Stable isotope signatures and nutritional sources of some dominant species from the PACManus hydrothermal area and the Desmos caldera.

Deep-sea hydrothermal vents in the western Pacific are increasingly explored for potential mineral extraction. The study of the composition of the food web plays an important guiding role in the ecological protection and restoration of potential mining areas. The general picture of the nutritional sources of species should be established to assess the potential impacts of future mining activities on the biological composition and food sources. To provide basic information, we analyzed the carbon and nitrogen stable isotope ratios of the dominant macrofauna (mussels, commensal scale worms, crustaceans, gastropods, and vestimentiferans) at three different sites in the PACManus hydrothermal area and the Desmos caldera. The δ13C ratio was significantly different between species: mussels and commensal scale worms showed lighter δ13C ratios, whereas crustaceans showed heavier ratios. In terms of δ15N, mussels had the lowest values and the crustaceans had the highest values. By taking into account these stable isotope signatures, we were able to develop inferences of the food sources for vent community organisms. We found that the food web was based on various species of chemoautotrophic bacteria. Mussels appeared to rely primarily on sulfur-based endosymbionts, which use the Calvin-Benson-Bassham (CBB) cycle and RuBisCO form I as the CO2-fixing enzyme. Commensal polychaetes mostly obtained their nutrition from their hosts. Crustacean species were omnivorous, feeding on chemosynthetic bacteria, sedimentary debris, or even animals according to the local environment. In contrast, gastropods relied mainly on symbiotic bacteria with some supplementary consumption of detritus. Vestimentiferans obtained food from symbiotic bacteria using the RuBisCO form II enzyme in the CBB cycle and may have several symbionts using different fixation pathways. Although most macrofauna relied on symbiotic chemoautotrophic bacteria, our study suggested a closer trophic relationship between animals. Therefore, to evaluate the potential impacts of deep sea mining, it is necessary to study the cascade effects on the food web of the whole ecosystem. Before exploiting deep-sea resources, further systematic investigations concerning the protection of deep-sea ecosystems are necessary.