Foraminiferal holobiont thermal tolerance under climate change – Roommates problems or successful collaboration?

Abstract. Understanding the response of marine organisms to expected future warming is essential. Large Benthic Foraminifera (LBF) are symbiont bearing protists considered to be major carbonate producers and ecosystems engineers. We examined the thermal tolerance of two main types of LBF holobionts characterized by different algal symbionts and shell types (resulted from alternative biomineralization mechanisms): The hyaline diatom bearing, Amphistegina lobifera, and the porcelaneous dinoflagellate bearing, Sorites orbiculus. To assess the relative contribution of host and symbiont algae to the holobiont thermal tolerance we separately evaluated their response by measuring calcification rates and photosynthetic activity under present-day and future warming scenarios. Our results show that both holobionts exhibit thermal resilience up to 32 °C and sensitivity to 35 °C. This sensitivity differs in the magnitude of their response: calcification of A. lobifera was completely inhibited while it was only reduced in S. orbiculus. Thus, future warming will significantly shift the relative contribution of the two species as carbonate producers. Moreover, A. lobifera exhibited a synchronized response of the host and symbionts. In contrast, in S. orbiculus the symbionts responded prior to the host, possibly limiting its resilience. Our results also demonstrate the role of pre-exposure and acclimation processes of host, symbionts or both in mitigating future warming. It highlights the possibility that while pre-exposure to moderate temperatures benefits the holobiont, in cases of extreme temperature it might reduce its thermal tolerance.