Experimental Inoculation of Coral Recruits With Marine Bacteria Indicates Scope for Microbiome Manipulation in Acropora tenuis and Platygyra daedalea

Abstract

Coral-associated microorganisms are essential for maintaining the health of the coral holobiont by participating in nutrient cycling and protecting the coral host from pathogens. Under stressful conditions, disruption of the coral prokaryotic microbiome is linked to increased susceptibility to diseases and mortality. Inoculation of corals with beneficial microbes could confer enhanced stress tolerance to the host and may be a powerful tool to help corals thrive under challenging environmental conditions. Here, we explored the feasibility of coral early life stage microbiome manipulation by repeatedly inoculating coral recruits with a bacterial cocktail generated in the laboratory. Co-culturing the two species Acropora tenuis and Platygyra daedalea allowed us to simultaneously investigate the effect of host factors on the coral microbiome. Inoculation cocktails were regularly prepared from freshly grown pure bacterial cultures, which were hence assumed viable, and characterized via the optical density measurement of each individual strain put in suspension. Coral early recruits were inoculated seven times over 3 weeks and sampled once 36 h following the last inoculation event. At this time point, the cumulative inoculations with the bacterial cocktails had a strong effect on the bacterial community composition in recruits of both coral species. While the location of bacterial cells within the coral hosts was not assessed, metabarcoding using the 16S rRNA gene revealed that two and six of the seven bacterial strains administered through the cocktails were significantly enriched in inoculated recruits of A. tenuis and P. daedalea , respectively, compared to control recruits. Despite being reared in the same environment, A. tenuis and P. daedalea established significantly different bacterial communities, both in terms of taxonomic composition and diversity measurements. These findings indicate that coral host factors as well as the environmental bacterial pool play a role in shaping coral-associated bacterial community composition. Host factors may include microbe transmission mode (horizontal versus maternal) and host specificity. While the long-term stability of taxa included in the bacterial inocula as members of the host-associated microbiome remains to be evaluated, our results provide support for the feasibility of coral microbiome manipulation, at least in a laboratory setting.

Deciphering Coral Disease Dynamics: Integrating Host, Microbiome, and the Changing Environment

“…Evidence for genetic-based heritability exists in the coral host (Meyer et al, 2009) and their symbiotic algae (Csaszar et al, 2010) in response to thermal stress, however, little is known about other environmental conditions of heritability (van Oppen et al, 2015). Damjanovic et al (Damjanovic et al, 2019) showed that inoculation of Acropora tenuis and Platygyra daedalae recruits with cocktails of bacterial cultures influenced the coral microbiome, demonstrating that the host bacterial community may be manipulated for the purposes of enhancing coral resilience. Other probiotic studies have shown that bacterial addition to corals prior to stress events may mitigate some of the negative effects; what the mechanisms behind these effect are remains unknown (Peixoto et al, 2017).…”

Development of a free radical scavenging probiotic to mitigate coral bleaching

“…Inoculated bacteria can be acquired by developing coral larvae [25] and probiotic mixes applied to cnidarians have been shown to be effective in inhibiting disease progression [40] and increasing resistance to the negative effects of oil [41] and heat exposure [42]. The E. diaphana probiotics generated in this study were assembled from 12 isolated bacterial strains selected based on their FRS ability since free radical production, specifically ROS, is relevant in coral bleaching.…”

“…Probiotics are preparations of viable microorganisms that are introduced to alter a microbial community in a way that is beneficial to the host. Microbiome engineering through the addition of probiotics has been postulated as a key strategy to manipulate host phenotypes and ecosystem functioning for coral reefs [23][24][25][26][27][28]. The differences in the bacterial species composition of healthy and thermally stressed corals [29][30][31][32][33][34] and the coral model Exaiptasia diaphana [35][36][37] suggest a role for microbiome engineering in cnidarian health.…”

Probiotics for corals

“…The feasibility of coral early life stage microbiome manipulation (probiotics) was investigated by repeatedly inoculating coral recruits (Acropora tenuis and Platy. daedalea) with a mixture of seven marine bacterial isolates, which had no specific targeted phenotypes19 . taxa as members of the host-associated microbiome remains to be evaluated20 , the findings provided support for the feasibility of coral microbiome manipulation, at least in a laboratory setting.…”