Oceanic Nitrogen Cycle Research Articles

Riverine nutrient impact on global ocean nitrogen cycle feedbacks and marine primary production in an Earth system model

Abstract. Riverine nutrient export is an important process in marine coastal biogeochemistry and also impacts global marine biology. The nitrogen cycle is a key player here. Internal feedbacks are shown to regulate not only nitrogen distribution, but also primary production and thereby oxygen concentrations. Phosphorus is another essential nutrient and interacts with the nitrogen cycle via different feedback mechanisms. After a previous study of the marine nitrogen cycle response to riverine nitrogen supply, here we include phosphorus from river export with different phosphorus burial scenarios and study the impact of phosphorus alone and in combination with nitrogen in a global 3D ocean biogeochemistry model. Again, we analyse the effects on near-coastal and open-ocean biogeochemistry. We find that riverine export of bioavailable phosphorus alone or in conjunction with nitrogen affects marine biology on millennial timescales more than riverine nitrogen alone. Biogeochemical feedbacks in the marine nitrogen cycle are strongly influenced by additional phosphorus. Where bioavailable phosphorus increases with river input, nitrogen concentration increases as well, except for in regions with diminishing oxygen concentrations. High phosphorus burial rates decrease biological production significantly. Globally, the addition of riverine phosphorus in the modelled ocean leads to elevated primary production rates in the coastal and open oceans.

Experimental evidence that nitrogen-fixing cyanobacterium Trichodesmium spp. supplies new nitrogen source to marine phytoplankton

In oligotrophic oceans, primary productivity is widely limited by nitrogen bioavailability. The broadly distributed and abundant nitrogen-fixing cyanobacterium Trichodesmium plays an important role in the oceanic nitrogen and carbon cycles by providing a “new” source of nitrogen to many non-diazotrophic microbes, thereby driving new primary production in the ocean. However, the underlying process and mechanism of nitrogen supply from Trichodesmium to other phytoplankton remain unclear. Here, our results demonstrated that the fixed nitrogen released by Trichodesmium could sustain the growth of a non-nitrogen-fixing cyanobacterium Synechococcus sp. PCC 7002, including a mutant strain (Mut-ureA) that cannot use urea. However, the growth rate of Mut-ureA was approximately 20% lower than that of the wild strain when Trichodesmium filtrate was used for nitrogen supply. This result was consistent with the composition of the Trichodesmium exudate, in which urea comprised more than 20% of the total fixed nitrogen that was released. It is evident from the experiments that a fraction of the Trichodesmium-derived nitrogen was not available to Mut-ureA. Our results suggested that Trichodesmium produces dissolved organic nitrogen, especially a certain amount of urea as a “new” nitrogen source, benefiting in particular populations of surrounding phytoplankton species.

Nitrogen isotope gradient on continental margins during the late Paleoproterozoic

