Bacterial Dynamics in Newly Settled Acropora kenti: Insights From Inoculations With Individual Probiotic Candidates

Bacterial probiotics hold promise for enhancing coral health and stress resilience; however, their application to coral juveniles, which are critical for restorative aquaculture, remains underexplored. Here, we show that the microbiome of newly settled Acropora kenti is amenable to manipulation, with individual probiotic candidates inducing distinct microbiome responses that differed in their stability after the final inoculation. Three of the eight tested strains, Halomonas smyrnensis, Endozoicomonas acroporae and Roseivivax lentus, remained prevalent in the microbiome at least 5 days post-inoculation. Notably, aggregates of Endozoicomonas cells were observed exclusively in the corresponding treatment, marking the first report of coral-associated microbial aggregates (CAMAs) in early life stage acroporids and the first induced through bacterial manipulation. In the Pseudoalteromonas rubra treatment, the inoculum was detected at low relative abundance but induced a microbiome shift that persisted beyond the final inoculation. While no significant coral health benefits were observed in this short 17-day experiment, the Ruegeria arenilitoris strain unexpectedly induced tissue regression and mortality. The described experimental system offers a systematic approach for tracking bacterial dynamics in newly settled corals, facilitating the identification of potentially beneficial strains and the exclusion of harmful ones to inform the design of probiotic consortia for coral aquaculture.