Microsensor studies of photosynthesis and respiration in the larger symbiont bearing foraminifera Amphistegina lobifera, and Amphisorus hemprichii

Abstract

The photosynthesis and respiration of the larger foraminifera Amphistegina lobiferaand Amphisorus hemprichiiwas studied with O₂, CO₂, and pH microsen-sors, and with a miniature gas exchange chamber. The diffusive transport of O₂ and CO₂ through both perforate (A. lobifera) and imperforate (A. hemprichii) calcite shells of the foraminifera was fast and allowed investigations of endosymbiont photosynthesis by microsensor measurements at the shell surface. Gross photosynthesis versus scalar irradiance (P vs. E_o) curves showed onset of light saturation (E_k) at 95-198 μmol photons m^-2 s^-1. No photoinhibition was observed up to an irradiance of 2000 μmol photons m^-2 s^-1. Net photosynthesis (at saturating irradiance) and dark respiration rates were 3.7-22.7 and 5.6-14.3 nmol O₂ foraminifer^-1 h^-1, respectively. Simultaneous CO₂, pH and O₂ measurements at the shell surface of A. hemprichiiduring experimental light-dark cycles showed rapid concentration changes of all three variables upon light-dark or dark-light shifts. The dynamics of O₂ and CO₂ at the shell surface of A. hemprichiishowed unequal net conversion rates of O₂and CO₂ during experimental light-dark cycles. The molar O₂/CO₂ conversion ratio at the shell surface of the foraminifera was ∼2 in darkness and ∼6 at saturating irradiance, pointing to a large internal supply of CO₂ in the host-symbiont association and the use of bicarbonate as source for inorganic carbon. The carbonate chemistry in the vicinity of symbiont-bearing larger foraminifera is thus strongly affected by the combined action of photosynthesis, respiration and calcification, and cannot be considered in equilibrium with the surrounding sea water. This has important implications for paleoenvironmental analysis and interpretation of the stable isotope composition of foraminiferal calcite shells and the derived models used for reconstructing climate and productivity in the past.

Keywords:

• photosynthesis
• inorganic carbon sources
• respiration
• larger foraminifera
• symbiosis
• P vs. E_o curves
• Amphestigina lobifera
• Amphisorus hemprichii