Calcification and photobiology in symbiont-bearing benthic foraminifera and responses to a high CO2 environment

Abstract

The present study investigates impacts of ocean acidification on calcification rates and light responses of large benthic foraminifera (LBF). Studies were conducted on diatom-bearing Amphistegina radiata and Heterostegina depressa and dinoflagellate-bearing Marginopora vertebralis in controls and manipulated seawater pCO2 conditions (467–1925μatm pCO2). In a six week experiment, calcification and photobiology were investigated for all three species. Additionally, short-term experiments were carried out on H. depressa and M. vertebralis to determine photosynthetic rates in several pCO2 environments and impacts of elevated pCO2 in increasing light intensities (photosynthesis irradiance “P–I” curves) on M. vertebralis. In the long-term experiment, positive growth (inferred through cross-sectional surface area) was measured in all control and acidification conditions but growth rates of A. radiata and H. depressa were not affected by increased pCO2 (linear models, p>0.05). However, M. vertebralis displayed significantly (planned comparison t=2.61, p<0.05) increased calcification rates (63%) in elevated pCO2 regimes. Increased pCO2 did not affect maximum quantum yield (measured by pulse amplitude modulation “PAM” fluorometry) and chlorophyll a content in any species investigated. Photosynthetic measurements (oxygen evolution) on H. depressa and M. vertebralis revealed positive net production under experimental light conditions (10 and 29μmolphotonsm−2s−1, respectively), however no significant effect of elevated pCO2 on net production and dark respiration after both long- and short-term exposure was observed. M. vertebralis measured under nine different light conditions displayed typical P–I curves with light saturation points of app. 500μmolphotonsm−2s−1. However, Pmax, α and Ek did not vary under different pCO2 conditions (496 and 1925μatm). Thus, foraminiferal species investigated in the present study did not show negative effects in exposures up to 1925μatm pCO2. However, previous field studies from natural CO2 vents showed that LBF disappear at pCO2 conditions predicted for the near future (pHTotal<7.9). This indicates that the short term ability of the holobiont or symbiont to cope or even benefit from elevated pCO2 is no guarantee for their survival in the long-term.