The microcystin-producing Microcystis is a genus of freshwater cyanobacteria and thrives in warm water conditions, especially in stagnant waters. The EPA predicted in 2013 that climate change and changing environmental conditions may lead to harmful algae growth and may negatively impact human health. Algal growth is also encouraged through the process of eutrophication (oversupply of nutrients). In particular, dissolved reactive phosphorus promotes algal growth.
Microcystins may have evolved as a way to deal with low iron supply in cyanobacteria: the molecule binds iron, and non-producing strains are significantly worse at coping with low iron levels. Low iron supply up-regulates McyD, one of the microcystin synthetic operons. Sufficient iron supply, however, can still boost microcystin production by making the bacterium better at photosynthesis, therefore producing sufficient ATP for MC biosynthesis.
Microcystin production is also positively correlated with temperature. Bright light and red light increases transcription of McyD, but blue light reduces it. A wide range of other factors such as pH may also affect MC production, but comparison is complicated due to a lack of standard testing conditions.