Geochemical constraints on the distribution and rates of debromination in the deep subseafloor biosphere

Abstract

Organic matter in marine sediments is degraded through a range of diverse metabolic pathways which are dependent on substrate availability, environmental conditions, and microbial ecology. The rates and systematics of these metabolic pathways affect long-term global geochemical cycles and the degradation of organic matter in the subsurface marine environment. Organohalide respiration is one of these pathways that has been hypothesized to be widely active in the deep biosphere, with the carbon-halogen bonds being broken through microbially-mediated redox reactions. Besides directly providing energy to microbes in marine sediments and allowing bromine to cycle back into the overlying ocean, organobromine respiration may also be closely linked to nitrogen and carbon cycling in anoxic marine sediments. Here we investigate the distribution and rates of debromination by tracking the production of dissolved bromide (Br-) with depth in pore water sampled at several continental margins. Pore water profiles of Br- and ammonium (NH4+) concentrations from the Krishna-Godavari (K-G) basin on the southeastern margin of India indicate a common distribution of rates of debromination and NH4+ production in continental margin sediments, and suggest that the pools of bioavailable nitrogen and organobromine compounds are likely geochemically associated at these sites. Dissolved Br- and total solid-phase bromine concentration profiles from the K-G basin and Costa Rica margin indicate the most rapid debromination occurs in the upper 10-20m of the sediment column. The rates of debromination in the sediment column from the Costa Rica, Cascadia, and Nankai convergent margins are estimated using numerical reaction-transport modeling of pore water Br- concentration profiles to constrain the maximum amount of metabolic energy that could be provided to the microbial communities through organobromine respiration. The modeled rates of debromination provide an upper limit to organobromine respiration activity because other debromination processes may also be responsible for an unknown fraction of these geochemically-derived rates. Modeled rates of debromination on the order of 101-103μmolm-2y-1 indicate that the maximum amount of energy that is potentially provided through organobromine respiration is low relative to other metabolic pathways such as sulfate reduction and methanogenesis, however organobromine respiration may still serve an important niche in the microbial community and debromination is an important part of the oceanic bromine cycle.