The late Paleoproterozoic was a key period in the evolution of Earth’s surface environments and life. It is generally accepted that during the 2.43–2.06 Ga Great Oxidation Episode, the atmospheric oxygen level rose to an intermediate state (0.1% to 10% of the present atmospheric level) and the chemical composition of the oceans changed significantly. The paleontological record indicates that eukaryotes appeared during the late Paleoproterozoic. Nitrogen, an essential nutrient for life, is thought to have been one of the key factors in eukaryote evolution. However, nitrogen bioavailability and spatial heterogeneity in nitrogen cycle in the late Paleoproterozoic oceans remain controversial. Here, we examine carbon and nitrogen cycling in the late Paleoproterozoic ocean, via analysis of redox-sensitive element contents and organic carbon and nitrogen isotope compositions of black shales of the ca. 1.9 Ga Nuvilic Formation of the Povungnituk Group, Cape Smith Belt, Canada. Two diamond drill-holes (DDHs) 718–3333 (∼50 m-long) and 4G8069 (∼90 m-long) investigated in this study contain turbiditic greywackes and black shales deposited on the northern margin of the Archean Superior craton below the storm wave-base. Sedimentary rocks of DDH 4G8069 might have been deposited closer to the continental source area, as indicated by sandstone abundance, than those of DDH 718–3333. Redox-sensitive element (V, U, and Mo) contents in black shales of DDH 4G8069 are low and show positive correlation with Al contents, whereas those of DDH 718–3333 are irregularly elevated. Fe/Al ratios of DDH 718–3333 are low, which likely reflect Fe loss from sediments under anoxic water-column conditions. Depositional environment is inferred to be above and below the redoxcline for DDHs 4G8069 and 718–3333, respectively. Organic carbon isotope values of DDH 718–3333 are approximately 1.7‰ lighter than those of DDH 4G8069, which might reflect methane cycling at the redoxcline. Nitrogen isotope values are positive (> +3‰) for all the black shale samples, corresponding to aerobic nitrogen cycling and presence of bioavailable nitrate on the northern margin of the Superior craton during the late Paleoproterozoic. Furthermore, nitrogen isotope values for DDH 718–3333 are approximately 2‰ heavier than those for DDH 4G8069. The observed isotopic heterogeneity is similar to that in modern oxygen-minimum zones and could represent intense denitrification at the redoxcline. Although no ca. 1.9 Ga microfossils reported to date are considered unambiguously eukaryotic, the aerobic nitrogen cycling inferred for the Nuvilic Formation, as well as for the previously studied Rove, Virginia, and Menihek formations, suggests that nitrogen limitation was unlikely to have inhibited biological evolution during the late Paleoproterozoic, a finding that is consistent with molecular clock studies suggesting that the emergence of eukaryotes occurred during the Paleoproterozoic.

Prophage enhances the ability of deep-sea bacterium Shewanella psychrophila WP2 to utilize D-amino acid.

Prophages are prevalent in the marine bacterial genomes and reshape the physiology and metabolism of their hosts. However, whether and how prophages influence the microbial degradation of D-amino acids (D-AAs), which is one of the widely distributed recalcitrant dissolved organic matters (RDOMs) in the ocean, remain to be explored. In this study, we addressed this issue in a representative marine bacterium, Shewanella psychrophila WP2 (WP2), and its integrated prophage SP1. Notably, compared to the WP2 wild-type strain, the SP1 deletion mutant of WP2 (WP2ΔSP1) exhibited a significantly lower D-glutamate (D-Glu) consumption rate and longer lag phase when D-Glu was used as the sole nitrogen source. The subsequent transcriptome analysis identified 1,523 differentially expressed genes involved in diverse cellular processes, especially that multiple genes related to inorganic nitrogen metabolism were highly upregulated. In addition, the dynamic profiles of ammonium, nitrate, and nitrite were distinct between the culture media of WP2 and WP2ΔSP1. Finally, we provide evidence that SP1 conferred a competitive advantage to WP2 when D-Glu was used as the sole nitrogen source and SP1-like phages may be widely distributed in the global ocean. Taken together, these findings offer novel insight into the influences of prophages on host metabolism and RDOM cycling in marine environments.IMPORTANCEThis work represents the first exploration of the impact of prophages on the D-amino acid (D-AA) metabolism of deep-sea bacteria. By using S. psychrophila WP2 and its integrated prophage SP1 as a representative system, we found that SP1 can significantly increase the catabolism rate of WP2 to D-glutamate and produce higher concentrations of ammonium, resulting in faster growth and competitive advantages. Our findings not only deepen our understanding of the interaction between deep-sea prophages and hosts but also provide new insights into the ecological role of prophages in refractory dissolved organic matter and the nitrogen cycle in deep oceans.

Better together? Lessons on sociality from Trichodesmium.

