Dr. Nick Peng will give a talk entitled Nitrogen cycling in strong redox gradients of marine environments: Oceanic oxygen minimum zones and salt marsh sediments at Speakers Club on Thursday, January 7, 2016 at 2:00 PM in Webb Hall 1100.

Abstract

            Nitrogen often limits primary production in the marine environment, so it is important to understand the budget and cycling of nitrogen. Microbial transformations between different forms of nitrogen at various reduction-oxidation states control the biological availability of nitrogen. Strong reduction-oxidation gradients in the environment are hotspots of microbial nitrogen cycling. This talk focuses on the microbial nitrogen cycling in two types of marine environments characterized by sharp oxyclines: oceanic oxygen minimum zones (OMZs) and salt marsh sediments.

            Nitrification, the oxidation of ammonia to nitrite and then nitrate, links the most reduced and the most oxidized forms nitrogen. Ammonia-oxidizing archaea (AOA) are considered as obligate aerobes, but high abundance of their functional gene were observed at anoxic depths of OMZs. Analysis of community composition of AOA genes using a DNA microarray showed that they were not different between anoxic and oxic depths. This suggests that AOA might possess some unknown metabolism that allows them to survive at anoxic conditions.

Nitrification produces substrates for denitrification, the sequential reduction of nitrate to dinitrogen gas, which is highly active in both OMZs and salt marsh sediments. The depth and spatial distribution of ammonia and nitrite oxidation rates were determined using incubations with ¹⁵N labeled substrates in two major OMZs: eastern tropical North and South Pacific. While organic matter flux exerts a first-order control on nitrification rates in OMZs, this study revealed that light, in situ concentrations of oxygen, nitrous oxide, and ammonium are all strongly correlated with ammonia oxidation rates. High rates of nitrite oxidation were measured at anoxic depths, and this remains unexplained.

            Salt marshes provide numerous valuable ecological services, including nitrogen removal before it enters the coastal ocean, thereby reducing eutrophication. The effect of long-term nitrogen fertilization was examined in a New England salt marsh, using slurry incubations with ¹⁵N labeled substrates. Nitrification and denitrification rates increased by more than ten-fold due to fertilization, whereas dissimilatory nitrate reduction to ammonium, which retains nitrogen, were high in unfertilized plots but not detected in fertilized plots. These results suggest that long-term fertilization shifted nitrogen cycling in salt marsh sediments from predominantly retention to removal.