The chemical microenvironment of the symbiotic planktonic foraminifer Orbulina universa

Abstract

Microsensor measurements of CO2, O2, pH and Ca2+ in the vicinity of the symbiont-bearing planktonic foraminifer Orbulina universa showed major light-modulated changes in the chemical microenvironment due to symbiont photosynthesis, respiration of the holobiont, and precipitation of the calcite shell. Under saturating light conditions, microprofiles measured towards the shell surface showed an O2 increase of up to 220% air saturation, a decrease in CO2 concentration to 4.9 μM, and a pH increase to 8.8 due to symbiont photosynthesis. The Ca2+ concentration decreased to ∼9.6 mM in two specimens, while it increased to 10.2–10.8 mM in three other specimens kept in light. In darkness, the respiration of the community decreased the O2 concentration to 82% of air saturation, CO2 increased up to 15 μM, the pH decreased to 8.0, and the Ca2+ concentration increased up to 10.4 mM. These data, and derived calculations of the distribution of HCO3 − and CO3 2− near the shell, showed that the carbonate system in the vicinity of O. universa was significantly different from conditions in the surrounding seawater, both in light and darkness, due to the metabolism of the foraminifer and its associated algae. Experimental light–dark cycles indicated a sufficient CO2 supply sustaining high carbon fixation rates of the symbiotic algae via conversion of HCO3 − or via CO2 release from calcification and host respiration. Our findings on irradiance-dependent CO2 and pH changes in the vicinity of symbiont-bearing planktonic foraminifera give direct experimental evidence for the predictions of isotope fractionation models used in palaeoclimatology stating that metabolic processes affect the isotopic carbon signal (δ13C) in foraminifera.