The budget of reactive nitrogen (Nr; oxidized and reduced inorganic and organic forms of nitrogen) has at least doubled since the preindustrial era due to human activities. Excess Nr causes ...significant detrimental effects on many terrestrial and aquatic ecosystems; less is known about the impact on the open ocean. Nr deposition may already rival biological N
2
fixation quantitatively and will likely continue to rise. However, it is unclear how much of the Nr currently deposited to the ocean is external in origin. Understanding the importance of ocean Nr emissions versus external Nr deposition is key to quantifying the influence of deposition on ocean biogeochemistry and climate. This article reviews our understanding of the impacts of Nr deposition on the open ocean and the emissions of Nr from the ocean, placing particular emphasis on stable isotopes as a tool to investigate the surface ocean-lower atmosphere Nr cycle and its variations over time.
The ocean has a dynamic exchange of reactive nitrogen with the atmosphere and is not just a passive recipient of nitrogen pollution from land.
Tracing anthropogenic nitrogen deposition to the ocean is a challenge due to overlapping geochemical signatures with other nitrogen inputs.
However, studies suggest an imprint of external (anthropogenic) nitrogen deposition in the Mediterranean Sea and North Pacific Ocean.
Climate change will impact nitrogen emissions from the ocean through warming, acidification, stratification, and changes in food webs.
Ammonia oxidation rates and ammonia-oxidizer community structure were examined in a depth profile (20–2000 m) at the Bermuda Atlantic Time-Series Study site in December 2009. Ammonia oxidation rates, ...measured from trace additions of
15
NH
4
+
(12–18 nmol L−1), ranged from undetectable at the surface and 2000 m to 2.0 ± 0.1 nmol L−1 d−1 at 120 m, the depth of the primary nitrite maximum (PNM). Nitrification was not detectable in the photic zone in December, perhaps in part due to the density structure of the upper water column at this time. Ammonium oxidation rates varied with ammonium concentration and yielded an estimate for the half-saturation constant of 65 ± 41 nmol L−1 for the assemblage at 100 m. This value is similar to that reported for the cultivated marine ammonia-oxidizing archeon Nitrosopumilus maritimus (134 nmol L−1), confirming the high affinity for ammonium of the in situ community in the Sargasso Sea. Ammonia-oxidizing archaeal (AOA) amoA gene copy numbers were two orders of magnitude higher than ammonia-oxidizing bacterial (AOB) amoA gene copy numbers at the PNM depth, suggesting that AOA were responsible for most of the ammonium oxidation, which was in turn responsible for the formation of the PNM. AOB abundance exceeded AOA at depths below 140 m, where both groups were much less abundant. Application of an AOA amoA functional gene microarray showed a diverse and even community distribution. A single archetype (AOA24, representing sequences originally from a coral reef) had the highest fluorescence ratio at depths of 0–1000 m.
Global models estimate that the anthropogenic component of atmospheric nitrogen (N) deposition to the ocean accounts for up to a third of the ocean’s external N supply and 10% of anthropogenic CO₂ ...uptake. However, there are few observational constraints from the marine atmospheric environment to validate these findings. Due to the paucity of atmospheric organic N data, the largest uncertainties related to atmospheric N deposition are the sources and cycling of organic N, which is 20–80% of total N deposition. We studied the concentration and chemical composition of rainwater and aerosol organic N collected on the island of Bermuda in the western North Atlantic Ocean over 18 mo. Here, we show that the water-soluble organic N concentration (WSON) in marine aerosol is strongly correlated with surface ocean primary productivity and wind speed, suggesting a marine biogenic source for aerosol WSON. The chemical composition of high-WSON aerosols also indicates a primary marine source. We find that the WSON in marine rain is compositionally different from that in concurrently collected aerosols, suggesting that in-cloud scavenging (as opposed to below-cloud “washout”) is the main contributor to rain WSON. We conclude that anthropogenic activity is not a significant source of organic N to the marine atmosphere over the North Atlantic, despite downwind transport from large pollution sources in North America. This, in conjunction with previous work on ammonium and nitrate, leads to the conclusion that only 27% of total N deposition to the global ocean is anthropogenic, in contrast to the 80% estimated previously.
Because nitrogen availability limits primary production over much of the global ocean, understanding the controls on the marine nitrogen inventory and supply to the surface ocean is essential for ...understanding biological productivity and exchange of greenhouse gases with the atmosphere. Quantifying the ocean’s inputs, outputs, and internal cycling of nitrogen requires a variety of tools and approaches, including measurements of the nitrogen isotope ratio in organic and inorganic nitrogen species. The marine nitrogen cycle, which shapes nitrogen availability and speciation in the ocean, is linked to the elemental cycles of carbon, phosphorus, and trace elements. For example, the majority of nitrogen cycle oxidation and reduction reactions are mediated by enzymes that require trace metals for catalysis. Recent observations made through global-scale programs such as GEOTRACES have greatly expanded our knowledge of the marine nitrogen cycle. Though much work remains to be done, here we outline key advances in understanding the marine nitrogen cycle that have been achieved through these analyses, such as the distributions and rates of dinitrogen fixation, terrestrial nitrogen inputs, and nitrogen loss processes.
