Microcystis

Microcystis aeruginosa forms the toxic algal blooms that close beaches, contaminate drinking water, and kill livestock worldwide. This colonial cyanobacterium produces microcystins—potent hepatotoxins that damage the liver of any animal that ingests them. Blooms can exceed billions of cells per liter, turning lakes bright green and creating conditions lethal to most other aquatic life. Microcystis represents cyanobacterial success taken to destructive extreme.

The bacterium dominates through a combination of capabilities. Gas vesicles enable rapid buoyancy adjustment—Microcystis rises to the surface for photosynthesis during calm periods, then descends to access deep nutrients. Competitors lacking this vertical mobility are shaded out. Toxin production deters grazing by zooplankton that would otherwise control populations. Colonial organization protects cells from viral infection and predation. Together these mechanisms create positive feedback: the more successful Microcystis becomes, the more it degrades conditions for competitors.

Microcystis blooms are intensifying globally due to nutrient pollution and climate change. Warmer temperatures favor cyanobacteria over other phytoplankton; nutrient loading from agriculture provides the fertilizer blooms need. Understanding Microcystis dynamics has become essential for water resource management. The organism exemplifies how climate and human activity can shift competitive balances toward organisms with dangerous characteristics—a cautionary tale about environmental change amplifying the success of harmful species.