Coccolithophores are unicellular phytoplankton that produce calcium carbonate coccoliths as an exoskeleton. Emiliania huxleyi, the most abundant coccolithophore in the world’s ocean, plays a major role in the global carbon cycle by regulating the exchange of CO₂ across the ocean-atmosphere interface through photosynthesis and calcium carbonate precipitation. As CO₂ concentration is rising in the atmosphere, the ocean is acidifying and ammonium (NH₄⁺) concentration of future ocean water is expected to rise. The latter is attributed to increasing anthropogenic nitrogen (N) deposition, increasing rates of cyanobacterial N₂ fixation due to warmer and more stratified oceans, and decreased rates of nitrification due to ocean acidification. Thus future global climate change will cause oceanic phytoplankton to experience changes in multiple environmental parameters including CO₂, pH, temperature and nitrogen source. This study reports on the combined effect of elevated pCO₂ and increased NH₄⁺ to nitrate (NO₃^–) ratio (NH₄⁺/NO₃^–) on E. huxleyi,maintained in continuous cultures for more than 200 generations under two pCO₂ levels and two different N sources. Here we show that NH₄⁺ assimilation under N-replete conditions depresses calcification at both low and high pCO₂, alters coccolith morphology, and increases primary production. We observed that N source and pCO₂ synergistically drive growth rates, cell size and the ratio of inorganic to organic carbon. These responses to N source suggest that, compared to increasing CO₂ alone, a greater disruption of the organic carbon pump could be expected in response to the combined effect of increased NH₄⁺/NO₃^– ratio and CO₂ level in the future acidified ocean. Additional experiments conducted under lower nutrient conditions are needed prior to extrapolating our findings to the global oceans. Nonetheless, our results emphasize the need to assess combined effects of multiple environmental parameters on phytoplankton biology in order to develop accurate predictions of phytoplankton responses to ocean acidification.

Lefebvre S. C., Benner I., Stillman J. H., Parker A. E., Drake M. K., Rossignol P. E., Okimura K. M., Komada T., & Carpenter E. J., in press. Nitrogen source and pCO2 synergistically affect carbon allocation, growth and morphology of the coccolithophore Emiliania huxleyi: potential implications of ocean acidification for the carbon cycle.

Global Change Biology doi:10.1111/j.1365 2486.2011.02575.x. Article (subscription required).