Growth rate, short-term N2 fixation rate, K1/2(DIC), cytosolic pH, and protein abundance of Trichodesmium IMS101, and diazotroph community composition, N2 fixation rate and nifH gene transcription of Trichodesmium spp. collected from NSCS surface water

We found that growth and N2-fixation of the ubiquitous cyanobacterium Trichodesmium decreased under acidified conditions, notwithstanding a beneficial effect of high CO2. Acidification resulted in low cytosolic pH and reduced N2-fixation rates despite elevated nitrogenase concentrations.

Low cytosolic pH required increased proton pumping across the thylakoid membrane and elevated adenosine triphosphate production. These requirements were not satisfied under field or experimental iron-limiting conditions, which greatly amplified the negative effect of acidification.