Many of the mechanisms that mediate prokaryote–host symbioses are unknown or unclear. Both extrinsic and intrinsic factors are at play, with no clear dominant influence. The po- tential for gut microbial communities to effect bivalve digestive enantiostasis and pathogen accumulation is great. Thus, un- derstanding the natural spatial and temporal variation of these communities, the influence of the surrounding seawater and particulate-associated microbes, and the impact that dis- turbances in the microbiome have on bivalve enantiostasis is imperative. Building on this work will allow more direct probing of questions relating to the role of microbial communities in host physiological functioning and enantiostasis. The complex relationships between the envi- ronment, bivalves, and maintenance of their microbial communities have only begun to be probed in the past 30 years. Although this study and many others have contributed important information, an understanding of whether the host, the resident microbial communities, or both produce critical di- gestive enzymes is nascent, and many questions remain. For example, one unknown is the contribution of enzymes from lysed cells, which has been shown to impact digestion even after bacteria are eradicated (Harris 1993). A second unknown is the contribution of bacterial–bacterial or bacterial–animal hori- zontal gene transfer. Several examples within sessile marine invertebrate phyla have emerged to suggest that the in- corporation of bacterial genes into host animal genomes occurs and is related to improving metabolic function (Boto 2014, Degnan 2014). In this case, bacterial genes responsible for en- zyme production and other metabolic processes could be transferred to the eukaryotic host genome. Finally, functional redundancy of enzyme production genes could occur within the microbial community. As such, the antibiotic elimination of certain bacteria may not result in an overall disruption of en- zyme production.
As with most research, results only generate more questions. There are many more topics to examine relating to the bivalve microbiome. For example, do core microbiome trends hold true across bivalve species? Do they hold true with other suspension feeders (e.g., Crepidula spp.)? Are observed similarities between oyster and mussel microbiomes a result of their shared feeding mechanism? Are observed differences between oyster and mussel microbiomes a result of their genetic differences? Do other marine filter feeders harbor the same numbers and types of bacteria as bivalves? Do they share a core? With continued reference to the eastern oyster, do Crassostrea virginica from other locations (i.e., Chesapeake Bay and Gulf of Mexico) harbor the same bacteria as those in Long Island Sound? How far do spatial trends extend until they are broken? Does the environment impart more of an influence in some locations than in others? Do the core microbiota contribute to specific host physiological functions? Does a high diversity in the gut microbiome inhibit pathogen colonization in the long term? Investigating such questions would be beneficial, resulting in an enhanced understanding of bivalve host–bacterial interactions.
Conclusion
Understanding natural variation in the genetic and func- tional diversity of the oyster-associated microbial communities is vital for establishing a baseline to which the effects of extrinsic and intrinsic factors can be compared. The presence of atrazine in the Chesapeake Bay may be selecting for pathogenic bacterial groups to reside within the oysters of the Chesapeake Bay and its tributaries. The effects of this compositional shift remain unclear, therefore, future research efforts should focus on understanding the relationship between biocidal herbicides and oyster-prokaryote interactions. Extended experiments with more time points, as well as repeated challenges, would help to further elucidate the role of oyster-associated microbial communities to the overall physiological functioning of the host. This research provides a vital baseline for future research aimed at understanding the role gut microbes have in oyster physiology. \(\)
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\(\)Acknowledgements
We would like to thank Horn-point laboratory for providing the spat used throughout the experiment. Dr. Tara Scully for acting as the lead investigator for the project. The George Washington University, for housing the project, and providing the funds necessary for it to be carried out. And finally, to each of the authors for their hard work and perseverance.
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