Xingchen Wang, Ph.D.

Education: Princeton University, Ph.D., Geosciences
Institution: California Institute of Technology (laboratories of Alex Sessions, Woodward Fischer and Jess Adkins)

SCOL Project: N Isotopes in Stromatolites: Linking the N Cycle to the Origins of Life

Nitrogen is a key element of life and its availability on the early Earth played a crucial role in the origins and expansion of life. Although nitrogen is abundant in the atmosphere in the form of dinitrogen, only specialized organisms that express a nitrogenase enzyme (‘nitrogen fixers’) can break the triple bonds of dinitrogen and use them as a nitrogen source for growth. In the modern oceans, the entire ecosystem is sustained by the nitrogen fixed by nitrogen fixers and its later recycling. Thus, the appearance of nitrogenase on the early Earth would have been critical for sustaining the earliest biosphere. However, the evolutionary timing and type of the first nitrogenase are widely debated, primarily because we lack reliable proxy data from the earliest rocks. Natural variations in the ratio of the two stable nitrogen isotopes (14-N and 15-N) recorded in fossil stromatolites (one of the earliest fossils) could provide a useful means to addressing these questions, because nitrogenases alter their relative abundance in predictable ways. Thus far, however, analytical techniques have targeted nitrogen isolated from bulk rocks and raise concerns about how well this measurement represents original organic matter. This proposal aims to apply a novel technique that I developed during my Ph.D. to measure nitrogen isotopes in organic nitrogen protected by carbonate minerals, which should provide a more robust record. My proposed study of nitrogen isotopes in fossil stromatolites should help constrain the evolutionary timing and type of the first nitrogenase.