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.

Keywords:

• Calcification
• carbon isotope fractionation
• carbonate system
• CO2, O2, pH, and Ca2+ microenvironment
• DBL
• inorganic carbon sources

Acknowledgments

We thank the staff of the Caribbean Marine Research Station (CARMABI) for providing laboratory facilities, and especially Brian Leysner for his help and support. Gaby Eickert is acknowledged for excellent technical assistance. We thank Hans-Peter Grossart for diving assistance and sampling of foraminifera. Brad Bebout, Volker Meyer and Roland Thar are thanked for their development of the microsensor data acquisition software and hardware used in the experiments. This study was financed by the Max-Planck-Gesellschaft (Germany) (SKR) and the Danish Natural Science Research Council (MK). Christoph Hemleben and an anonymous reviewer are thanked for constructive criticism.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Editorial responsibility: Tom Fenchel