Under anoxic conditions, the bacteria inhabiting algae cultures break down the organic material and produce hydrogen sulfide and ammonia which causes the odor.
This reduces the levels of oxygen within the sediment often resulting in partially anoxic conditions, which can be further exacerbated by limited water flux.
While biomethanation usually only occurs under anoxic conditions, the generation of methane in an oxygenated environment under near-ambient conditions was discovered in 2006.