Methane is an important greenhouse gas in Earth’s atmosphere. Methane accumulates in oxygenated, surface freshwaters with high amounts of cyanobacteria. Cyanobacteria are photosynthetic bacteria, that grow using the same light-dependent, oxygen-producing process that plants use. However, the exact methane source in these environments are unknown, and many researchers believed that methanogenic archaea that live in anaerobic “pockets” or attached to cyanobacteria may be responsible for the methane produced in these waters.  

A recent study tested if cyanobacteria directly contribute to global methane budgets. The researchers grew 13 different kinds of cyanobacteria from various environments in the laboratory and fed them heavy carbon ("Carbon-13"). The heavy carbon was then incorporated into the cyanobacteria biomass, "labelling" the compounds that the cyanobacteria produced by making them heavier than usual. The researchers saw that methane was labelled with heavy carbon, even in cultures in which the cyanobacterium was the only organism present, which shows that cyanobacteria directly produce methane.

Because cyanobacteria blooms are expected to increase with global climate change, and methane is a potent greenhouse gas, the observation that cyanobacteria directly produce methane suggests that cyanobacteria blooms may be a previously unrecognized positive feedback loop on global climate change. However, the controls on methane production by cyanobacteria in the environment and the contribution of cyanobacteria to global methane budgets are still poorly understood.