Cyanobacteria are capable of differentiating into heterocysts, which are able to fix nitrogen. Due to the prevalence of cyanobacteria in oceans, their presence is able to modify the ecosystems they are found in.
Upwelling and eutrophication can occur where high levels of cyanobacteria are present. This can decrease the oxygen levels in the surrounding water, leading to the death of other organisms.
Some species of cyanobacteria produce cyanotoxins (such as anatoxin and saxitoxin). These are highly toxic and can kill in low concentrations. This is the reason ‘blue-green algae’ is deemed dangerous.
Mass blooms of cyanobacteria can increase the localised concentration of fixed nitrogen. Due to this being by a photosynthetic process, the cost (both economically and energetically) of nitrates / nitrites may be far lower, if it is possible to recover the fixed nitrogen – the Haber process is very energetically intensive. The concentration of phosphorous and iron in the water controls the concentration of prochloron cyanobacteria – this could be controlled by humans by ‘geoengineering’ to give efficient nitrogen fixation. Some people have reservations; there were some small trials in oceans many years ago (ocean fertilisation), however this has not been explored recently.