Symbiont diversity within Loripes orbiculatus and the case for multiple hosts.

Seagrasses support immense biodiversity and are critical for maintaining coastal ecosystem health. These foundation species benefit from a "three-way" facultative relationship with one of the common inhabitants of seagrass meadows, lucinid bivalves, which host specific bacterial Candidatus Thiodiazotropha symbionts. Relatives of the bivalve symbionts have been detected on seagrass roots, raising the possibility that these symbionts may colonize both animals and plants; however, no study has yet compared bivalve- and seagrass-associated symbionts at the same site and time. Our combination of 16S ribosomal RNA (rRNA) gene amplicon and metagenome sequencing revealed a greater diversity than was previously observed within both lucinid bivalves and on seagrass roots from the Adriatic Sea and resulted in the closed genome of one prominent symbiont species. We show that two of the Ca. Thiodiazotropha ASVs found on seagrass roots are identical to those found in bivalve hosts at the same site. This suggests that symbiont sharing may occur in the seagrass habitat between these two host species, which has important evolutionary and ecological implications for both hosts and symbionts.