Collaborative research: Non-Local Bacterial Transport of Nitrate within Sediments Underlying Oxygen Deficient Zones: A New Twist in the N Cycle

University Of Southern California, Los Angeles CA

Investigators

Abstract

In this project, researchers at the University of Southern California and Princeton University will investigate the link between non-local bacterial nitrate transport and regional nitrogen losses in oxygen deficient basins off the Southern California coast and regions of the Eastern Subtropical North Pacific. Recent studies suggest remarkably rapid benthic nitrate consumption in the Santa Barbara Basin (Sigman et al., 2003; Hammond et al., 2006). Yet, budgets for N, S and C fluxes in this basin may be only satisfied if a large fraction (up 100 % potentially) of nitrate uptake in SBB sediments does not involve the respiration of organic carbon, but instead is coupled to oxidation of sulfides. The bacteria Thioploca and Beggiatoa, found in this region, are known to accumulate up to 500 mM of nitrate intracellularly. They transport nitrate into the sediments, up to depths of tens of centimeters where nitrate is used for hydrogen sulfide oxidation. This non-local nitrate transport (within laminated sediments) clearly ?breaks? the rules of diagenetic sequence. Preliminary work in SBB and Santa Monica and San Pedro Basins produced geochemical and isotopic evidence which is consistent with bacterial transport and subsequent transformation of nitrate to dinitrogen (N2) at depth in the sediments. The researchers on this project hypothesize that non-local bacterial transport of nitrate may be a significant but previously overlooked sink in the regional and global N budgets of oxygen deficient zones. They will (1) examine the geochemical framework favorable for non-local bacterial transport of nitrate; (2) determine the relative contribution of this process to net regional losses of fixed N; (3) identify and quantify the effects of this phenomenon on the ä15N of nitrate in the overlying water column; and (4) investigate the possible coupling of microbial nitrate transport to other reactions that reduce nitrate, such as Anammox. This project, which includes one long cruise and four brief cruises to San Pedro Basin, will provide a framework for potential involvement of USC and Princeton undergraduate and graduate students in oceanographic research. The tell-tale signs of nitrate-transporting bacteria have been and will continue to be a topic of interest in the ongoing International Geobiology summer course on nearby Catalina Island. Finally, this project will support the scientific development of a young female researcher who spear-head this proposal.