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Respiration, photosynthesis, and δ13C signature in symb

Stable isotope composition of foraminifera tests, and especially δ18O and δ13C, are commonly used as proxies to reconstruct past ocean conditions, including paleotemperature and paleoproductivity. δ18O is used as a proxy for paleotemperature, whereas δ13C is a proxy for productivity. Metabolic theory predicts that at the community level there is a positive correlation between temperature and both respiration and photosynthesis, albeit it at different rates (Brown et al., 2004; Regaudie-de-Gioux and Duarte, 2012). The Q10 temperature coefficient is the measure in the rate of change as a consequence of an increasing temperature by 10°C. The Q10 of respiration is ~2.5, and for photosynthesis ~1.5. This has a twofold impact on mixotrophic organisms (organisms that use a mix of autotrophy and heterotrophy). Under the same nutrient conditions, but at higher temperature relative nutrient availability is lower, and the mixotroph will rely more on the contribution of the photosymbionts. The latter is expected to result in an increased δ13C.