Bacteria to the rescue: helping corals survive warming oceans


Breaking down the microbiology world one bite at a time

Bacteria to the rescue: helping corals survive warming oceans

Coral reefs are often called the rainforest of the sea and for good reason; they host over 25% of all marine life despite covering only 0.2% of the total seafloor. Not only do coral reefs have a remarkable biodiversity, they also provide many ecosystem services to humans such as fisheries, tourism and recreation. 

Corals live in symbiosis with photosynthetic algae (zooxanthellae) living in their tissue. The coral protects the algae which return the favor by producing and giving coral cells the nutrients necessary to survive. The changing climate and rapidly warming sea surface temperatures increase the frequency and intensity of coral bleaching events. When stressed, corals expel their endosymbiotic algae and lose their colors (bleaching) but without the algae, the coral cannot survive and eventually dies. In this context, slowing down climate change is not only essential, but finding ways to mitigate its impact on coral reefs is key. 

Figure1: The steps to coral death. A heatwave stresses otherwise healthy corals, which release their symbiotic algae (zooxanthellae) and become bleached. While corals may recover, if the heatwave persists too long or reoccurs, corals can die. 

In their study, Santoro and colleagues explored the possibility of using beneficial microorganisms for corals (BMCs) to improve heat stress tolerance and increase survival during extreme heat events.

To investigate this, the researchers simulated a heatwave in laboratory aquariums. First, they cultivated bacterial strains directly from the corals’ own microbiome and screened each of these 133 individual strains for beneficial traits for corals. These traits include nitrogen fixation or protection against coral diseases. After screening, six strains were selected to create a consortium of beneficial microbes, called a probiotic. They administered the probiotic to the corals and increased the water temperature to 30°C for 10 days before cooling it back down to 26°C. They monitored coral health, metabolism and changes in the coral microbiome over the course of 75 days. 

While all corals bleached during the simulated heatwave, 100% of the corals treated with the probiotic recovered and survived while 40% of the non-treated corals died. The probiotic helped with the recovery of the heat-stressed corals by inducing changes in the microbiome. Indeed, while the probiotic strains did not colonise the coral microbiome long-term, it still restructured the coral microbiome which positively impacted the metabolism and gene expression. For example, genes involved in apoptosis (cell death) were down-regulated while genes involved in heat protection were up-regulated.

Overall, the researchers suggest that adding probiotics could improve coral survival in times of heat stress. While this is only a laboratory experiment and the effects need to be tested in natural complex ecosystems, the implications for coral protection and rehabilitation could be important.

Link to the original post: E. P. Santoro, R. M. Borges, J. L. Espinoza, M. Freire, C. S. M. A. Messias, H. D. M. Villela, L. M. Pereira, C. L. S. Vilela, J. G. Rosado, P. M. Cardoso, P. M. Rosado, J. M. Assis, G. A. S. Duarte, G. Perna, A. S. Rosado, A. Macrae, C. L. Dupont, K. E. Nelson, M. J. Sweet, C. R. Voolstra, R. S. Peixoto, Coral microbiome manipulation elicits metabolic and genetic restructuring to mitigate heat stress and evade mortality. Sci. Adv. 7, eabg3088 (2021)

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