Non-lethal effects of ocean acidification on two symbiont-bearing benthic foraminiferal species

Abstract. We conducted experiments to assess the effect of elevated atmospheric carbon dioxide concentrations on survival, fitness, shell microfabric and growth of two species of symbiont-bearing coral-reef benthic foraminifera, using pCO₂ Ievels similar to those likely to occur in shallow marine pore waters in the decades ahead. Foraminifera were cultured at constant temperature and controlled pCO₂ (385 ppmv, 1000 ppmv, and 2000 ppmv) for six weeks, and total alkalinity and dissolved inorganic carbon were measured to characterize the carbonate chemistry of the incubations. Foraminiferal survival and cellular energy levels were assessed using Adenosine Triphosphate (ATP) analyses, and test microstructure and growth were evaluated using high resolution SEM and image analysis. Fitness and survival of Amphistegina (A.) gibbosa and Archaias (A.) angulatus were not directly affected by elevated pCO₂ and the concomitant decrease in pH and calcite saturation states (Ω_c values) of the seawater (pH and Ω_c values of 8.12, 7.86, and 7.50, and 5.4, 3.4, and 1.5, for control, 1000 ppmv, and 2000 ppmv, respectively). In A. gibbosa, a species precipitating low-Mg calcite, test growth was not affected by elevated pCO₂, but areas of dissolved calcium carbonate were observed even though Ω_c was >1 in all treatments; the fraction of test area dissolved increased with decreasing Ω_c. Similar dissolution was observed in offspring produced in the 2000 ppmv pCO₂ treatments. In A. angulatus, whose tests are more-solubile high-Mg calcite, growth was greatly diminished in the 2000 ppmv pCO₂ treatment compared to the control. These non-lethal effects of ocean acidification – reduced growth in A. angulatus, and enhanced dissolution in A. gibbosa – may reflect differences in test mineralogy for the two species; the long-term ecological consequences of these effects are not yet known.