A glimpse into the unseen: Homogeneity of marine sediment bacterial communities surrounding healthy Porites spp. corals at One Tree Island Reef.

Given the increased frequency and severity of coral mass bleaching events, understanding how each coral reef component contributes to the stability and function of the coral reef ecosystem is crucial for reef health assessments. Reefal sediments and their associated bacteria are essential for reef functionality and ecological stability but are often overlooked in reef assessments. To understand drivers of bacterial composition in sediments adjacent to undisturbed Porites spp. corals, the trophic stage of 20 colonies was assessed during the austral spring of 2023 using incubation experiments at One Tree Island Reef. Sediments were sampled, and bacterial composition was assessed extracting the DNA and amplifying and sequencing the V3 – V4 region of the 16S ribosomal RNA gene. The findings show a homogeneous sediment bacterial composition and a habitat-specific community structure, driven by sediment nitrogen levels and granulometry. Coral surface area and trophic stage did not impact bacterial structure significantly, but the high presence of organic matter decomposing bacteria in sediments suggests high bioactivity surrounding corals. Therefore, shifts in the trophic stage of Porites spp. could alter sediment bacterial community composition and structure under stress events. This study serves as a baseline to identify possible shifts in the composition of sediment bacterial communities linked to a change in the health of Porites spp. corals due to environmental perturbations. Moreover, future studies should assess the active bacterial community to gain a deeper understanding of the contribution of these microorganisms to the ecological stability and function of coral reef ecosystems.