The molybdenum isotope signature of microbial nitrogen utilization

Many chemosynthesis-based communities prospering in deep-sea environments rely on the metabolic activity of sulfur-oxidizing bacteria. This is also the case for vestimentiferan siboglinid tubeworms, whose demand for nutrition is entirely satisfied by their endosymbiotic bacteria harbored in the trophosome. Such chemosymbiosis leads to a significantly lower nitrogen isotope composition of the trophosome than in other types of soft tissue. However, the specific process of nitrogen utilization by siboglinids remains unclear. As a key element in the relevant enzymes (nitrogenase, nitrate reductase), molybdenum (Mo) is indispensable in the biogeochemical cycling of nitrogen. The Mo isotope composition (δ⁹⁸Mo) of siboglinids is thus a potential proxy to decode the mode of nitrogen utilization. In this study, we found that δ⁹⁸Mo along the chitinous tube of the vestimentiferan siboglinid Paraescarpia echinospica from the Haima seep of the South China Sea yields values as negative as -4.59‰ (-1.13 ± 1.75‰, n = 19) – the lowest δ⁹⁸Mo signature ever reported for any kind of natural material. It is suggested that this extremely negative Mo isotope composition is caused by preferential utilization of isotopically light Mo by the tubeworm symbionts during nitrate reduction. Such Mo isotope signature could provide a means to identify siboglinid tubeworms in the rock record, a group of annelids that has previously escaped unambiguous identification due to the lack of mineralized skeleton.