New Processes and Microbes in the Marine Nitrogen Cycle
A Department of Environmental Sciences Seminar
Primary production in most of the surface ocean is limited by fixed nitrogen. Diverse groups of microbes transform nitrogen between fixed and gaseous forms. Oxygen is a key variable in determining the fixed nitrogen budget because the loss of fixed nitrogen can only occur under low oxygen conditions, while the retention of nitrogen usually requires oxygen. Marine oxygen minimum zones (OMZs) are unique regions with a full spectrum of oxygen conditions. Nitrite oxidation and N2O consumption occurring across natural oxygen gradients in OMZs were investigated using a combination of chemical, microbiological, and modeling methods. Nitrite oxidation is thought to be restricted to oxic environments. However, we found two novel nitrite-oxidizing bacteria from anoxic seawaters and presented the first direct experimental evidence of anaerobic nitrite oxidation. Including anaerobic nitrite oxidation in a biogeochemical model reduced the estimate of fixed nitrogen loss by up to 62%. N2O consumption, on the other hand, was assumed to occur only under anoxic conditions. We found transcriptionally active 2O-consuming microbes in oxygenated seawater. These microbes initiated N2O consumption rapidly after switching to anoxic conditions, and their consumption rates were much higher than those from anoxic seawaters. These results contribute to the accurate prediction of the marine nitrogen budget in a changing ocean and confirm that much is yet to be learned about the marine nitrogen cycle.