At a Glance
The fixation of nitrogen gas by specialized organisms such as Trichodesmium is key to controlling photosynthetic production in marine ecosystems and may be impaired by ocean acidification. Recent studies sought to untangle the separate effects of high CO2 and low pH on Trichodesmium and found that the former accelerates photosynthesis and N2-fixation whereas the latter impairs these functions. Low ambient pH results in low intracellular pH, which decreases the efficiency of the nitrogenase enzyme.
The photosynthetic production of organic carbon that supports marine ecosystems is limited by the bioavailability of nitrogen in large regions of the oceans. As a result, the fixation of nitrogen gas (N2) by specialized organisms (diazotrophs, which synthesize the nitrogenase enzyme) is a key process controlling marine productivity. There have been numerous recent publications on the effect of ocean acidification, caused by the CO2 emitted by fossil fuel burning, on N2-fixation by marine organisms. In particular, the cyanobacterium Trichodesmium, which is ubiquitous in low latitude oceans, has been reported to increase N2-fixation under acidic conditions.
A fundamental complication in assessing biological responses to ocean acidification is the need to unravel the differential effects of the simultaneous decrease in pH and increase in CO2 concentration. Experiments in which only one of these two parameters is varied at a time demonstrate that increasing CO2 while maintaining pH constant accelerates the rates of photosynthesis and N2-fixation by Trichodesmium. In contrast, decreasing pH at constant CO2 impairs photosynthesis and N2-fixation.
The negative effect of low pH, which overwhelms the positive effect of high CO2 when both co-vary, is enhanced under conditions of low iron availability, which are prevalent in large regions of the open ocean. Notably, the detrimental effect of low pH can be masked under experimental conditions if the culture medium is contaminated by traces of ammonium as acidification decreases the concentration of the uncharged and toxic ammonia (lower NH3/NH4+ ratio).
Mechanistic explanations for the positive effect of high CO2 and negative effect of low pH on Trichodesmium are provided by physiological measurements and proteomic analyses of the organism under various conditions. In particular, low ambient pH results in a decreased intracellular pH despite increased energy devoted to proton (H+) export. In turn, the low intracellular pH impairs the efficiency of the nitrogenase enzyme and leads to a decrease in N2-fixation rate despite an increase in enzyme concentration. Results of acidification experiments with field populations of Trichodesmium in the South China Sea are wholly consistent with laboratory results.
This study reconciles previous studies that gave conflicting results on the response of Trichodesmium to acidification. Extrapolation to the conditions expected for year 2100 suggests a potentially significant decline in the supply of new nitrogen to oceanic ecosystems.