Trace and Rare Earth Element Fingerprints of Aerobic Oxidation of Methane in Seep-Dwelling Bivalves

Aerobic methanotrophic bacteria are pivotal in the global carbon cycle by converting methane into biomass and inorganic carbon species. Light rare earth elements (light-REE; La, Ce) are part of the metalloenzymes mediating the biochemical processes in methanotrophs. However, the partitioning of trace metals and REE in chemosymbiotic megafauna with methanotrophic endosymbionts remains largely unknown. Here we determine stable isotope compositions (delta 13C, delta 15N) of soft tissues (gill, mantle, foot) as well as trace metal and REE contents of soft tissues and shells of chemosymbiotic bivalves dwelling at methane seeps (Site F and Haima seeps) of the South China Sea. Isotopic compositions of soft tissues are found to reflect the mode of chemosymbiosis (i.e., methanotrophy vs. thiotrophy). Particularly gill tissues of bivalves with methanotrophic endosymbionts display marked light-REE and trace metal enrichments, which is interpreted to reflect the enzymatic activity of endosymbionts. Additionally, correlations between light-REE and copper (Cu), zinc (Zn), and molybdenum (Mo) contents in soft tissues of chemosymbiotic mussels are ascribed to the uptake of these elements by the methanotrophic symbionts. An observed higher trace metal content in the tissue of the semi-infaunal and infaunal bivalves with thiotrophic endosymbionts is believed to reflect the uptake of metals associated with sulfide particles. This study documents diagnostic enrichment of trace metals and light-REE in the soft tissues of bivalves with chemotrophic endosymbionts and provides new constraints for future identification of chemosymbiosis at ancient seeps. Hydrocarbon seeps, sites on the seafloor where methane-rich fluids leak into seawater, are an important source of methane. Yet, the transfer of this greenhouse gas to the ocean is reduced by microbial activity, particularly aerobic and anaerobic oxidation of methane. Some trace metals and rare earth elements (REE) are key to such methanotrophic activity, representing cofactors of metalloenzymes involved in methane oxidation. Although the actual enzymatic activity is well understood, the modes of biological uptake and the partitioning of these elements in seep ecosystems remain largely unconstrained. Here, chemosymbiotic bivalves, harboring chemotrophic bacteria in their soft tissues, were collected from two active seep provinces in the South China Sea. Comprehensive geochemical analyses have been conducted to characterize nutritional sources and trace element enrichment. The observed element patterns are best explained by simultaneous uptake of the trace elements by methanotrophic endosymbionts. Higher sulfide levels in the environment of bivalves with thiotrophic endosymbionts are held responsible for the observed greater trace metal enrichment in the tissues of bivalves harboring sulfide oxidizers. This study deepens our understanding of the behavior of trace metals and REE in methane-seep ecosystems. It will also help to better understand the adaptation of metazoans to ancient chemosynthesis-based environments. Stable isotopic composition of bivalve tissue reflects nutritional sources Correlations between light-rare earth elements and trace metal enrichment in mussel tissues reflect in vivo uptake by endosymbiotic methanotrophic bacteria Elemental distributions have the potential to identify chemosymbiosis in fossil bivalves