English: Changes to the marine nitrogen cycle under future ocean acidification conditions.
As additional CO2 is absorbed by the ocean and reacts with water molecules (H2O), carbonic acid is formed and broken down into both bicarbonate (H2CO3) and hydrogen (H+) ions (gray arrow), which reduces bioavailable carbonate and decreases ocean pH (black arrow). This is likely to enhance nitrogen fixation by diazatrophs (gray arrow), which utilize H+ ions to convert N2 into bioavailable forms such as ammonia (NH3) and ammonium ions (NH+4). However, as pH decreases, and more ammonia molecules are converted to ammonium ions (gray arrow), there is less oxidation of ammonia to nitrite (NO−2), resulting in an overall decrease in nitrification and denitrification (black arrows). This in turn would lead to a further build up of fixed nitrogen in the ocean, with the potential consequence of eutrophication. Gray arrows represent an increase while black arrows represent a decrease in the associated process.