Biogeochemistry, microbiology and analysis of sulfur compounds
The sulfur cycle in marine sediments exerts a major control on the redox state of the ocean and atmosphere. The overall driver in the sulfur cycle is the microbial mediated reduction of sulfate to sulfide. Using the highly sensitive radio-label technique to make direct measurement of sulfate reduction rates (SRR), we know that more than 90% of the sulfate turnover (not to be confused with total sulfate flux) can be typically be attributed to oxidation of compounds other than methane , and that only a small fraction of the sulfate reduced becomes buried as pyrite. Most of the sulfide produced in surface marine sediments via microbial sulfate reduction is eventually oxidized back to sulfate via sulfur compounds of intermediate oxidation state in a complex web of competing chemical and biological reactions. Improved handling, derivatization, and chromatographic techniques allow us to more closely examine the occurence and fate of sulfur intermediates such as elemental suflur, polysulfides, thiosulfate, tetrathionate, and sulfite.
In a paper by Milucka et al. (Nature, 2012) we provide insights into sulfate reduction and associated microbially-mediated zerovalent sulfur transformations in sediments associated with anaerobic oxidation of methane coupled to sulfate reduction (AOM) and AOM is thought to be mediated by a consortium of methanotrophic archaea (ANME) and sulfate-reducing Deltaproteobacteria. We showed that zerovalent sulfur compounds (S0) are formed during AOM-coupled SR and conclude that the S0 is a product of a novel pathway for sulfate reduction performed by the ANME (as indicated in the dotted red line in the figure above). We could further show that the produced S0 in the form of hydrodisulfide can serve as a substrate for disproportionation by the Deltaproteobacteria associated with the ANME. These observations have significant implications for role of sulfur intermediates in our understanding of the biogeochemical carbon and sulfur cycle in modern and past environments.
Coupling of phosphorus, iron and sulfur cycles
While the broad outlines of phosphorus cycling in the ocean are known, a mechanistic understanding of P cycling, especially at or near the sediment-water interface, is lacking. This has far-reaching implications, in part because fundamental conclusions about nutrient cycling and nitrogen fixation are based on the N:P ratios and Fe:P ratios. Slight changes in the C:P ratios of particulate material delivered to the sediment, or the C:P ration of particles exported to the euphotic zone may strongly propagate through the microbial food webs (Deutsch et al., 2012). Much of what is known about the P associated with particles comes from NMR studies of particulate and dissolved organic matter and from basic operationally defined chemical extraction schemes.
A tool for examining P biogeochemistry is these systems is the use of 33P radiotracer on both surface sediments (see Goldhammer et al., Nature Geoscience, 2010) and in the subeuphotic water column. In a recent article published in Geophysical Research Letters, (Sokoll et al., GRL 2017; doi:10.1002/2016GL072183) we describe experiments using radiolabeled phosphate that show the rapid uptake of phosphate into particles in sub-euphotic waters off the coasts of Mauritania and Namibia. The experiments and analysis demonstrate that this uptake is biologically mediated, and thus P cycling in the dark ocean is more intense than originally thought. In other words, P associated with sinking organic matter in the ocean is not simply degraded and released into the water column, but cycles several times through a microbial loop. Such intensified P cycling in sinking particles may have implications for the composition of phosphorus bearing organic matter reaching the seafloor, and for global ocean biogeochemical models.
Biogeochemistry of the ultra-oligotrophic South Pacific Gyre
Due to its extreme remoteness from any continents, the surface waters of the South Pacific Subtropical Gyre (SPG) are the most oligotrophic in the global ocean, with the clearest waters and lowest sea surface chlorophyll a concentrations. Recent studies indicate that microbial nutrient and carbon cycling is especially adapted for these ultraoligotrophic waters, and SPG may be a significant region of nitrogen fixation. As part of the UltraPac SO-245 Expedition onboard the RV Sonne, we conducted a cross-gyre transect to investigate the controls on nitrogen, phosphorus and organic carbon cycling, trace element isotope geochemistry, and microbioal ecology in the water column and surface sedments at 8 main stations, and 7 intermediate stations. The transect proceeded east to west over three sections: a) 25°30' S along the northern side of the gyre from 84°33'W to 110°00'W, b) southwest to 39°S 140°W though the heart of the gyre, and c) further westward along 39°S to 170°W where we linked up with GeoTraces station GR11.