Deep-marine brine seeps in the modern ocean are considered analogues for settings that favoured the formation of sedimentary-exhalative zinc and lead deposits in deep time. Microbial activity plays an important role in the accumulation of ore minerals, meaning that the extent of mineralization is at least indirectly dependent on nutrient fluxes. Here, we investigated the biogeochemical nitrogen cycle in shallow (15-50 cm) sediment cores from the Orca Basin brine pool and surrounding sites, as well as from an active brine seep area near Dead Crab Lake in the Gulf of Mexico, with the aim of constraining the effect of brine seepage on this bio-essential element. We find high porewater ammonium concentrations in the millimolar range, paired with elevated ratios of organic carbon to nitrogen in sediments, which confirm previous hypotheses that the brine recycles ammonium from sedimentary strata back into the water column. Within Orca Basin, we note tentative evidence of microbial ammonium utilization. At the active seep, ammonium is mixed into the overlying water column and likely undergoes oxidation. Isotopic data from sediments and dissolved ammonium, paired with previously published genomic data, suggest the presence of dissimilatory nitrate reduction to ammonium (DNRA) at the brine-seawater interface. We conclude that brine seeps can stimulate biological nitrogen metabolisms in multiple ways. Our results may help calibrate studies of biogeochemical cycles around brine seeps that are archived in the rock record.

Microbial transformation and decomposition of organic matter in sediments constitutes one of the largest fluxes of carbon in marine environments. Mineralization of sedimentary organic matter by microorganisms results in selective degradation such that bioavailable or accessible compounds are rapidly metabolized while more recalcitrant, complex compounds are preserved and buried in sediment (Mahmoudi et al., 2017). Recent studies have found that the ability to use different carbon sources appears to vary among microorganisms, suggesting that the availability of certain pools of carbon can be specific to the taxa that utilize the pool. This implies that organic matter mineralization in marine environments may depend on the metabolic potential of the microbial populations that are present and active. The goal of our study was to investigate the extent to which organic matter availability and transformation may be species-specific using sediment from Guaymas Basin (Gulf of California). We carried out time-series incubations using bacterial isolates and sterilized sediment in the IsoCaRB system (Beaupre et al., 2016) which allowed us to measure the production rates and natural isotopic signatures (δ13C and Δ14C) of microbially-respired CO2. Separate incubations using two different marine bacterial isolates (Vibrio sp. and Pseudoalteromonas sp.) and sterilized Guaymas Basin sediment under oxic conditions showed that the rate and total quantity of organic matter metabolized by these two species differs. Approximately twice as much CO2 was collected during the Vibrio sp. incubation compared to the Pseudoalteromonas sp. incubation. Moreover, the rate at which organic matter was metabolized by the Vibrio sp. was much higher than the Pseudoalteromonas sp. indicating the intrinsic availability of organic matter in sediments may depend on the species that is present and active. Isotopic analyses of microbially respired CO2 will be used to constrain the type and age of organic matter that is accessible to each species. Moreover, molecular analysis of subsamples collected from each incubation will link carbon utilization with the underlying gene expression. Our study sheds light on the degree to which the metabolic capacities of microorganisms affect carbon transformation in sedimentary environments.