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

At two natural volcanic seeps in Papua New Guinea, the partial pressure of carbon dioxide (pCO(2)) 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 pCO(2), 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 pCO(2), 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 pH(total) = 8.1, pCO(2) = 323 mu atm) and CO2 seep sites (mean pH(total) = 7.8, pCO(2) = 803 mu atm) each at two reefs. In Porites spp., only one of the biochemical parameters investigated (the ratio of photoprotective to light-harvesting pigments) responded to pCO(2), 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 pCO(2) reef interactions. In A. millepora, most pigments investigated were reduced at elevated pCO(2), while other parameters (e.g. tissue biomass, total proteins, total lipids, protein carbonyls, some fatty acids and pigments) differed between reefs or showed pCO(2) 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 pCO(2) up to 800 mu atm, 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.