Geochemical pattern of chemoautotrophic mussels from cold seeps and hydrothermal vents: Implications for fluid and trophic type

Hydrothermal vents and cold seeps occur in tectonically active areas and along continental margins. They harbor diverse chemoautotrophic organisms, providing valuable insights into fluid types and trophic dynamics. Interestingly, certain mussel species can inhabit both ecosystems, deriving energy from the aerobic oxidation of methane (AeOM) or sulfur oxidation. However, the geochemical fingerprinting of methanotrophic mussels in hydrothermal vents areas remains largely unexplored, especially compared with cold seeps. In this study, stable isotopes (δ¹³C, δ¹⁵N, and δ³⁴S) and rare earth elements (REEs) in Gigantidas platifrons collected from hydrothermal vents at Iheya North Knoll in the Okinawa Trough and the cold seep at Site F in the South China Sea were examined. The δ¹³C values demonstrate carbon fixation via AeOM in both environments. The average δ¹³C_tissue values of seep G. platifrons (−75.4‰, n = 28) is similar to δ¹³C_methane. However, the average δ¹³C_tissue of hydrothermal G. platifrons (−25.8‰, n = 13) exhibits enhanced fractionation compared with δ¹³C_methane, potentially because of the abundance of methanotrophs. The δ³⁴S and δ¹⁵N values of mussels indicate two different sulfur and nitrogen sources. In both environments, G. platifrons consistently exhibits positive lanthanum (La) anomalies because it serves as an enzyme co-element in AeOM. Seep mussels have more positive La/La^⁎ ratios compared with hydrothermal mussels, indicating an increased number of methanotrophs on their gills. Positive europium (Eu) anomalies exist in hydrothermal mussels and are influenced by ascending hydrothermal fluids. Four REE pattern fingerprints are summarized, highlighting the potential of stable isotopes and REEs as powerful analytical proxies for studying the complex interactions between fluid sources, microbial symbiosis, and trophic structures near hydrothermal vents and cold seeps.