Coral symbioses under prolonged environmental change: living near tolerance range limits

Abstract

As climate change progresses, understanding the long-term response of corals and their endosymbionts (Symbiodinium) to prolonged environmental change is of immediate importance. Here, a total of 1152 fragments from 72 colonies of three common coral species (Stylophora pistillata, Pocillopora damicornis, Seriatopora hystrix) underwent a 32-month reciprocal depth transplantation. Genetic analysis showed that while S. hystrix maintained its generalist symbiont, some S. pistillata and P. damicornis underwent temporary changes in resident symbionts immediately after stress (transplantation; natural bleaching). These temporary changes were phylogenetically constrained to ‘host-compatible’ symbionts only and reversion to original symbionts occurred within 7 to 12 months, indicating long-term fidelity and stability of adult symbioses. Measurements of symbiont photo-physiology (dark adapted yield, pressure over photosystem II) and coral health (host protein, bleaching status, mortality) indicated a broad acclimatory capacity. However, this came at an apparent energetic expense as disproportionate mortality amongst symbioses that persisted outside their distribution range was observed following a natural bleaching event. As environmental changes due to climate change become more continuous in nature, sub-lethal effects linked to the existence near tolerance range limits coupled with the inability of adult coral colonies to change resident symbionts makes corals particularly susceptible to additional environmental fluctuations or stress events and reduces the resilience of coral populations.

In vivo Microscale Measurements of Light and Photosynthesis during Coral Bleaching: Evidence for the Optical Feedback Loop?

“…It is also possible that the observed dynamics reflect photoacclimation of Symbiodinium , as high light acclimation involves an increase in the functional absorption cross section of PSII that leads to a lowering of F v /F m (Hennige et al, 2008; Suggett et al, 2009). Additionally, it is possible that bleaching affected the symbiont community, potentially favoring more efficient photosynthesisers under high temperature (Roth, 2014; Sampayo et al, 2016). …”

Rapid adaptive responses to climate change in corals

“…Thus, although evidence is accumulating that some hostassociated microbes might facilitate adaptive responses in corals, the fitness consequences of climate-change-induced evolution of the coral microbiome are unclear. There is also uncertainty around the extent to which increased stress tolerance might involve physiological trade-offs that compromise host health and fitness 104 , and whether selection occurs at the level of individuals or the holobiont.…”

Environmental latitudinal gradients and host‐specificity shape Symbiodiniaceae distribution in Red Sea Porites corals

“…Shallow water tropical and subtropical corals rely on their association with microscopic endosymbiotic dinoflagellates of the family Symbiodiniaceae. Providing the corals with up to 95% of their nutritional needs (Falkowski, Dubinsky, Muscatine, & Porter, 1984), these photosynthetic symbionts are crucial for the growth and functioning of coral reefs (Hughes et al, 2017(Hughes et al, , 2018Sampayo et al, 2016).…”