Dissolved organic phosphorus (DOP) concentration distributions in the global surface ocean inform our understanding of marine biogeochemical processes such as nitrogen fixation and primary ...production. The spatial distribution of DOP concentrations in the surface ocean reflect production by primary producers and consumption as an organic nutrient by phytoplankton including diazotrophs and other microbes, as well as other loss processes such as photolysis. Compared to dissolved organic carbon and nitrogen, however, relatively few marine DOP concentration measurements have been made, largely due to the lack of automated analysis techniques. Here we present a database of marine DOP concentration measurements (DOPv2021) that includes new (n = 730) and previously published (n = 3140) observations made over the last ~30 years (1990-2021), including 1751 observations in the upper 50 m. This dataset encompasses observations from all major ocean basins including the poorly represented Indian, South Pacific, and Southern Oceans and provides insight into spatial distributions of DOP in the ocean. It is also valuable for researchers who work on marine primary production and nitrogen fixation.
The Southern Ocean plays a critical role in regulating global climate as a major sink for atmospheric carbon dioxide (CO2), and in global ocean biogeochemistry by supplying nutrients to the global ...thermocline, thereby influencing global primary production and carbon export. Biogeochemical processes within the Southern Ocean regulate regional primary production and biological carbon uptake, primarily through iron supply, and support ecosystem functioning over a range of spatial and temporal scales. Here we assimilate existing knowledge and present new data to examine the biogeochemical cycles of iron, carbon and major nutrients, their key drivers and their responses to, and roles in, contemporary climate and environmental change. Projected increases in iron supply, coupled with increases in light availability to phytoplankton through increased near-surface stratification and longer ice-free periods, are very likely to increase primary production and carbon export around Antarctica. Biological carbon uptake is likely to increase for the Southern Ocean as a whole, whilst there is greater uncertainty around projections of primary production in the Sub-Antarctic and basin-wide changes in phytoplankton species composition, as well as their biogeochemical consequences. Phytoplankton, zooplankton, higher trophic level organisms and microbial communities are strongly influenced by Southern Ocean biogeochemistry, in particular through nutrient supply and ocean acidification. In turn, these organisms exert important controls on biogeochemistry through carbon storage and export, nutrient recycling and redistribution, and benthic-pelagic coupling. The key processes described in this paper are summarised in the graphical abstract. Climate-mediated changes in Southern Ocean biogeochemistry over the coming decades are very likely to impact primary production, sea-air CO2 exchange and ecosystem functioning within and beyond this vast and critically important ocean region.
Reduced forms of the C56S and C60S variants of the thioredoxin-like Clostridium pasteurianum Fe2S2 ferredoxin (CpFd) provide the only known examples of valence-delocalized Fe2S2+ clusters, which ...constitute a fundamental building block of all higher nuclearity Fe–S clusters. In this work, we have revisited earlier work on the CpFd variants and carried out redox and spectroscopic studies on the Fe2S22+,+ centers in wild-type and equivalent variants of the highly homologous and structurally characterized Aquifex aeolicus ferredoxin 4 (AaeFd4) using EPR, UV–visible–NIR absorption, CD and variable-temperature MCD, and protein–film electrochemistry. The results indicate that the Fe2S2+ centers in the equivalent AaeFd4 and CpFd variants reversibly interconvert between similar valence-localized S = 1/2 and valence-delocalized S = 9/2 forms as a function of pH, with pK a values in the range 8.3–9.0, because of protonation of the coordinated serinate residue. However, freezing high-pH samples results in partial or full conversion from valence-delocalized S = 9/2 to valence-localized S = 1/2 Fe2S2+ clusters. MCD saturation magnetization data for valence-delocalized S = 9/2 Fe2S2+ centers facilitated determination of transition polarizations and thereby assignments of low-energy MCD bands associated with the Fe–Fe interaction. The assignments provide experimental assessment of the double exchange parameter, B, for valence-delocalized Fe2S2+ centers and demonstrate that variable-temperature MCD spectroscopy provides a means of detecting and investigating the properties of valence-delocalized S = 9/2 Fe2S2+ fragments in higher nuclearity Fe–S clusters. The origin of valence delocalization in thioredoxin-like ferredoxin Cys-to-Ser variants and Fe–S clusters in general is discussed in light of these results.
