Microcystis aeruginosa is a species of freshwater cyanobacteria which can form harmful algal blooms (HABs) that are of economic and ecological importance. They are the most common toxic cyanobacterial bloom in eutrophic freshwater.
Cells usually are organized into colonies (large colonies of which may be viewed with the naked eye) that begin in a spherical shape, but losing their coherence to become perforated or irregularly shaped over time.
The coloration of the protoplast is a light blue-green, 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 (Oberholster), 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 Microcystis aeruginosa can form persistent microcystin toxins under the right environmental conditions, it has come to be a source of drinking water pollution (which can be rendered hepatotoxic). This can lead to increased economic costs such as water quality mitigation measures in the form of increased costs at water filtration facilities, as well as damage to local tourism caused by lake or other waterway closures due to toxicity concerns.
In 2009 unprecedented mammal mortality in the southern part of the Kruger National Park led to an investigation which implicated Microcystis aeruginosa. The dead animals included grazers and browsers which preferred drinking from the leeward side of two man-made dams, a natural point of accumulation for drifting Microcystis blooms. Mammals such as elephant 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, the most abundant cyanobacterial genus in South Africa, may occur in toxic and harmless strains. 
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