The N2-fixing cyanobacterium Trichodesmium is an important player in the oceanic nitrogen and carbon cycles. Trichodesmium occurs both as single trichomes and as colonies containing hundreds of trichomes. In this review, we explore the benefits and disadvantages of colony formation, considering physical, chemical, and biological effects from nanometer to kilometer scale. Showing that all major life challenges are affected by colony formation, we claim that Trichodesmium's ecological success is tightly linked to its colonial lifestyle. Microbial interactions in the microbiome, chemical gradients within the colony, interactions with particles, and elevated mobility in the water column shape a highly dynamic microenvironment. We postulate that these dynamics are key to the resilience of Trichodesmium and other colony formers in our changing environment.

Nitrogen uptake by methanotrophic consortia in deep-water gas hydrate-bearing sediments

Methane-consuming microbes inhabiting marine methane seeps have recently been found to have the capacity to assimilate inorganic nitrogen, suggesting a previously unaccounted role in the global nitrogen cycle. Despite ex-situ experimental observations, definitive evidence of this process under in-situ conditions remains elusive, hindering the complete understanding of the controlling factors and magnitude of this process. We present the isotopic variations of organic carbon δ13Corg and total nitrogen δ15N values in two sediment cores collected from the gas hydrate-bearing Håkon Mosby Mud Volcano, SW Barents Sea (72°N, ∼1260 m water depth). We identified a stratigraphic interval containing methane-derived carbonates directly overlying a gas hydrate layer at 67 cm and typified by δ13Corg and δ15N as low as −42.0‰ and 1.2‰, respectively. Stable isotope mixing models confirm in-situ nitrogen uptake by methanotrophic consortia, contributing to up to 49.1 wt% of the local bulk sedimentary organic matter – a finding calling for reevaluation of the role of methane seeps in the oceanic nitrogen cycle.

Characterization of Plebeiobacterium marinum gen. nov., sp. nov. and Plebeiobacterium sediminum sp. nov., revealing the potential nitrogen fixation capacity of the order Marinilabiliales

Biological nitrogen fixation plays a crucial role in the marine nitrogen cycle, impacting global marine productivity and related carbon fluxes. The strains were analyzed by gene annotation, growth conditions and phylogenetic analysis of 16S rRNA gene sequences.These two strains were isolated from the coastal sediment at Xiaoshi Island in Weihai, China. The strains were analyzed by gene annotation, growth conditions and phylogenetic analysis of 16S rRNA gene sequences. It was revealed that strains D04T and AATT contain a set of nif gene clusters responsible for nitrogen fixation. Cell are yellow-colored, Gram-stain-negative, facultatively anaerobic, and rod-shaped bacteria. The optimal growth conditions for strain D04T were found to be at 33 °C, pH 7.0, and in 2% (w/v) NaCl, while strain AATT prefers growth conditions at 33 °C, pH 6.5, and in 3% (w/v) NaCl. The highest similarity of strains D04T and AATT was to Saccharicrinis fermentans NBRC 15936T, with a similarity of 94.1% and 94.8%, respectively. The 16S rRNA gene sequence similarity between the two strains was 96.6%. These novel strains were found to represent new taxa of the Marinilabiliaceae family, and we propose the names Plebeiobacterium marinum gen. nov., sp. nov. and Plebeiobacterium sediminum sp. nov. with type strains D04T (MCCC 1H00493T = KCTC 92026T) and AATT (MCCC 1H00485T = KCTC 92028T), respectively. In this study, nitrogen fixation genes were predicted for 53 strains from the whole order Marinilabiliales and it was found that nitrogen fixation gene clusters were present in 26 strains. These gene clusters were found in every family in the order, highlighting that the presence of nitrogen-fixing gene clusters in the order is common. Nitrogen-fixing bacteria in sediments play an important role in various biogeochemical cycles. Thus, understanding the oceanic nitrogen cycle can provide insights into the energy flow of marine systems.

