Rare occurrences of free-living bacteria belonging to <i>Sedimenticola</i> from subtidal seagrass beds associated with the lucinid clam, <i>Stewartia floridana</i>

Lucinid clams and their sulfur-oxidizing endosymbionts comprise two compartments of a three-stage, biogeochemical relationship among the clams, seagrasses, and microbial communities in marine sediments. A population of the lucinid clam, Stewartia floridana, was sampled from a subtidal seagrass bed at Bokeelia Island Seaport in Florida to test the hypotheses: (1) S. floridana, like other lucinids, are more abundant in seagrass beds than bare sediments; (2) S. floridana gill microbiomes are dominated by one bacterial operational taxonomic unit (OTU) at a sequence similarity threshold level of 97% (a common cutoff for species level taxonomy) from 16S rRNA genes; and (3) the dominant OTU retrieved from S. floridana gill tissues represents less than 1% of all sediment and pore water OTUs from the S. floridana habitat. Population densities for S. floridana at Bokeelia ranged from 0 to 2354 individuals per cubic meter and were significantly higher with high seagrass coverage compared to bare sediments. Sediment and pore water microbial communities were dominated by Delta- and Gammaproteobacteria. Over 97% of 16S rRNA gene sequences recovered from five S. floridana gill specimens, as well as gills of two other lucinid clams recovered from Bokeelia, Ctena orbiculata and Lucinisca nassula, were closely related to the previously described gammaproteobacterium, Sedimenticola, and one Sedimenticola OTU dominated the tissue communities. OTUs affiliated with Sedimenticola were also shared by sediment, pore water, and all host tissues, but represented < 0.5% of all the OTUs from free-living bacterial communities. The results from this study provide tentative identification of the endosymbiont of several lucinid clams from one habitat, and characterize the abundance of putative endosymbiont OTUs in the free-living environment, which has not been done previously for S. floridana.