Calcification and photobiology in symbiont-bearing benthic foraminifera and responses to a high CO 2 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 pCO 2 conditions (467-1925μatm pCO 2). 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 pCO 2 environments and impacts of elevated pCO 2 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 pCO 2 (linear models, p>0.05). However, M. vertebralis displayed significantly (planned comparison t=2.61, p<0.05) increased calcification rates (63%) in elevated pCO 2 regimes. Increased pCO 2 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 pCO 2 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, P max, α and E k did not vary under different pCO 2 conditions (496 and 1925μatm). Thus, foraminiferal species investigated in the present study did not show negative effects in exposures up to 1925μatm pCO 2. However, previous field studies from natural CO 2 vents showed that LBF disappear at pCO 2 conditions predicted for the near future (pH Total<7.9). This indicates that the short term ability of the holobiont or symbiont to cope or even benefit from elevated pCO 2 is no guarantee for their survival in the long-term. © 2012 Elsevier B.V.