Quantifying respiration and photosynthesis rates in Amphistegina lobifera at different temperatures

Current rising temperatures in the oceans change marine habitats and faunal compositions as well as physiological and metabolic processes in marine organisms. While thermal stress is potentially threatening existing benthic communities it can be advantageous for invasive species that are better adapted to high temperatures. One such example of a successfully expanding species is the symbiont-bearing large benthic foraminifera Amphistegina lobifera Larsen, 1976 originating from the Red Sea, that has spread throughout the coastal ecosystems of the eastern Mediterranean. Studies on A. lobifera have shown its high tolerance to increasing temperature with regards to survivorship and photochemistry in temperature ranges from 24 °C to 36 °C. Interestingly, little is known about the species’ actual metabolic and photosynthetic activity with respect to oxygen consumption and production under different temperatures, especially towards the lower boundaries of its optimal environment. This study addresses this gap with a quantitative assessment of A. lobifera’s respiration rates that also allows for better comparison with other species and environmental factors. Amphistegina lobifera was permanently cultivated in the laboratory at University of Vienna in artificial seawater (ASW) at 24 °C and 38 psu with a day:night light cycle of 8:16 hours and ~ 10 µmol photons/m²/s light intensity. A non-invasive method was used to analyse oxygen respiration rates. The method involved placing an Oxygen Sensor Spot in a small, 2 ml airtight glass vial filled with ASW alongside the foraminifera. Oxygen concentrations under dark and light conditions (~ 30 µmol photons/m²/s) at different temperatures (16 °C, 20 °C, 24 °C, 28 °C, 32 °C, 36 °C) were recorded. Seventeen cleaned, living specimens were measured in triplicate after a 24-hour acclimation period. Respiration rates, normalized for biovolume (µm³), ranged from 3.73 × 10⁻⁹ nmol O₂/µm³/h at 16 °C to 2.83 × 10⁻⁸ nmol O₂/µm³/h at 32 °C under dark conditions. Oxygen production under light conditions consistently exceeded consumption. Gross photosynthesis was lowest at 36 °C (1.45 × 10⁻⁹ nmol O₂/µm³/h) compared to the overall mean of 4.06 × 10⁻⁸ nmol O₂/µm³/h. These results will give further insights into the ecological impacts and the contribution to biogeochemical cycles of A. lobifera in future ocean environments. Furthermore, the method provides a robust approach for comparing respiration rates across species and isolating the effects of specific environmental factors on metabolic rates.