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Effects of ocean acidification on the primary framework builders of coral reefs
Motivated by the growing number of studies reporting inter-specific differences for the effects of OA on calcifying taxa, this study tested the hypothesis that taxonomically diverse calcifying cnidarians show differential responses to OA. Phylogenetic analyses show that the Scleractinia consist of two clades (Robusta and Complexa), with the split between the two occurring prior to the evolution of calcification. Further, milleporine hydrocorals arose after the Scleractinia in seas of different chemical composition. The potential for the Robusta, Complexa, and milleporines to represent different responses to OA due to independent evolution of calcification contextualized the present analysis. Scleractinians representing Robusta (Pocillopora meandrina) and Complexa (Porites spp. and Acropora pulchra) as well as the milleporine hydrocoral (Millepora platyphylla) were grown in 3 PCO2 regimes (400, 750 & 900 μatm) crossed with 2 temperatures (28.0 & 30.1 °C), with calcification measured after 3 weeks. Porites spp. and A. pulchra (= Complexa) and M. platyphylla were unaffected by PCO2, while calcification in P. meandrina (= Robusta) declined 33% and 55% at high PCO2 (750 & 900 μatm respectively) and 30.1 °C. These patterns clearly demonstrate differential responses to OA among phylogenetically diverse taxa, and raise the intriguing possibility that differential evolution of calcification could modulate the impacts of OA on these taxa. Due to differences in the response between Pocillopora meandrina and Millepora platyphylla to OA, I tested the hypothesis that the scleractinian, Pocillopora meandrina, and the hydrocoral, Millepora platyphylla, respond differently to increased PCO2 due to differential utilization of particulate food. To test this hypothesis, PCO2 treatments of 380 μatm and 710 μatm, were crossed with feeding treatments created through the supply of seawater filtered to ~100 μm (control zooplankton), seawater filtered to ~2 μm (low zooplankton), or seawater enriched with natural zooplankton (high zooplankton). M. platyphylla was unaffected by increased PCO2 at control and low zooplankton treatments, but high PCO2 increased calcification 19% in the high zooplankton treatment. P. meandrina was unaffected by increased PCO2 at the control zooplankton treatment, but increased PCO2 decreased calcification 32% and 29% for high and low zooplankton treatments, respectively. In control and low feeding treatments, calcification of M. platyphylla, may be nutrient limited, with the effect removed with additional zooplankton. P. meandrina interacts with food in different ways, as calcification was depressed by PCO2 both when additional zooplankton were supplied and when the smallest particulates were removed, but not with access to control seawater. We speculate that the differences between scleractinian and hydrocorals in their response to PCO2 as a function of food supply may reflect the evolutionary origins of these taxa in ancient seas differing in chemical composition.