Article: Metabolomic Signatures of Coral Bleaching History by Roach and colleagues (2021)
Background: Coral bleaching: nearly everyone has heard about it, but most people do not understand exactly what it means. Coral reefs and specific algae types have a required mutually-beneficial relationship (i.e., obligate endosymbiosis). The algae provides nutrients to the coral, and the coral provides protection from predators and other nutrients for the algae. Both require each other or else they both would not survive. When ocean temperatures increase, the algae gets released from the corals-- preventing each from receiving their mutual benefits and often causing each die. The algae is responsible for the vibrant colors of corals, so the expulsion of the algae results in a white “bleached” color. Most of the world’s corals are expected to experience coral bleaching by 2050. Increasing our understanding of the metabolic impacts (i.e., how bleaching affects the metabolism of the coral and algae) of this process will allow us to provide more effective restoration techniques in response to climate change. Further, we would be able to predict the consequences of coral bleaching and determine if bleaching had occurred on a particular reef in the past.
Methods: In Kāneʻohe Bay, Hawaiʻi, Roach and colleagues collected 10 coral samples from a reef that partially experienced bleaching. Five samples of historically bleached coral (i.e., experienced bleaching in the past) and 5 samples from non-bleached coral were exposed to either heat-stress or normal conditions in-lab over a 5 day period (i.e., microcosm). Fragments were collected for biochemical analysis of metabolites (i.e., small nutrient particles) before and after treatment. After analyzing the in-lab grown corals, different coral samples were collected from a previously bleached reef to compare in-lab simulation results to naturally preserved coral samples.
Findings: Roach and colleagues found that the entire group of metabolites (i.e., metabolome) in the historically bleached coral was significantly different than that of the historically non-bleached coral. Specifically, betaine lipids were the strongest predictors of coral bleaching history and different variations in their chemical structures were either more abundant in historically bleached corals or historically non-bleached corals. Further, metabolomic characteristics of the coral host, algae, and shared metabolome, respectively, could determine bleaching history. The heat-stress simulations resulted in enhanced differences in metabolite composition compared to the non-heat-stressed coral.
Conclusions: Through these findings, Roach and colleagues have developed an effective novel method for identifying the bleaching history of coral reefs. For coral reefs to survive increased ocean warming, active restoration is needed. This includes propagating corals in labs, transplanting healthy corals to degraded reefs, and assisting evolution by using corals that show traits increasing resiliency. For example, thermally-resistant or resilient corals would be able to grow and restore bleached coral reefs. With an understanding of what metabolites and biochemicals are more abundant in historically bleached compared to historically non-bleached corals, we can identify and select the corals that will be most effective in the “transplantation” restoration method. Although unfortunate that we are left to apply these methods of restoration, it is a step closer to being able to fight the effects of climate change for our future coral reefs.
Figure: Bleached (right) and unbleached (left) coral from sampling site (Kāneʻohe Bay, Hawaiʻi) during a 2015 bleaching event.
Reference:
Roach, T.N.F., Dilworth, J., H., C.M. et al. Metabolomic signatures of coral bleaching history. Nat Ecol Evol (2021).
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