Nitrogen cycling in the sediments of Santa Barbara basin and Eastern Subtropical North Pacific:: Nitrogen isotopes, diagenesis and possible chemosymbiosis between two lithotrophs (Thioploca and Anammox) -: "riding on a glider"

This article presents data on the isotopic composition of pore water ammonium and Nor, in the anoxic sediments of the Santa Barbara Basin and the Eastern Subtropical North Pacific (ESNP) region. Ammonium delta N-15 behaves differently in the two regions, reflecting variability in bacterial processes of sedimentary nitrogen cycling. In Santa Barbara Basin, stoichiometry of TCO2 and NH4+ production defined by pore water chemistry indicates that the only reaction influencing pore water ammonium concentration in these sediments appears to be organic matter decomposition. Pore water ammonium was found to be 1-3 parts per thousand heavier than decomposing organic matter. In contrast, the upper 15-30 cm of ESNP pore waters have ammonium with delta N-15 that is more than 10 parts per thousand heavier than decomposing organic matter. At greater depth, the delta N-15 of deeper pore waters approaches that of decomposing organic matter. Mass balance calculations indicate that the observed isotopic enrichment is not due to the preferential loss of isotopically enriched organic matter. Pore water profiles show that the zone with heavy isotopes is characterized by a TCO2/NH4+ inputs ratio of 4 or less. We attribute this to the presence of Thioploca bacteria, who transport NO3- to depth and reduce it to NH4+ while oxidizing sulfide. Based on pore water stoichiometry, and reaction-diffusion modeling of pore water isotopic composition we propose that the N-15-enriched ammonium is produced as the result of chemosymbiosis between Thioploca and Anammox-like bacteria in these sediments. Our work provides the new insights into benthic microbial ecology, and the role of coupled individual metabolic pathways in sedimentary nitrogen cycling. In Santa Barbara Basin, factors which contribute to the isotopic fractionation of 1-3 parts per thousand between pore water ammonium and solid N-org were also considered. In this sediments, similar to 23% of the rain of organic nitrogen is lost to diagenesis, therefore, the isotopic effect of degradation on the residual N-org is estimated to be small, -0.7 parts per thousand. The most likely cause of fractionation is preferential degradation of an isotopically heavier, more labile marine fraction relative to an isotoopically lighter, more refractory terrestrial component of the organic matter, but other possibilities cannot be ruled out. When corrected for the observed fractionation, it appears that the delta N-15 of these sediments may represent non-steady state conditions. (c) 2005 Elsevier B.V. All rights reserved.