Testing in situ incubators to measure rates of microbial nitrogen transformations in the Eastern Tropical North Pacific oxygen deficient zone

Nitrogen cycling is in large part facilitated by microbes, both on land and at sea. Microbial denitrification is the successive reduction of nitrate (NO₃^-) to dinitrogen (N₂), by way of the greenhouse gas nitrous oxide (N₂O); it largely occurs in suboxic waters, where oxygen is inadequate for aerobic respiration. In contrast to the denitrification process, some nitrite (NO₂^-) can also be oxidized back to nitrate, which preserves it for further biological activity. We examined seawater samples collected from various stations and depths in the Eastern Tropical North Pacific (ETNP) oxygen-deficient zone, aboard the R/V Falkor in July 2018. To make direct measurements of nitrate reduction and nitrite oxidation rates, ¹⁵N tracer incubations were performed in situ by an autonomous incubator deployed at select target depths. More standard deckboard experiments were also performed in parallel for comparison. Timecourse samples were analyzed with the denitrifier method (Sigman et al. 2001) or the azide method (McIlvin and Altabet 2005) for δ¹⁵N-NO₃^- or δ¹⁵N-NO₂^-, respectively. The resulting δ¹⁵N values and NO_X concentrations were used to calculate the rates of N transformations. In situ incubation rates of both nitrate reduction and nitrite oxidation were similar to than their deckboard counterparts at matching depths, though each had relatively large uncertainties. Nitrite oxidation rates were also well matched to nitrate reduction rates in both in situ and deckboard incubations, consistent with the close coupling between these two processes observed in earlier studies.