Nitrite Cycling in Warming Arctic and Subarctic Waters

AbstractThe primary nitrite (NO2−) maximum (PNM) is a typical feature of oceanic nitrogen (N) cycle, yet its characterization in the world ocean remains a gap. By combining the natural abundance of NO2− isotopes with geochemical model, we report for the first time that the formation of Arctic and subarctic PNMs is dominated by ammonia oxidation, while the oxidation of NO2− is the main sink. Notably, NO2− oxidation plays a more important role in Arctic and subarctic waters than in low‐ and mid‐latitude waters. The residence time of NO2− in the PNM further suggests that the NO2− cycle in the Arctic Ocean is more dynamic than in other marine ecosystems. Our findings provide insights into N cycle dynamics in the upper Arctic Ocean ecosystem.

Mesoscale Eddy Effects on Nitrogen Cycles in the Northern South China Sea Since the Last Glacial

Archaeal ammonia oxidation is the most important intermediate pathway in regulating the oceanic nitrogen cycle; however, the study of its specific role on a geological time scale is restricted to a specific part of marginal seas; thus far, only in the southern South China Sea (SCS). To explore the spatial pattern of the role of archaeal ammonia oxidation in the SCS, the GDGT-[2]/[3] ratio (Glycerol Dialkyl Glycerol Tetraether), an indicator of the archaeal ammonia oxidation rate, was analyzed and examined from the collected data profiles since the last glacial period in the northern SCS. The results showed that the GDGT-[2]/[3] ratio in the northern SCS was opposite to that in the southern SCS, with higher GDGT-[2]/[3] values during the Holocene compared to the last glacial period. Based on existing published depths of thermocline (DOT) data in the northern SCS since 30 ka, we believe that hydrological structural variations induced by mesoscale eddies caused this difference. Therefore, physical processes are very important factors that control the nitrogen cycle over a long-time scale. This study may provide new insights into the understanding of the role of archaeal ammonia oxidation within the marine nitrogen cycle over geological time scale.

Microbial labilization and diversification of pyrogenic dissolved organic matter

Abstract. With the increased occurrence of wildfires around the world, interest in the chemistry of pyrogenic organic matter (pyOM) and its fate in the environment has increased. Upon leaching from soils by rain events, significant amounts of dissolved pyOM (pyDOM) enter the aquatic environment and interact with microbial communities that are essential for cycling organic matter within the different biogeochemical cycles. To evaluate the biodegradability of pyDOM, aqueous extracts of laboratory-produced biochars were incubated with soil microbes, and the molecular changes to the composition of pyDOM were probed using ultrahigh-resolution mass spectrometry (Fourier transform–ion cyclotron resonance–mass spectrometry). Given that solar irradiation significantly affects the composition of pyDOM during terrestrial-to-marine export, the effects of photochemistry were also evaluated in the context of pyDOM biodegradability. Ultrahigh-resolution mass spectrometry revealed that many different (both aromatic and aliphatic) compounds were biodegraded. New labile compounds were produced, 22 %–40 % of which were peptide-like. These results indicated that a portion of pyDOM has been labilized into microbial biomass during the incubations. Fluorescence excitation–emission matrix spectra revealed that some fraction of these new bio-produced molecules is associated with proteinaceous fluorophores. Two-dimensional 1H–1H total correlation nuclear magnetic resonance (NMR) spectroscopy identified a peptidoglycan-like backbone within the microbially produced compounds. These results are consistent with previous observations of peptidoglycans within the soil and ocean nitrogen cycles where remnants of biodegraded pyDOM are expected to be observed. Interestingly, the exact nature of the bio-produced organic matter was found to vary drastically among samples indicating that the microbial consortium used may produce different exudates based on the composition of the initial pyDOM. Another potential explanation for the vast diversity of molecules is that microbes only consume low molecular-weight compounds, but they also produce reactive oxygen species (ROS), which initiate oxidative and recombination reactions that degrade high molecular-weight compounds and produce new molecules. Some of the bio-produced molecules (212–308 molecular formulas) were identified in estuarine and marine (surface and abyssal oceanic), and 81–192 of these formulas were of molecular composition attributed to carboxyl-rich alicyclic molecules (CRAM). These results indicate that some of the pyDOM biodegradation products have an oceanic fate and can be sequestered into the deep ocean. The observed microbially mediated diversification of pyDOM suggests that pyDOM contributes to the observed large complexity of natural organic matter observed in riverine and oceanic systems. More broadly, our research shows that pyDOM can be substrate for microbial growth and be incorporated into environmental food webs within the global carbon and nitrogen cycles.

