A model‐based insight into the coupling of nitrogen and sulfur cycles in a coastal upwelling system

Abstract

The biogeochemical cycling in oxygen-minimum zones (OMZs) is dominated by the interactions of microbial nitrogen transformations and, as recently observed in the Chilean upwelling system, also through the energetically less favorable remineralization of sulfate reduction. The latter process is masked, however, by rapid sulfide oxidation, most likely through nitrate reduction. Thus, the cryptic sulfur cycle links with the nitrogen cycle in OMZ settings. Here, we model the physical-chemical water column structure and the observed process rates as driven by formation and sinking of organic detritus, to quantify the nitrogen and sulfur cycles in the Chilean OMZ. A new biogeochemical submodule was developed and coupled to the Regional Ocean Model System (ROMS). The model results generally agree with the observed distribution of reactive species and the measured process rates. Modeled heterotrophic nitrate reduction and sulfate reduction are responsible for 47% and 36%, respectively, of organic remineralization in a 150 m deep zone below mixed layer. Anammox contributes to 61% of the fixed nitrogen lost to N2 gas, while the rest of the loss is through canonical denitrification as a combination of organic matter oxidation by nitrite reduction and sulfide-driven denitrification. Mineralization coupled to heterotrophic nitrate reduction supplies ∼48% of the ammonium required by anammox. Due to active sulfate reduction, model results suggest that sulfide-driven denitrification contributes to 36% of the nitrogen loss as N2 gas. Our model results highlight the importance of considering the coupled nitrogen and sulfur cycle in examining open-ocean anoxic processes under present, past, and future conditions.

A year in the life of a thrombolite: comparative metatranscriptomics reveals dynamic metabolic changes over diel and seasonal cycles

“…under anaerobic conditions can use nitrate as the terminal electron acceptor rather than oxygen (Visscher and Taylor, 1993). For example, the oxidation of sulfur and thiosulfate can be linked to nitrate and nitrite reduction in both cultivars (Aminuddin and Nicholas, 1973) as well as in oceanic oxygen minimum zones (Azhar et al, 2014), suggesting that in the thrombolite-forming mats the sulfur and nitrogen cycle are closely coupled.…”

A single-column ocean biogeochemistry model (GOTM–TOPAZ) version 1.0

“…MOM was operated using CORE-II forcing data (Large and Yeager, 2009) from 1950 to 2008. We also used data from the Surface Ocean CO 2 Atlas (SOCAT) (Bakker et al, 2016) from the analysis period to verify the CO 2 airsea gas flux in TOPAZ. The time periods for which SOCAT observational data exist for point 102 are April 2001, January 2005, November 2008, and December 2008.…”

Tracking the Paleocene‐Eocene Thermal Maximum in the North Atlantic: A Shelf‐to‐Basin Analysis With a Regional Ocean Model

“…This study employed version 3.6 of ROMS (Shchepetkin & McWilliams, , ; Song & Haidvogel, ), with additional modifications to biogeochemistry that are detailed in Al Azhar et al (). ROMS is a terrain‐following, primitive equation regional ocean model.…”