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Abstract Biological nitrogen fixation is a key process balancing the loss of combined nitrogen in the marine nitrogen cycle. Its relevance in upwelling or high nutrient regions is still unclear, with the few available studies in these regions of the ocean reporting rates that vary widely from below detection limit to > 100 nmol N L⁻¹ d⁻¹. In the eastern tropical Atlantic Ocean, two open ocean upwelling systems are active in boreal summer. One is the seasonal equatorial upwelling, where the residual phosphorus associated with aged upwelled waters is suggested to enhance nitrogen fixation in this season. The other is the Guinea Dome, a thermal upwelling dome. We conducted two surveys along 23° W across the Guinea Dome and the Equator from 15° N to 5° S in September 2015 and August–September 2016 with high latitudinal resolution (20–60 nm between stations). The abundance ofTrichodesmiumcolonies was characterized by an Underwater Vision Profiler 5 and the total biological nitrogen fixation in the euphotic layer was measured using the¹⁵N₂technique. The highest abundances ofTrichodesmiumcolonies were found in the area of the Guinea Dome (9°–15° N) with a maximum of 3 colonies L⁻¹near the surface. By contrast, colonies were almost absent in the Equatorial band between 2° N and 5° S. The highest nitrogen fixation rate was measured at the northern edge of the Guinea Dome in 2016 (ca. 31 nmol N L⁻¹ d⁻¹). In this region, where diazotrophs thrived on a sufficient supply of both phosphorus and iron, a patchy distribution was unveiled by our increased spatial resolution scheme. In the Equatorial band, rates were considerably lower, ranging from below detection limit to ca. 4 nmol N L⁻¹ d⁻¹, with a clear difference in magnitude between 2015 (rates close to zero) and 2016 (average rates around 2 nmol N L⁻¹ d⁻¹). This difference seemed triggered by a contrasting supply of phosphorus between years. Our study stresses the importance of surveys with sampling at fine-scale spatial resolution, and shows unexpected high variability in the rates of nitrogen fixation in the Guinea Dome, a region where diazotrophy is a significant process supplying new nitrogen into the euphotic layer. 

Micronutrients control phytoplankton growth in the ocean, influencing carbon export and fisheries. It is currently unclear how micronutrient scarcity affects cellular processes and how interdependence across micronutrients arises. We show that proximate causes of micronutrient growth limitation and interdependence are governed by cumulative cellular costs of acquiring and using micronutrients. Using a mechanistic proteomic allocation model of a polar diatom focused on iron and manganese, we demonstrate how cellular processes fundamentally underpin micronutrient limitation, and how they interact and compensate for each other to shape cellular elemental stoichiometry and resource interdependence. We coupled our model with metaproteomic and environmental data, yielding an approach for estimating biogeochemical metrics, including taxon-specific growth rates. Our results show that cumulative cellular costs govern how environmental conditions modify phytoplankton growth. 

Abstract. Oxygen-deficient zones (ODZs) are major sites of net naturalnitrous oxide (N2O) production and emissions. In order to understandchanges in the magnitude of N2O production in response to globalchange, knowledge on the individual contributions of the major microbialpathways (nitrification and denitrification) to N2O production andtheir regulation is needed. In the ODZ in the coastal area off Peru, thesensitivity of N2O production to oxygen and organic matter wasinvestigated using 15N tracer experiments in combination with quantitative PCR (qPCR) andmicroarray analysis of total and active functional genes targeting archaeal amoAand nirS as marker genes for nitrification and denitrification, respectively.Denitrification was responsible for the highest N2O production with amean of 8.7 nmol L−1 d−1 but up to 118±27.8 nmol L−1 d−1 just below the oxic–anoxic interface. The highest N2O productionfrom ammonium oxidation (AO) of 0.16±0.003 nmol L−1 d−1occurred in the upper oxycline at O2 concentrations of 10–30 µmol L−1 which coincided with the highest archaeal amoA transcripts/genes.Hybrid N2O formation (i.e., N2O with one N atom from NH4+and the other from other substrates such as NO2-) was the dominantspecies, comprising 70 %–85 % of total produced N2O fromNH4+, regardless of the ammonium oxidation rate or O2concentrations. Oxygen responses of N2O production varied withsubstrate, but production and yields were generally highest below 10 µmol L−1 O2. Particulate organic matter additions increasedN2O production by denitrification up to 5-fold, suggesting increasedN2O production during times of high particulate organic matter export.High N2O yields of 2.1 % from AO were measured, but the overallcontribution by AO to N2O production was still an order of magnitudelower than that of denitrification. Hence, these findings show thatdenitrification is the most important N2O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positivefeedback of the total oceanic N2O sources in response to increasingoceanic deoxygenation.