Figure 5.

Hypothesised mechanism for coral symbiosis destabilisation based on proteomic analysis of Acropora millepora and its associated symbionts under thermal stress. Elevated temperature induces metabolic stress in the host causing protein destabilisation and increase in respiration. Protein degradation leads to the release of free amino acids, which together with lipids, are degraded to support the increased respiratory demand. Due to less energy available, the energetically expensive symbiosome membrane protein V-ATPase declines, reducing inorganic carbon availability to the symbiont, thus reducing photosynthetic activity. This leads to reduced activity in the central metabolic processes in the symbionts (carbohydrate and lipid metabolism). The overall lowered metabolic activity of the symbiont together with increased housekeeping requirement, due to increased temperature, reduces the pool of surplus energy and excess carbon available for transfer to the host, which subsequently experiences further energy limitation. The evidence obtained in this study supports the hypothesis presented in Rädeker et al. (2021), whereby the reduction in carbon transfer is the mechanism underlying the breakdown of the symbiotic relationship in corals. While we did not obtain direct evidence for the process of increased growth in the symbionts from availability of free nitrogen as proposed by Rädecker et al. (2021), the mechanism fits the scheme suggested here and would exacerbate the strength of the response. Ultimately, the lowered energy available in the host leads to degradation of major cellular structural components which could lead to expulsion of the symbionts from the host tissue. Blue indicates decrease, red indicates an increase. Solid lines indicate processes or pools for which evidence was obtained in this study, stippled lines indicate processes or pools that are hypothesised to change or have been shown to change by Rädecker et al. (2021). Arrows indicate positive effect and blocked lines negative effect.