Collaborative Research: Hydrogen isotope analysis of amino acids as a novel tracer of microbial processes in marine particles

The formation and sinking of organic particles into the deep ocean are important processes in global element cycles. These particles (also called ‘particulate organic matter’ or POM) carry carbon into the deep ocean where it is stored for hundreds of years. The storage of carbon in the deep ocean contributes to the drawdown of atmospheric carbon dioxide as well as loss of nutrients from the surface ocean.

Understanding the mechanisms and processes that govern the fluxes of POM is thus directly relevant to the global carbon cycle, marine ecology, and fisheries. This project uses new tools to measure the isotopes of three different elements (hydrogen, carbon, nitrogen). The approach shows promise for addressing a long-standing question: to what extent do bacteria replace the organic matter from algae (the original source of the POM) as particles sink.

Results from this study will be important for predicting the amount and dietary quality of POM in a rapidly changing ocean. The project will support Ph.D. students at both Caltech and University of Miami and educational activities for K-12 and college students.

The project takes advantage of a recent analytical development, the measurement of hydrogen isotopes in amino acids as a new tracer. Preliminary measurements suggest that there is a large (up to 20%) shift in the hydrogen isotope ratio of amino acids within POM over the upper 300 m of the ocean, consistent with the depth at which POM degradation is most intense.

The investigators hypothesize that the shift in hydrogen isotopes reflects the replacement of phytoplankton biomass with bacterial biomass. The project will test this hypothesis through 1) the analysis of archived POM samples from four different localities in the Atlantic and Pacific oceans, 2) measurements of diverse phytoplankton grown in culture, and 3) degradation experiments in which algal biomass is fed to bacteria and zooplankton.

In addition to cutting-edge hydrogen isotope measurements, the investigators will employ a suite of additional measurements to characterize the microbial communities and POM being studied. Collectively, this work aims to develop hydrogen isotopes in amino acids as a novel proxy for the turnover of organic matter within marine particles, which could be applied to studies of marine POM throughout the world’s oceans.

The project will also develop outreach activities to introduce K-12 and collegiate students to marine science. These will be implemented through the GO-Outdoors program in Pasadena, the Exploring Marine Science Day in Miami, and undergraduate student cruises in Miami.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.