Globally, coral reefs are under pressure from climate change, with cumulative impacts causing concerning declines in coral abundance. Active intervention measures are increasingly considered to buy time for coral reefs as the world transitions to a low-carbon economy, including aquaculture-production of coral stock that better tolerate environmental stress. Application of probiotics during aquaculture production and deployment of coral stock has potential to modulate the naturally occurring bacteria associated with corals, and may confer health benefits.
As part of the Reef Restoration and Adaptation Program, the Life Space probiotics project has isolated more than 850 bacterial strains representing over 50 bacterial genera from a range of coral species and life stages using a variety of culturing strategies to capture a high bacterial diversity. In vitro screening of strains for traits expected to benefit coral health is ongoing, including antibacterial activity, biofilm formation on coral mucus, and production of digestive enzymes. In vivo screening experiments were performed with recruits of Acropora tenuis and Platygyra daedalea, and samples were collected to assess the effect of individual probiotic candidates on recruit growth, symbiont density, immune response, and microbiome composition. Image analyses provided evidence of differential colouration and symbiont densities between treatments. Using phenoloxidase assays as a proxy for elicited immune response, differences in immune response stimulation in the coral host were also observed. The microbiome analyses demonstrated increased abundances and retention over time for several probiotic candidates. These experiments provide further evidence that juvenile coral microbiomes can be manipulated and have various health outcomes including early variation in algal symbiont densities and stimulation of early immune response. While the spatial organization of bacteria inside the host remains to be investigated, these results provide support for the development and implementation of probiotics in aquaculture production of corals. The continued use of effective probiotics is a scalable cost-effective tool that could greatly aid in the mass production of coral in aquaculture systems with potential for field-based applications.