The marine nitrogen cycle: new developments and global change.

The ocean is home to a diverse and metabolically versatile microbial community that performs the complex biochemical transformations that drive the nitrogen cycle, including nitrogen fixation, assimilation, nitrification and nitrogen loss processes. In this Review, we discuss the wealth of new ocean nitrogen cycle research in disciplines from metaproteomics to global biogeochemical modelling and in environments from productive estuaries to the abyssal deep sea. Influential recent discoveries include new microbial functional groups, novel metabolic pathways, original conceptual perspectives and ground-breaking analytical capabilities. These emerging research directions are already contributing to urgent efforts to address the primary challenge facing marine microbiologists today: the unprecedented onslaught of anthropogenic environmental change on marine ecosystems. Ocean warming, acidification, nutrient enrichment and seawater stratification have major effects on the microbial nitrogen cycle, but widespread ocean deoxygenation is perhaps the most consequential for the microorganisms involved in both aerobic and anaerobic nitrogen transformation pathways. In turn, these changes feed back to the global cycles of greenhouse gases such as carbon dioxide and nitrous oxide. At a time when our species casts a lengthening shadow across all marine ecosystems, timely new advances offer us unique opportunities to understand and better predict human impacts on nitrogen biogeochemistry in the changing ocean of the Anthropocene.

Constraining uncertainties of diazotroph biogeography from nifH gene abundance

AbstractMarine diazotrophs fix dinitrogen gas into bioavailable nitrogen that drives the ocean nitrogen cycle; yet, efforts to infer global diazotroph distributions have been limited by a sparsity of observations. In situ measurements of nifH gene abundance (essential for nitrogen fixation) are increasingly being used to inform the biogeography of diazotrophs. However, comparing such gene abundances spatially, temporally and between diazotroph species remains difficult. We synthesize existing data on gene‐to‐cell and cell‐to‐biomass conversions for four major diazotroph groups to convert nifH gene counts to abundance‐ and biomass‐based biogeographic “currencies.” Results suggest up to two orders of magnitude uncertainty converting from nifH gene abundance to cell abundance, and up to four orders of magnitude uncertainty from nifH gene abundance to biomass. Uncertainty arises due to large taxonomic variation in cell size and presumed polyploidy, that is, variability in the number of genomes per cell. Such uncertainties hinder comparing biogeographies of different species. Additionally, numerical models need biogeographies for validation, typically in the currency of carbon biomass. Here, we show that conversion uncertainty from nifH gene abundance to biomass overwhelms biomass variability simulated in such models. These results demonstrate a basic currency problem in converting gene abundance observations to biogeographically meaningful quantities for synthesizing studies and modeling approaches. Such issues may also have relevance to other genes and organisms beyond diazotrophs. To avoid biases in interpreting gene counts as a measure of abundance, we suggest converting gene counts to a binary presence/non‐detect metric to map broad biogeographical distributions more robustly.

Two co-dominant nitrogen-fixing cyanobacteria demonstrate distinct acclimation and adaptation responses to cope with ocean warming.

The globally dominant N2 -fixing cyanobacteria Trichodesmium and Crocosphaera provide vital nitrogen supplies to subtropical and tropical oceans, but little is known about how they will be affected by long-term ocean warming. We tested their thermal responses using experimental evolution methods during 2 years of selection at optimal (28°C), supra-optimal (32°C) and suboptimal (22°C) temperatures. After several hundred generations under thermal selection, changes in growth parameters, as well as N and C fixation rates, suggested that Trichodesmium did not adapt to the three selection temperature regimes during the 2-year evolution experiment, but could instead rapidly and reversibly acclimate to temperature shifts from 20°C to 34°C. In contrast, over the same timeframe apparent thermal adaptation was observed in Crocosphaera, as evidenced by irreversible phenotypic changes as well as whole-genome sequencing and variant analysis. Especially under stressful warming conditions (34°C), 32°C-selected Crocosphaera cells had an advantage in survival and nitrogen fixation over cell lines selected at 22°C and 28°C. The distinct strategies of phenotypic plasticity versus irreversible adaptation in these two sympatric diazotrophs are both viable ways to maintain fitness despite long-term temperature changes, and so could help to stabilize key ocean nitrogen cycle functions under future warming scenarios.

