Experimental inoculations show that environmental bacteria and host factors affect bacterial community composition in coral recruits. Coral metagenome

Coral-associated microorganisms are essential for maintaining the health of the 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 microbiome manipulation by repeatedly inoculating coral recruits with a bacterial cocktail generated in the laboratory. By co-culturing the two coral species Acropora tenuis and Platygyra daedalea, we were also able to investigate the effect of coral host species on microbiome community composition.The repeated inoculation of coral recruits with a bacterial cocktail had a strong effect on the bacterial community composition in early recruits of both coral species. Metabarcoding using the 16S rRNA gene revealed that the seven bacterial single nucleotide variants (SNVs) administered through the cocktails were enriched in inoculated coral recruits. Moreover, significant differences between inoculated and control recruits were solely driven by SNVs present in the inocula. 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 indices.Results from our co-culture experiment confirm coral host factors as well as the environmental bacterial pool play a role in shaping 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 assessed, we provide proof-of-concept for the feasibility of coral microbiome manipulation as a first step towards developing probiotics for coral reef restoration.