Coupled cycling of metals with nitrogen and carbon in marine sediments

jennifer Karolewski, Ph.D., 2024
Scott D. Wankel, Advisor

Cross element processes are complex and often understudied components within biogeochemical cycles. In this thesis, I use stable isotopes of carbon, nitrogen, and oxygen as the primary tools to interrogates these complex reactions. First, I report abiotic oxidation of nitrite to nitrate by manganese(III)-pyrophosphate. This reaction can occur even in the absence of oxygen, unlike biological nitrite oxidation. Reaction rates were measured at a range of environmentally relevant pH values (5-8) with the reaction proceeding more quickly at lower pH. Reaction order was second order with respect to manganese(III) and first order with respect to nitrous acid. No reversibility of reaction was observed upon addition of isotopically distinct nitrate. An inverse kinetic isotope effect of +19.9 ± 0.7‰ was calculated, which was comparable in magnitude and direction to that of biological nitrite oxidation. In natural waters, such as estuaries, this reaction could potentially play an important role in the nitrogen cycle. Next, I report an abiotic reaction between hydroxylamine and manganese(III)-pyrophosphate which forms nitrous oxide, nitrite, and likely dinitrogen gas. In artificial seawater (pH = 8), this reaction proceeds rapidly, with the ratio of products highly dependent on the reactant ratio. Nitrous oxide site preference (SP) of +35.5 ± 0.6‰ was observed, consistent with the isotopic signatures of several marine nitrifying organisms. This suggests that “leakage” of intermediate hydroxylamine from nitrifier cells could potentially react with manganese(III) in a mixed biotic-abiotic process without previously being noticed. Finally, I performed experiments using carbon-13 labelling to measure rates of anaerobic oxidation of methane (AOM) in cold seep sediments collected at Cascadia Margin. Four forms of oxidized manganese and four forms of oxidized iron were added to treatments in order to evaluate how these altered rates of AOM. In contrast to previous work, addition of metals did not overall increase rates of AOM above those of an unamended control and some treatments in fact reduced it. However, energy yields from microbes using metal as an electron acceptor are higher per mole of methane reduced than that of using sulfate so even with these lower rates, energy yields would have exceeded those of controls. Additionally, doubling times for the archaea performing AOM are long enough that the microbial community may not have been able to adapt on the timescale of the experiment. Overall, the results of this thesis illuminate the need for further study of abiotic and coupled cycling reactions when considering biogeochemical cycles.