Project: IODP Expedition 336- Mid-Atlantic Ridge Microbiology

This project was funded by a C-DEBI small grant to SW and WZ, by a C-DEBI Postdoctoral Fellowship to CB, and by NSF award OCE-1131671 to WZ.

Project title (SW and WZ) "Autotrophy and heterotrophy supported by microbial nitrogen cycling in sediments underlying the oligotrophic ocean: A stable isotope study of North Pond porewaters"

Description from C-DEBI:
As a key element in fundamental biomolecules, the cycling and availability of nitrogen is a central factor governing the extent of ecosystems across the Earth. In the organic-lean sediment porewaters underlying the oligotrophic ocean, where low levels of microbial activity persist despite exceedingly energy-depleted conditions, the extent and modes of nitrogen transformations have not been widely investigated. We used the N and O isotopes of porewater nitrate from the North Pond site in the oligotrophic North Atlantic (North Pond) to provide constraints on the extent of both nitrate production (via nitrification) and consumption (via denitrification). Nitrate accumulates far above bottom seawater concentrations (~21 uM) throughout the sediment column (up to almost ~50 uM) and persists down to the oceanic basement as deep as 90 mbsf, indicating the predominance of aerobic nitrification and remineralization in these sediments. However, large changes in the δ15N and δ18O of nitrate also reveal variable influence of nitrate respiration across the three boreholes.

Combining the N and O isotopes and using an inverse porewater diffusion-reaction model, we estimate rates of nitrification and denitrification throughout the sediment column. Results indicate a range of rates across the three boreholes and are generally consistent with variations observed in profiles of dissolved oxygen at this site. The model also estimates values of the N isotopic composition of newly produced nitrate, which were generally lower than measurements of sinking particulate nitrogen in this region. We suggest that this must be the result of sedimentary-hosted nitrogen fixation supplementing the relatively small organic matter pool derived from the overlying euphotic zone. These findings indicate that the production of organic matter by in situ autotrophy (by both nitrification and nitrogen fixation) must supply a large fraction of the biomass and organic substrate for heterotrophy in these sediments. This work sheds new light on an active nitrogen cycle operating, despite exceedingly low carbon inputs, in the deep sedimentary biosphere.

Project title (CB) "Determining the rates of denitrification, nitrification, and nitrogen fixation using natural abundance isotope profiles in North Pond sediments"

Description from C-DEBI:
The sedimentary contribution to the global ocean fixed nitrogen budget is currently under debate. Early studies using the isotopic balance of 15N of nitrate, predicted a large nitrogen loss through sedimentary denitrification and yielding an enormous imbalance in the budget, with nitrogen loss far outweighing sources of nitrogen to the ocean. The major roadblock preventing our ability to estimate the rates of nitrogen metabolisms within the deep biosphere is the difficulty sampling and measuring rates while keeping in situ conditions. Even with the recent technological advances in collecting deep sediments, there is still perturbation of the microbial community when conducting bottle incubations and adding artificial isotope tracers. To overcome these issues, I plan to use natural abundance stable isotope profiles of multiple nitrogen species to more quantitatively constrain rates of microbial nitrogen metabolism. Stable isotope profiles have the potential to tease apart multiple processes that the concentration profiles alone cannot. In order to do this, however, we need a more in-depth understanding of the isotope systematics of each process under conditions that more closely approximate the low-energy systems manifested in sediments underlying the oligotrophic ocean. Here I plan to conduct multiple incubation experiments designed to measure the isotope systematics during a) coupled denitrification and nitrification and b) benthic nitrogen fixation, under conditions relevant to the deep biosphere.