Foraminiferal holobiont thermal tolerance under future warming – roommate problems or successful collaboration?

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 ecosystem engineers. We examined the thermal tolerance of two main types of LBF holobionts characterized by different algal symbionts and shell types (resulting from alternative biomineralization mechanisms): the hyaline diatom-bearing Amphistegina lobifera and the porcellaneous-dinoflagellate-bearing Sorites orbiculus. In order to assess the holobiont thermal tolerance we separately evaluated foraminiferal calcification rates and symbionts' net photosynthesis under present-day and future warming scenarios. Our results show that both holobionts exhibit progressive loss-of-life functions between 32 and 35 ∘C. This sensitivity differs in the magnitude of their response: calcification of A. lobifera was drastically reduced compared with S. orbiculus. Thus, future warming may significantly shift the relative contribution of the two species as carbonate producers. Moreover, A. lobifera exhibited a synchronous response of calcification and net photosynthesis. In contrast, in S. orbiculus the symbionts decreased net photosynthesis prior to calcification. This implies that algal symbionts limit the resilience of the halobiont.