The potential of parrotfish faeces in replenishing reefs with coral-associated microbiome

Abstract Sparisoma viride is the most abundant parrotfish in the Caribbean and is considered as the most important excavator due to corallivore behaviour. Parrotfishes are a keystone group that favour the growth and resilience of coral reefs removing macroalgae and structuring the benthic communities. The microbial symbiotic communities are involved in multiple functions related to nutrition and immunity maintaining corals health. Because S. viride scrape coral tissues, the skeleton and the algae on coral, it could be important reservoirs or vectors of microorganism for the corals through the faeces dispersion, however, the role of parrotfishes as reservoirs are poorly studied. Establishing microbial communities present in parrotfish faeces will contribute to understand the ecological impact of parrotfishes in coral resilience. We investigated the composition of disseminated bacteria and the extent to which the cell integrity of dinoflagellate photosymbionts (Symbiodiniaceae) is maintained in the faeces and compare with sediments and water column controls. Then, we analysed diversity and structure of bacterial communities at family level and search similarities between faeces of the study and coral associated microbiome reported in the literature. Similar levels of structural integrity and photosynthetic health of Symbidiodinaceae cells were found in both faeces and reef sediments. Besides, the sediments microbiome echoes the parrotfish faecal microbiome by sharing high diversity and a similar bacterial community composition. Several bacterial families were present in parrotfish faeces and in coral microbiome reported in the literature highlighting the dispersal potential of parrotfishes replenishing coral reefs. Despite the sampling limitations, these findings uncover the potential role of the excavator parrotfish in enriching environmental reservoirs, especially reef sediments, with coral-associated bacteria and photosynthetic microalgae. Parrotfishes could reinforce the coral microbiome and facilitate coral symbiont acquisition, key features critical to maintaining the fitness of one of the most threatened ecosystems on Earth. This finding could be considered as a first step in uncovering a mechanism for reef-microbiome maintenance.