Biochemical responses to ocean acidification contrast between tropical corals with high and low abundances at volcanic carbon dioxide seeps

AbstractAt two natural volcanic seeps in Papua New Guinea, the partial pressure of carbon dioxide (pCO2) in the seawater is consistent with projections for 2100. Here, the cover of massive scleractinian corals Porites spp. is twice as high at elevated compared with ambient pCO2, while that of branching corals such as Acropora millepora is greater than twofold reduced. To assess the underlying mechanisms for such community shifts under long-term exposure to elevated pCO2, biochemical parameters related to tissue biomass, energy storage, pigmentation, cell protection, and cell damage were compared between Porites spp. and A. millepora from control (mean pHtotal = 8.1, pCO2 = 323 µatm) and CO2 seep sites (mean pHtotal = 7.8, pCO2 = 803 µatm) each at two reefs. In Porites spp., only one of the biochemical parameters investigated (the ratio of photoprotective to light-harvesting pigments) responded to pCO2, while tissue biomass, total lipids, total proteins, and some pigments differed between the two reefs, possibly reflecting differences in food availability. Furthermore, some fatty acids showed pCO2 –reef interactions. In A. millepora, most pigments investigated were reduced at elevated pCO2, while other parameters (e.g. tissue biomass, total proteins, total lipids, protein carbonyls, some fatty acids and pigments) differed between reefs or showed pCO2–reef interactions. Tissue biomass, total lipids, and cell-protective capacities were distinctly higher in Porites spp. than in A. millepora, indicating higher resistance to environmental stress in massive Porites. However, our data suggest that important biochemical measures remain relatively unaffected in these two coral species in response to elevated pCO2 up to 800 µatm, with most responses being smaller than differences between species and locations, and also when compared with responses to other environmental stressors such as ocean warming.