| Summary: | Extensive characterization of a single lucinid bivalve habitat was conducted to characterize the relationship between host bivalve and thiotrophic bacterial endosymbionts. For lucinids, the ecological and evolutionary relationships between hosts and endosymbionts are poorly understood. Reconstructing the evolutionary history of lucinid endosymbiosis, and the geologic significance of the association, has been hampered by insufficient knowledge of endosymbiont ecology and taxonomic diversity. Host organisms (Lucinisca nassula and Phacoides pectinatus) were collected from Cedar Keys, Florida, within the top 15-20 cm of the sediment in sea grass beds. PCR amplification and sequencing of bacterial 16S rRNA genes from lucinid gills and sediment cores retrieved ~900 sequences. Based on comparative phylogenetic methods, gill endosymbiont sequences were most closely related to uncultured Gammaproteobacteria associated with symbiosis, and specifically to lucinid endosymbionts (97-99% sequence similarity) and not to free-living organisms. Not all gill sequences were genetically identical, with intra- and inter-gill sequence diversity. Sediment diversity was high, represented by 13 major taxonomic groups, including equally dominant Chloroflexi, and Delta- and Gammaproteobacteria. Other organisms included the Bacteroidetes, Acidobacteria, Spirochetes, and Firmicutes. Rare (<0.3%) sequences from the sediment were related to lucinid gill endosymbionts. Results support the hypothesis that recruitment of free-living organisms is likely. Based on habitat geochemistry, however, the bacteria are constrained to reducing conditions, and this may be reflected in the habitat types colonized by the host. Habitat-host-symbiont diversity was evaluated from other locations from Florida and The Bahamas. 16S rRNA gene sequences retrieved from those hosts revealed that not all lucinid endosymbionts belong to the Gammaproteobacteria, because some sequences were most closely related to Alphaproteobacteria. One sequence was most closely related to Methylobacterium spp., which may indicate that dual symbiosis (thiotrophy and methanotrophy) in lucinid bivalves may be possible. Together, these results are significant to paleoecological and evolutionary studies using lucinids in the fossil record (e.g. isotope studies).
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