Nitrogen addition modulates the response of Hydrolithon onkodes to ocean acidification

A growing body of evidence suggests that continued acidification of surface waters of the ocean will have significant negative impacts on calcifying organisms and this will be particularly important for taxa that build coral reefs. Calcified crustose coralline algae are important primary producers on coral reefs that bind the reef framework together and often serve as settlement cues for coral larvae. The results of previous experiments indicate that rates of calcification of Hydrolithon onkodes are reduced significantly by elevated pCO2, although the magnitude of the response appears to be habitat-dependent. We conducted an experiment in Moorea, French Polynesia to test the hypothesis that nitrogen availability might modulate calcification responses of H. onkodes to elevated pCO2. Using a crossed- factor design with two levels of pCO2 (390, 800 μatm) and two levels of ammonium concentration (ambient, 10x ambient), coralline algal samples were incubated for two weeks. Samples exposed to elevated ammonium exhibited significantly higher photosynthetic performance regardless of the pCO2 treatment. Rates of net calcification were reduced under elevated pCO2 at ambient ammonium concentrations, but calcification increased significantly with ammonium addition under both pCO2 levels. The mechanism underlying the effect of ammonium enrichment on the calcification response is not known, but is consistent with the stimulation of photosynthesis by nutrient addition and a close coupling between rates of photosynthesis and calcification. These results suggest that the effects of ocean acidification on calcification likely will be variable and determined, at least in part, by habitat-related environmental conditions.