As rising atmospheric carbon dioxide levels cause the pH of oceans to decrease, an outstanding question is how ocean acidification will affect marine ecosystems. Nitrogen fixation is the process by which atmospheric N2 is converted to NH3 or other molecules to make the nutrient available to living organisms. The cyanobacterium Trichodesmium fixes as much as 50% of N2 for marine ecosystems, but studies of how ocean acidification affects Trichodesmium N2 fixation have yielded conflicting results. New research indicates that the conflicts likely arose because of growth media contamination and that ocean acidification will decrease Trichodesmium growth and N2 fixation (Science 2017, DOI: 10.1126/science.aal2981). A team led by Dalin Shi of Xiamen University replicated previous experiments, ultimately finding that a toxic metal and ammonium in the growth media affected the results. Shi and colleagues’ lab studies and field work demonstrate that if other variables remain constant, higher CO2 levels increase Trichodesmium growth and N2 fixation. However, the pH decrease that accompanies an increase in CO2 overrides that effect and leads to a net decrease in growth and N2 fixation. The effect is amplified when iron is a limiting nutrient, as in the South China Sea.