While increased carbon dioxide levels theoretically boost the productivity of nitrogen-fixing bacteria in the world's oceans, because of its "fertilizing" effect, a new study reveals how increasingly acidic seawater featuring higher levels of this gas can overwhelm these benefits, hampering the essential service these bacteria provide for marine life. The new data help explain disparities in previous studies exploring the effects of ocean acidification on nitrogen fixation. The abundant cyanobacteria Trichodesmium is estimated to contribute up to 50% of marine nitrogen fixation; therefore, understanding how this species will respond to a changing environment is critical. Some studies have reported that, under acidified conditions, Trichodesmium significantly increases its rates of nitrogen fixation, photosynthesis and growth, whereas others have documented significant decreases in these processes. Haizheng Hong et al. studied Trichodesmium under controlled conditions, correcting for ammonium and copper contamination (which they say affected some previous results). They found that increasingly acidic water negatively impacted the bacterium's ability to fix nitrogen. The negative impacts were even more pronounced if iron, an essential nutrient for Trichodesmium, was limited. Further analysis of key bacterial proteins revealed that acidification under iron-limited conditions requires a reallocation of iron among proteins to compensate for the loss of nitrogen-fixation efficiency. The researchers also sampled Trichodesmium at three stations in the northern South China Sea, where surface iron concentrations are very low; nitrogen fixation was also limited in these locations, they report.