The Gill Microbiota of Scallop Is Dominated by Symbiotic Campylobacterota and Upwelling Intensification Differentially Affects Their Abundance.

Despite the great importance of gills for bivalve mollusks (respiration, feeding, immunity), the microbiota associated with this tissue has barely been characterized in scallops. The scallop is an important economic resource that is cultivated in areas where coastal upwelling is intensifying by climate change, potentially affecting host-microbiota interactions. Thus, we first characterized the bacterial community present in gills from cultivated scallops (by 16S rRNA gene amplicon sequencing) and assessed their stability and functional potential in animals under farm and laboratory conditions. Results showed that under both conditions the gill bacterial community is dominated by the phylum Campylobacterota (57%), which displays a chemoautotrophic potential that could contribute to scallop nutrition. Within this phylum, two phylotypes, namely symbionts A and B, were the most abundant; being, respectively, taxonomically affiliated to symbionts with nutritional functions in mussel gills, and to uncultured bacteria present in coral mucus. Additionally, in situ hybridization and scanning electron microscopy analyses allowed us to detect these symbionts in the gills of . Given that shifts in upwelling phenology can cause disturbances to ecosystems, affecting bacteria that provide beneficial functions to the host, we further assessed the changes in the abundance of the two symbionts (via qPCR) in response to a simulated upwelling intensification. The exposure to combined decreasing values in the temperature, pH, and oxygen levels (upwelling conditions) favored the dominance of symbiont B over symbiont A; suggesting that symbiont abundances are modulated by these environmental changes. Overall, results showed that changes in the main Campylobacterota phylotypes in response to upwelling intensification could affect its symbiotic function in under future climate change scenarios. These results provide the first insight into understanding how scallop gill-microbial systems adapt and respond to climate change stressors, which could be critical for managing health, nutrition, and scallop aquaculture productivity.