Quantifying the temperature responses of symbiotic nitrogen fixation and its carbon cost

How does temperature affect physiological processes? This fundamental question helps people understand how organisms live in extreme environments, adjust to daily/seasonal cycles, and respond to climate change. For many processes, like photosynthesis, the basics have been known for decades. For one process, though, knowledge has been severely limited by technology. The process, 'nitrogen fixation,' takes nitrogen gas (78% of air) and makes it available to organisms. It is done by certain bacteria, often in symbiosis with certain plants (beans, peas, and some wild herbs, shrubs, and trees), and is the main natural way that nitrogen gets into ecosystems. Given that nitrogen is an essential part of protein and DNA, nitrogen fixation is critical for life. It also plays a major role in climate change. According to models, plants will store more carbon if nitrogen fixation provides enough nitrogen to balance their nutrition. Here's where temperature comes in: models currently assume that nitrogen fixation always works best at 25 degrees Celsius, leading to predictions that tropical forests will fix less nitrogen as the world warms. Recent work in the labs of investigators Menge and Griffin used new technology to show that, in a handful of species, nitrogen fixation works best at 29-37 degrees Celsius and adjusts to recent temperature. The proposed work will use this new technology to test whether these findings hold across a wide range of species and growing conditions. This work will train scholars, diversify ecology, and improve models that are used to set climate policy.

The temperature response of symbiotic N fixation (SNF) is poorly understood. What's the basic shape of the curve? How does it vary across taxa? How ubiquitous is acclimation to growing temperatures? Answers to these basic questions are fundamental to biology. They are also critical for understanding global environmental change, given that different ways of modeling SNF modify C storage by 50 Pg C by the end of the century. The models that include a temperature response of SNF use a function that comes from a small set of measurements, nearly all of which were from asymbiotic bacteria. The function does not include acclimation. This function has been applied at the global scale to both the rate of SNF and, despite zero data, to the carbon cost of SNF. Preliminary data from the principal investigator's labs suggest that the temperature response of SNF is drastically different than the function currently in use. Furthermore, the preliminary data reveal the potential for acclimation to growing temperature. The proposed work will address the questions: What are the temperature responses of SNF and the respiratory carbon cost of SNF? How do they vary across taxa and growing temperatures? How do they compare to the temperature response of photosynthesis? To do so it will use continuous ethylene and carbon dioxide analyzers to measure SNF activity, respiratory carbon dioxide fluxes from nodules, and photosynthesis on many N-fixing symbioses grown at different temperatures in growth chambers. This work will make major progress toward fundamental understanding of a woefully under-studied process.

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.