The response of the marine nitrogen cycle to ocean acidification

Ocean acidification, arising from the influx of anthropogenically generated carbon, poses a massive threat to the ocean ecosystems. Our knowledge of the effects of elevated anthropogenic CO2 in marine waters and its effect on the performance of single species, trophic interactions and ecosystems is increasing rapidly. However, our understanding of the biogeochemical cycling of nutrients such as nitrogen is less advanced and lacks a comprehensive overview of how these processes may change under OA conditions. We conducted a systematic review and meta –analysis of eight major nitrogen transformation processes incorporating 49 publications to synthesize current scientific understanding of the effect of OA on nitrogen cycling in the future ocean. The following points were identified by our meta-analysis: (1) diazotrophic nitrogen fixation is likely enhanced by 29 ± 4 % under OA. (2) species and strain-specific response of nitrogen fixers to OA was detectable, which may result in alterations in microbial community composition in the future ocean. (3) nitrification processes were reduced by a factor of 29 ± 10%. (4) declines in nitrification rates were not reflected by nitrifier abundance. (5) contrasting results in uni-specific culture experiments versus natural communities were apparent for nitrogen fixation and denitrification.

The net effect of the nitrogen cycle process responses also suggests there may be a shift in the relative nitrogen pools, with excess ammonium originating from CO2 fertilized diazotrophs. This regenerated inorganic nitrogen can recycle in the upper water column increasing the relative importance of the ammonium- fueled regenerated production.

However, several feedback mechanisms with other chemical cycles, such as oxygen, and interaction with other climate change stressors may counteract these findings. Finally, our review highlights the shortcomings and gaps in current understanding of the potential changes in nitrogen cycling under future climate and emphasises the need for further ecosystem studies.

Wannicke N., Frey C., Law C. S. & Voss M., in press. The response of the marine nitrogen cycle to ocean acidification. Global Change Biology. Article (subscription required).

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