives on glyphosate, although high concentrations may inhibit it. M. aeruginosa has shown glyphosate resistance as result of preselective mutations, and glyphosate presence serves as an advantage to this and other microbes that are able to tolerate its effects, while killing those less tolerant.
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