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The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium

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Science  05 May 2017:
Vol. 356, Issue 6337, pp. 527-531
DOI: 10.1126/science.aal2981

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Reconciling pH and future productivity

The differential effects of reduced seawater pH and increased carbon dioxide on marine phytoplankton productivity have not been resolved. Hong et al. found that previous experimentation did not account for variable metal concentrations or for ammonia contamination. After controlling for these variables, experimentation, protein expression analysis, and field data showed that low pH, coupled with the low ambient iron availability in the open ocean, inhibits nitrogen fixation, whereas elevated CO2 is fertilizing. Overall, the deleterious effects of decreased pH trump the beneficial effects of increased CO2. Thus, it seems that in a future, more acidic ocean, phytoplankton productivity is likely to be suppressed.

Science, this issue p. 527

Abstract

Acidification of seawater caused by anthropogenic carbon dioxide (CO2) is anticipated to influence the growth of dinitrogen (N2)–fixing phytoplankton, which contribute a large fraction of primary production in the tropical and subtropical ocean. 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.

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