Amino acid δ15N identifies mixed nitrogen source utilization in chemosymbiotic mussels (Bathymodiolus childressi) from deep-sea methane seeps along the NE Atlantic Margin

Natasha L Vokhshoori, University of California Santa Cruz, Ocean Sciences, Santa Cruz, CA, United States, Matthew McCarthy, University of California Santa Cruz, Ocean Sciences Department, Santa Cruz, CA, United States, Hilary G Close, Rosenstiel School of Marine, Atmospheric, and Earth Science, University of Miami, Department of Ocean Sciences, Miami, United States, Amanda W Demopoulos, US Geological Survey, Wetland and Aquatic Research Center, Gainesville, United States and Nancy Prouty, USGS, Santa Cruz, United States
Abstract:
The deep-sea mussel Bathymodiolus childressi primarily uses methane-derived carbon as a source of energy and nutrition by housing methanotrophic endosymbionts in its gills, while retaining its ability to filter-feed. However, holobiont nitrogen sources and uptake mechanisms are poorly understood. B. childressi is capable of taking up both dissolved inorganic nitrogen (e.g. ammonium) and particulate organic nitrogen (e.g. algal and/or detrital) but assessing the degree of mixotrophy and nitrogen source availability in these heterogenous seep environments. Comparing nitrogen sources in actively venting seeps with dense mussel bed populations to areas where seep activity is low and mussel bed density is sparse is difficult with only bulk isotope analysis. Here we employ a more precise isotopic approach to investigate nitrogen sources and assimilation and address if mixotrophy allows this species to adapt to variable venting conditions. We measure the δ15N value of specific amino acids (AA) in phytoplankton, sediment, and mussel tissues from three seeps fields with varying venting characteristics along the U.S. Atlantic Margin and compare results to non-chemosynthetic littoral mussels. We test a suite of δ15NAA trophic and microbial indices to elucidate the degree of bacterial influence versus particle feeding. Despite overlapping bulk isotope values, δ15NAA varied significantly between sites. Based on δ15NAA, seep mussels from Norfolk had on average higher trophic level and lower microbial influence than Baltimore and Chincoteague seep fields. Within Norfolk, densely populated mussel beds had higher bacterial influence than sparsely populated mussel beds. In addition, specific AAs (e.g. Proline and Isoleucine) show promise as sensitive proxies for chemoautotrophic influence and may be a tracer for chemosynthetic based production to the larger food web.
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