NSFGEO-NERC: Understanding the consequences of changing phytoplankton elemental use efficiencies for global ocean biogeochemistry

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This project is jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (UKRI/NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own ivestigators and component of the work.Earth system computer models that simulate the global cycles of carbon and other elements are one of our most valuable tools to predict environmental change in the future ocean. However, existing modeling methods cannot realistically test how increasing ocean temperatures will affect the needs of marine micro-organisms for the limiting nutrients that usually control their growth, including nitrogen, phosphorus and iron. Because these limiting nutrients govern how much photosynthesis occurs in the ocean, they strongly influence the fate of carbon dioxide produced by human fossil-fuel burning. For this reason, determining how nutrient use by plankton will respond to greenhouse warming of the surface ocean is a priority for scientists and policy makers. This project supports a collaboration between two marine microbiologists in the United States and an Earth system modeler from the United Kingdom that is aiming to better understand how oceanic limiting nutrient cycles and the biological communities they support will react to global warming. The U.S. investigators are using laboratory culture experiments, field work in the coastal ocean, and existing collections of past open ocean chemistry and biology measurements to generate quantitative estimates of rising temperature effects on nutrient elemental use efficiencies (EUEs). An EUE is defined as the rate at which a marine microbe can take up carbon dioxide via photosynthesis, per unit of limiting nutrient in the cell. Preliminary results show that EUEs are highly sensitive to changing temperature, which in turn has far-reaching consequences for how ocean ecosystems function. The U.K. investigator is using these experimentally-determined values to build a new Earth system model that is centered on how warmer conditions will affect EUEs for important groups of marine micro-organisms. This novel approach has the potential to yield much better estimates of the ways that ocean carbon uptake and nutrient cycling will be altered as the ocean continues to warm in the future. Broader Impacts include breaking new scientific ground in oceanography and climate science through improved Earth system models, providing educational opportunities for graduate students, undergraduate researchers, K-12 students, and postdoctoral investigators, and strengthening collaborative ties between the marine science communities of the U.S. and the U.K.Quantitative relationships between phytoplankton nutrient limitation, primary production and globally-rising sea surface temperatures are still not well understood, largely due to a lack of mechanistic experimental data. This is problematic for Earth system modelers who are trying to project future oceanic productivity levels that underpin predicted changes in ocean ecosystems, information that is urgently needed to inform environmental policy makers. This project addresses this knowledge gap in a closely coordinated, collaborative project between two U.S. marine microbiologists and a biogeochemical modeler from the U.K. The international team of interdisciplinary researchers are applying the emergent concept of Elemental Use Efficiencies (EUEs) to integrate flexible thermal and resource limitation responses into a new generation of Earth system models. EUEs are defined as the amount of carbon fixed per unit time, normalized to the amount of a limiting resource in a phytoplankton cell, as represented by the cellular nutrient quota. Preliminary work suggests that EUEs can be very sensitive to future ocean warning, with major downstream consequences for nutrient-limited marine plankton assemblages and thus also for ocean carbon and nutrient biogeochemistry. The investigators are using high-throughput thermal block methodology to measure EUEs for the limiting nutrients nitrogen, phosphorus and iron in a diverse set of four different functional groups of laboratory phytoplankton isolates across their entire thermal ranges. EUEs obtained for these three key limiting nutrient resources as a function of temperature and degree of nutrient stress are being applied in an iterative manner to inform a novel EUE-centric version of the established NEMO-PISCES ocean model. Field EUE data from coastal and open ocean regimes are being collected for comparison to and ground-truthing of both the laboratory culture results, and the modeling output. This project will advance the discipline of ocean global change biology by providing unique new insights into how biogeochemically-critical marine microbial functional groups may respond to simultaneous shifts in temperature and limiting nutrient resource availability both today, and in the future changing ocean.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.