Abstract:
Nitrification, the oxidation of ammonium (NH4+) to nitrite (NO2-) and nitrate (NO3-), produces the substrates for denitrification, thus fueling the anaerobic nitrogen loss processes in oxygen minimum zones (OMZs). Incubations with 15NH4+ and 15NO2- were performed to measure NH4+ and NO2- oxidation on cruises in the eastern tropical North (March/April 2012) and South (July 2013) Pacific (ETNP and ETSP, respectively) OMZs. We investigated the depth distribution of both processes, as well as their sensitivities to substrate concentration and light. 15NO2- and 15NO3- production were determined using isotopic ratio mass spectrometry, with the azide and the denitrifier methods, respectively. Both archaeal and β-proteobacterial amoA genes were quantified using qPCR. Subsurface maxima of both rates and amoA gene abundances were consistently found in the oxycline above the anoxic layer in both OMZs. The observed tight correlation between NH4+ oxidation and nitrous oxide concentration in the oxycline suggests that NH4+ oxidation was an important source of nitrous oxide. At anoxic depths, substantial number of amoA genes were detected, but NH4+ oxidation were undetectable or negligible, whereas NO2- oxidation rates were sometimes high. At an offshore station in the ETSP, NH4+ oxidation displayed an extremely high affinity for NH4+, with a half-saturation concentration of 27 nM. At 10% surface irradiance, NH4+ oxidation rates were detectable but lower than in the dark. No significant effect of light on NO2- oxidation was found. These results indicate a highly dynamic internal nitrogen cycling in OMZs, while the high rates of NO2- oxidation at anoxic depths remain a conundrum.
Xuefeng “Nick” Peng’s Brief CV:
EDUCATION
Sept. 2015 Ph.D. Department of Geosciences, Princeton University, Princeton, NJ
Dissertation: Nitrogen cycling in strong redox gradients of marine environments: oceanic oxygen minimum zones and salt marsh sediments
Advisor: Bess Ward
2014 Microbial Diversity Course, Marine Biological Laboratory, Woods Hole, MA
Project: Investigating microbial communities in salt marsh sediments using CARD-FISH
May 2010 B.A., Economics, Environmental Studies double major (Summa Cum Laude), Connecticut College, New London, CT
Honors Thesis: The effect of long-term fertilization on the population dynamics of ammonia-oxidizing bacteria and archaea in Great Sippewissett Marsh
Advisor: Anne Bernhard
Fall 2008 Semester in Environmental Science, Marine Biological Laboratory, Woods Hole, MA
Project: Nitrogen cycling in sediments in two stretches of the Coonamessett River
Advisor: Anne Giblin
2003-2006 High School, Shenzhen Middle School, Shenzhen, China
Focus: Mathematics, Physics
RESEARCH INTERESTS
Biogeochemistry of nitrogen in marine environments, particularly in oxygen minimum zones and salt marsh sediments, microbial and molecular methods to study the link between nitrogen cycling processes and the microorganisms responsible for them.
PUBLICATIONS
l Peng, X., Yando, E., Hildebrand, E., Dwyer, C., Kearney, A., Waciega, A., Valiela, I., and Bernhard, A.E. (2013). Differential responses of ammonia-oxidizing archaea and bacteria to long-term fertilization in a New England salt marsh. Frontiers in Microbiology 3: 445.
l Peng, X., Jayakumar, A., and Ward, B.B. (2013). Community composition of ammonia-oxidizing archaea from surface and anoxic depths of oceanic oxygen minimum zones. Frontiers in Microbiology 4: 177.
l Jayakumar, A., Peng X., and Ward, B.B. (2013). Community composition of bacteria involved in fixed nitrogen loss in the water column of two major oxygen minimum zones in the ocean. Aquatic Microbial Ecology 70: 245-259.
l Ji, Q., Babbin, A.R., Peng, X., Bowen, J.L., and Ward, B.B. Nitrogen substrate-dependent nitrous oxide cycling in salt marsh sediments. Journal of Marine Research 73: 71-92.
l Peng, X, Fuchsman, C.A., Martens-Habbena, W., Jayakumar, A., and Ward, B.B. Ammonium and nitrite oxidation in the Eastern Tropical North Pacific. Global Biogeochemical Cycles, in revision.