A source of isotopically light organic carbon in a low-pH anoxic marine zone

Cristian A Vargas,Jennifer Walker,Julio Sepúlveda,Brett Walker,Joe Salisbury,Xiaomei Xu,Wolfgang Schneider,Sebastian I Cantarero,Marcela Cornejo D’Ottone,Alexander Galán,Laura Farías,Ricardo De Pol-Holz

doi:10.1038/s41467-021-21871-4

Abstract

Geochemical and stable isotope measurements in the anoxic marine zone (AMZ) off northern Chile during periods of contrasting oceanographic conditions indicate that microbial processes mediating sulfur and nitrogen cycling exert a significant control on the carbonate chemistry (pH, AT, DIC and pCO2) of this region. Here we show that in 2015, a large isotopic fractionation between DIC and POC, a DIC and N deficit in AMZ waters indicate the predominance of in situ dark carbon fixation by sulfur-driven autotrophic denitrification in addition to anammox. In 2018, however, the fractionation between DIC and POC was significantly lower, while the total alkalinity increased in the low-pH AMZ core, suggesting a predominance of heterotrophic processes. An isotope mass-balance model demonstrates that variations in the rates of sulfur- and nitrogen-mediated carbon fixation in AMZ waters contribute ~7–35% of the POC exported to deeper waters. Thus, dark carbon fixation should be included in assessments of future changes in carbon cycling and carbonate chemistry due to AMZ expansion.

Highlights

Geochemical and stable isotope measurements in the anoxic marine zone (AMZ) off northern Chile during periods of contrasting oceanographic conditions indicate that microbial processes mediating sulfur and nitrogen cycling exert a significant control on the carbonate chemistry of this region
Organic carbon oxidation in AMZ waters is driven by microorganisms able to utilize alternative electron acceptors, which results in excess CO2 and lower pH (< 7.8)[12]
Under such O2limited conditions, chemolithoautotrophic processes that consume dissolved inorganic carbon (DIC) are dominated by bacterial sulfur-driven autotrophic denitrification (SDAD) and anaerobic ammonium oxidation (Anammox), oxygen-dependent nitrifying microorganisms operate, but at lower rates[13,14]

Summary

Introduction

Geochemical and stable isotope measurements in the anoxic marine zone (AMZ) off northern Chile during periods of contrasting oceanographic conditions indicate that microbial processes mediating sulfur and nitrogen cycling exert a significant control on the carbonate chemistry (pH, AT, DIC and pCO2) of this region. Recent work has shown that the nitrite-rich AMZ in the ETSP off northern Chile is an extreme end-member within the spectrum of AMZ ecosystems, where O2 is mostly absent, except during sporadic intrusions[10] Gene surveys in this region have shown that significant fractions of picoplankton in AMZ waters are chemoautotrophs associated with the cycling of sulfur and nitrogen, such as SDAD and anammox[16,17]. Models suggest that a doubling of surface water pCO2 could lead to a doubling or more of pCO2 in OMZ/AMZ waters[21], mostly due to the reduction of seawaters buffer capacity by the acidic components of the normal Redfield cycle[22] These projections ignore the impact of microbially driven processes linked to sulfur and/or nitrogen on biogeochemical cycles, including carbon cycling. The extent to which O2 concentrations, the sulfur and nitrogen cycles, and the carbonate system are linked in AMZs remains poorly understood[12,15]

Methods

Results

Conclusion