Abstract
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 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.
Highlights
Since the beginning of the industrial revolution, anthropogenic activity has led to rapid ocean warming
Among our experiments the activity baselines of both calcification and net photosynthesis are significantly different between A. lobifera and S. orbiculus
Our study separately describes the thermal sensitivity of the foraminifera and the algal symbionts in two types of holobiont systems: A. lobifera-hosting diatoms, mostly from the order Fragilariales (Barnes, 2016; Prazeres et al, 2017; Schmidt et al, 2016b, 2018), and S. orbiculus-hosting dinoflagellates, Symbiodiniaceae (Merkado et al, 2013; Pawlowski et al, 2001; Pochon et al, 2007)
Summary
Since the beginning of the industrial revolution, anthropogenic activity has led to rapid ocean warming. This negatively affects marine ecosystems and symbiontbearing calcifiers (Kawahata et al, 2019). The Mediterranean can be presented in biogeographic models as a “miniature ocean”, providing indications of global patterns in marine ecosystems in a warmer world (Lejeusne et al, 2010). Warming in the Eastern Mediterranean is expected to rise almost 4 times more rapidly than the global forecast (Macias et al, 2013). The Eastern Mediterranean is expected to be one of the regions most affected by global warming
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