Non-lethal effects of ocean acidification on the symbiont-bearing benthic foraminifer Amphistegina gibbosa

The responses of marine taxa to ocean acidification are varied, with, for example, some exhibiting decreased and some increased calcification rates. Experiments were conducted to assess the effect of elevated atmospheric carbon dioxide concentrations on the survival, fitness, shell microfabric and growth of Amphistegina gibbosa, a symbiont-bearing, coral-reef dwelling, benthic foraminiferal species that precipitates low-Mg calcite tests, using CO2 partial pressure (pCO(2)) levels similar to those likely to occur in shallow marine pore waters in the decades ahead. Specimens were cultured at constant temperature and controlled pCO(2) (ambient, 1000 parts per million by volume [ppmv], and 2000 ppmv) for 6 wk, and total alkalinity and dissolved inorganic carbon were measured every 2 wk to characterize the carbonate chemistry of the incubations. Foraminiferal survival and cellular energy levels were assessed using adenosine triphosphate analyses, and test microstructure and growth were evaluated using high resolution scanning electron microscopy and image analysis. Fitness and survival were not directly affected by elevated pCO(2) and the concomitant decrease in pH and calcite saturation states (Omega(c)). Test growth was not affected by elevated pCO(2). However, areas of dissolution were observed after 6 wk, even though Omega(c) was >1 in all treatments; the fraction of test area dissolved increased with decreasing Omega(c). Test dissolution occurred only in small, well defined patches that appeared to be distributed randomly over the whole test surface. Similar dissolution was observed in offspring produced in the 2000 ppmv pCO(2) treatments. The long-term ecological consequences of the effects observed are not yet known.