Food availability is a primary factor determining species distribution and success, especially for offshore benthic organisms that rely on sinking particles as their main food supply. Temporal ...changes in the quantity and quality of food sources, often linked to shifts in the prevailing hydrographic conditions, thus have important consequences for benthic community composition and ecosystem functioning. Here, we investigate variability in the biochemical composition of offshore benthic organisms and their food sources across different time-scales (3 weeks, 1 year, 2 years) at the Sub-Antarctic Prince Edward Islands (PEIs) using nitrogen and carbon stable isotope (SI; δ15N and δ13C, respectively) and fatty acid (FA) analyses. The δ15N of suspended particulate matter (SPM) did not show a consistent trend over time, probably because the primary driver of δ15NSPM variability appears to be seasonality and we sampled at roughly the same time each year. In contrast, the δ13CSPM increased with time, likely as a result of the changing proportion of macroalgal detritus relative to offshore phytoplankton biomass. The δ15N and δ13C of select trophic groups varied with time, but not necessarily in synchrony with the SPM, possibly due to differences in the SI turnover rates of the food sources versus the consumers. In contrast, the FA composition of most taxa did change over time, reflecting the shorter integration period for FA compared to SI. While taxa showed different FA patterns across the time-scales investigated, the dominant trend was one of decreasing food quality with time. Combined, the SI and FA results indicate that the response of the benthos to the factor Time is not straightforward. We conclude that the two techniques provide different yet complementary temporal information, with animal SI ratios reflecting the longer-term and FA composition the shorter-term response of the benthos to changing food availability. Our findings suggest that different species at the PEIs will respond differently to climate-driven changes in food quality and quantity, increasing the uncertainty facing this vulnerable ecosystem.
•Temporal variation in the diet of benthos was investigated at Sub-Antarctic islands.•The δ13C of suspended material varied with time but its δ15N did not.•Trophic groups had different responses to the factor Time.•Organismal fatty acid composition indicates decreasing food quality with time.•Benthos may have complex responses to long-term changes at the islands.
Net primary production (NPP) fueled by nitrate is often equated with carbon export, providing a metric for CO2 removal to the deep ocean. This “new production paradigm” assumes that nitrification, ...the oxidation of regenerated ammonium to nitrate, is negligible in the sunlit upper ocean. While surface layer nitrification has been measured in other oceanic regions, very few data exist for the Southern Ocean. We measured NPP, nitrogen (N) uptake, and nitrification in the upper 200 m across the Atlantic Southern Ocean in winter and summer. Rates of winter mixed‐layer nitrate uptake were low, while ammonium uptake was surprisingly high. NPP was also low, such that NPP and total N (nitrate+ammonium) uptake were decoupled; we attribute this to ammonium consumption by heterotrophic bacteria. By contrast, NPP and total N uptake were strongly coupled in summer except at two stations where an additional regenerated N source, likely dissolved organic N, apparently supported 30–45% of NPP. Summertime nitrate uptake rates were fairly high and nitrate fueled >50% of NPP, indicating the potential for significant carbon export. Nitrification supplied <10% of the nitrate consumed in summertime surface waters, while in winter, mixed‐layer nitrification was on average 16 times higher than nitrate uptake. Despite the near‐zero nitrification rates measured in the summer mixed layer, the classically defined f ratio does not well‐represent Southern Ocean carbon export potential annually. This is because some fraction of the nitrate regenerated in the winter mixed layer is likely supplied to phytoplankton in summer; its consumption cannot, therefore, be equated with export.
Key Points
The dominant biological process acting on the surface Southern Ocean nitrate pool is assimilation in summer and nitrification in winter
Primary production and nitrogen uptake are coupled in summer but not winter, likely due to ammonium assimilation by heterotrophic bacteria
15N‐based estimates of the f ratio do not well‐represent Southern Ocean carbon export potential on an annual basis
Ice shelves cover ~1.6 million km
of the Antarctic continental shelf and are sensitive indicators of climate change. With ice-shelf retreat, aphotic marine environments transform into new open-water ...spaces of photo-induced primary production and associated organic matter export to the benthos. Predicting how Antarctic seafloor assemblages may develop following ice-shelf loss requires knowledge of assemblages bordering the ice-shelf margins, which are relatively undocumented. This study investigated seafloor assemblages, by taxa and functional groups, in a coastal polynya adjacent to the Larsen C Ice Shelf front, western Weddell Sea. The study area is rarely accessed, at the frontline of climate change, and located within a CCAMLR-proposed international marine protected area. Four sites, ~1 to 16 km from the ice-shelf front, were explored for megabenthic assemblages, and potential environmental drivers of assemblage structures were assessed. Faunal density increased with distance from the ice shelf, with epifaunal deposit-feeders a surrogate for overall density trends. Faunal richness did not exhibit a significant pattern with distance from the ice shelf and was most variable at sites closest to the ice-shelf front. Faunal assemblages significantly differed in composition among sites, and those nearest to the ice shelf were the most dissimilar; however, ice-shelf proximity did not emerge as a significant driver of assemblage structure. Overall, the study found a biologically-diverse and complex seafloor environment close to an ice-shelf front and provides ecological baselines for monitoring benthic ecosystem responses to environmental change, supporting marine management.
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