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Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation.

ABSTRACT: In many oceanic regions, growth of phytoplankton is nitrogen-limited because fixation of N2 cannot make up for the removal of fixed inorganic nitrogen (NH4+, NO2-, and NO3-) by anaerobic microbial processes. Globally, 30-50% of the total nitrogen loss occurs in oxygen-minimum zones (OMZs) and is commonly attributed to denitrification (reduction of nitrate to N2 by heterotrophic bacteria). Here, we show that instead, the anammox process (the anaerobic oxidation of ammonium by nitrite to yield N2) is mainly responsible for nitrogen loss in the OMZ waters of one of the most productive regions of the world ocean, the Benguela upwelling system. Our in situ experiments indicate that nitrate is not directly converted to N2 by heterotrophic denitrification in the suboxic zone. In the Benguela system, nutrient profiles, anammox rates, abundances of anammox cells, and specific biomarker lipids indicate that anammox bacteria are responsible for massive losses of fixed nitrogen. We have identified and directly linked anammox bacteria to the removal of fixed inorganic nitrogen in the OMZ waters of an open-ocean setting. We hypothesize that anammox could also be responsible for substantial nitrogen loss from other OMZ waters of the ocean.

SUBMITTER: Kuypers MM

PROVIDER: S-EPMC556276 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Abstract Despite their role in modulating the marine ecosystem, variability and drivers of low‐oxygen events in the offshore northern Benguela Upwelling System (BenUS) have been rarely investigated due to the events' episodicity which is difficult to resolve using shipboard measurements. We address this issue using 4 months of high‐resolution glider data collected between February and June 2018, 100 km offshore at 18°S. We find that oxygen (O2) concentrations in the offshore northern Benguela are determined by the subsurface alternation of low‐oxygen Angola‐derived water and oxygenated water from the south at 100–500 m depth. We observe intermittent hypoxia (O2 < 60 μmol kg−1) which occurs on average for ∼30% of the 4 months deployment and is driven by the time‐varying subsurface pulses of Angola‐derived tropical water. Hypoxic events are rather persistent at depths of 300–450 m, while they are more sporadic and have weekly duration at shallower depths (100–300 m). We find extreme values of hypoxia, with O2 minima of 16 μmol kg−1, associated with an anticyclonic eddy spinning from the undercurrent flowing on the BenUS shelf and showing no surface signature. Fine‐scale patchiness and water mass mixing are associated with cross‐frontal stirring by a large anticyclone recirculating tropical water into the northern BenUS. The dominance of physical drivers and their high variability on short time scales reveal a dynamic coupling between Angola and Benguela, calling for long‐term and high‐resolution measurements and studies focusing on future changes of both tropical O2 minima and lateral fluxes in this region. Key Points Alternating subsurface physical fluxes of Angola‐ and Benguela‐derived waters drive mesopelagic oxygen (O2) variability in the offshore Benguela Hypoxic events (O2 < 60 μmol) are intermittent and are associated with weekly pulses of tropical water in the upper mesopelagic Mesoscale eddies further modulate the O2 field and are associated with severe hypoxia, fine‐scale O2 patchiness and water mass mixing

Project description:Due to their lithotrophic metabolisms, morphological complexity and conspicuous appearance, members of the Beggiatoaceae have been extensively studied for more than 100 years. These bacteria are known to be primarily sulfur-oxidizing autotrophs that commonly occur in dense mats at redox interfaces. Their large size and the presence of a mucous sheath allows these cells to serve as sites of attachment for communities of other microorganisms. But little is known about their individual niche preferences and attached microbiomes, particularly in marine environments, due to a paucity of cultivars and their prevalence in habitats that are difficult to access and study. Therefore, in this study, we compare Beggiatoaceae strain composition, community composition, and geochemical profiles collected from sulfidic sediments at four marine stations off the coast of Namibia. To elucidate community members that were directly attached and enriched in both filamentous Beggiatoaceae, namely Ca. Marithioploca spp. and Ca. Maribeggiatoa spp., as well as non-filamentous Beggiatoaceae, Ca. Thiomargarita spp., the Beggiatoaceae were pooled by morphotype for community analysis. The Beggiatoaceae samples collected from a highly sulfidic site were enriched in strains of sulfur-oxidizing Campylobacterota, that may promote a more hospitable setting for the Beggiatoaceae, which are known to have a lower tolerance for high sulfide to oxygen ratios. We found just a few host-specific associations with the motile filamentous morphotypes. Conversely, we detected 123 host specific enrichments with non-motile chain forming Beggiatoaceae. Potential metabolisms of the enriched strains include fermentation of host sheath material, syntrophic exchange of H2 and acetate, inorganic sulfur metabolism, and nitrite oxidation. Surprisingly, we did not detect any enrichments of anaerobic ammonium oxidizing bacteria as previously suggested and postulate that less well-studied anaerobic ammonium oxidation pathways may be occurring instead.

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Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation.

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