Resident and Transient Microbial Taxa in the Gill Microbiome of the Mussel Mytilus galloprovincialis

Microorganisms associate with marine bivalves, forming microbial communities collectively known as the bivalve’s microbiome. These communities may include resident taxa, associated with the bivalve host throughout its life, and transient taxa, whose presence is temporary and attributed to the passage of, for example, seawater or food through the host. To assess the gill microbiome of the Mediterranean mussel, Mytilus galloprovincialis Lamarck, 1819, and its resident and transient taxa, individuals of this important aquaculture species were sampled at two locations on the eastern coast of the Adriatic Sea, Lim Bay and Mali Ston Bay, over one year. The microbiome was characterised using Illumina MiSeq sequencing of the V4 region of the 16S rRNA gene. For comparison, the microbial community in the surrounding seawater and sediment was also assessed at the same time and locations. The structure of the gill microbiome, determined at the OTU level, was distinct from the microbial community in both the surrounding seawater and sediment and did not differ between the two locations. However, it showed differences between months, indicating temporal dynamics. Taxonomic analysis revealed the presence of both resident and transient microbial taxa in the gill microbiome. Members without known relatives within Endozoicomonadaceae (i.e. unclassified Endozoicomonadaceae ) were present in the gills of mussels from both locations and throughout the study period and appeared to be resident members. In contrast, Endozoicomonas and “ Candidatus Endoecteinascidia” were transient, appearing in the gill microbiome of mussels from both locations only in certain months, as were Vibrio and members without known relatives within Vibrionaceae (i.e. unclassified Vibrionaceae ), with the distinction that these were specific to Lim Bay. Overall, the results of this study show that the gill microbiome of the mussel Mytilus galloprovincialis differs from the microbial community in the surrounding seawater and sediment, exhibits temporal dynamics, and contains both resident and transient microbial taxa.