Primary productivity is defined as the rate at which photosynthetic organisms produce organic compounds in an ecosystem. Primary production of organic compounds by photosynthetic algae and cyanobacteria in the ocean provide the fundamental building blocks of marine ecosystems, from zooplankton all the way up to fish and mammals. Primary productivity in the ocean also photosynthetically produces more than half of the oxygen in the atmosphere, making it an important source of the oxygen life needs to survive. Understanding how primary productivity is distributed spatially and temporally in the ocean is important when characterizing global biogeochemical cycles and climate. Marine primary productivity is also a major sink for fossil fuel carbon emissions and is dramatically affected by other human activities, so better constraining the magnitude and distribution of primary productivity in the ocean today is important to when trying to understand how human actions will affect health of marine ecosystems in the future.
Marine primary productivity can be estimated by determining the productivity of photosynthetic oxygen in the ocean, because oxygen and organic compounds are produced through photosynthesis. The triple oxygen isotope (16O, 17O, and 18O) proxy of marine productivity estimates gross oxygen productivity (GOP), the total rate of photosynthetic O2 production without depletion by respiration, by determining the difference between the triple oxygen isotope composition of atmospheric and biologically produced O2 and the rate of air-sea O2 exchange. Measurements can be made from dissolved oxygen samples throughout the ocean, which can then be interpreted into gross oxygen productivity rates. Measurements of the clumped oxygen isotope (17O18O and 18O18O) signature of dissolved oxygen can be used in a mathematically similar way, by comparing the clumped oxygen isotope composition of atmospheric and biologically produced O2, to estimate gross oxygen productivity. Both of these proxies need high-quality experimental measurements of the fractionation of the rare isotopologues of oxygen by marine respiration in order to be applied effectively.
In my current research project, I will incubate a set of important marine microbes, and measure the values of the five isotopologues of oxygen as these experiments progress. This data will be used to constrain the fractionation of the rare isotopologues of oxygen through respiration and/or photosynthesis, which can be used to improve applications of the triple oxygen isotope and clumped oxygen isotope proxies of gross oxygen productivity in natural systems.