Physiological and Biochemical Characterization of Isolated Bacteria from a Coccolithophore Chrysotila dentata (Prymnesiophyceae) Culture

Coccolithophores are involved in oceanic carbon and nitrogen cycles, and they also have an impact on global climate change. Chrysotila dentata have a complex and close relationship with phycosphere bacteria. In this study, culturable phycosphere bacteria (free-living bacteria and attached bacteria) are isolated from C. dentata by a gradient dilution method and identified based on the 16S rRNA gene sequencing analysis. The phylogenetic tree (neighbor-joining tree, N-J tree) was constructed using the bacterial sequences and closest related sequences from GenBank. Colony characteristics, Gram nature, and physiological and biochemical characteristics were obtained based on a series of tests, such as the sugar utilization (glucose, arabinose, xylose, maltose, and mannitol) test, Voges–Proskauer reaction, urease tests, gelatin liquefaction, Gram test, starch hydrolysis, among others. In this study, seven strains (CF1, CF2, CF3, CF5, CF6, and CF7) of free-living bacteria (CF) and five strains (CA1, CA2, CA3, CA4, and CA5) of attached bacteria (CA) are isolated and identified. We found that the culturable phycosphere bacteria of C. dentata were mainly α-proteobacteria and γ-proteobacteria, with a small part of the CFB (Cytophaga-Flexibacter-Bacteroides) group bacteria and firmicutes. In this study, most α-proteobacteria can utilize malonate and positive in the urease test, meanwhile they can grow in a 7% NaCl medium. Differently to α-proteobacteria, γ-proteobacteria are more reactive, and can utilize maltose, glucose, arabinose, malonate, aesculin, and starch hydrolysis. Meanwhile, γ-proteobacteria can growth in a 7% NaCl and pH 5.7 medium, and some bacteria of this strain were positive in nitrate reduction. Firmicutes are similar to γ-proteobacteria: they are similar in reactivity, as they can utilize maltose, glucose, arabinose, malonate, aesculin, and starch hydrolysis, and can growth in a 7% NaCl and pH 5.7 medium. The difference is that some of firmicutes were positive in gelatin liquefaction and can utilize mannitol. The CFB group of bacteria were only positive in malonate, aesculin, and starch hydrolysis. The above results provide basic experimental data for further studies on the relationship between the coccolithophores and culturable phycosphere bacteria.

Impact of intensifying nitrogen limitation on ocean net primary production is fingerprinted by nitrogen isotopes

The open ocean nitrogen cycle is being altered by increases in anthropogenic atmospheric nitrogen deposition and climate change. How the nitrogen cycle responds will determine long-term trends in net primary production (NPP) in the nitrogen-limited low latitude ocean, but is poorly constrained by uncertainty in how the source-sink balance will evolve. Here we show that intensifying nitrogen limitation of phytoplankton, associated with near-term reductions in NPP, causes detectable declines in nitrogen isotopes (δ15N) and constitutes the primary perturbation of the 21st century nitrogen cycle. Model experiments show that ~75% of the low latitude twilight zone develops anomalously low δ15N by 2060, predominantly due to the effects of climate change that alter ocean circulation, with implications for the nitrogen source-sink balance. Our results highlight that δ15N changes in the low latitude twilight zone may provide a useful constraint on emerging changes to nitrogen limitation and NPP over the 21st century.

Labilization and diversification of pyrogenic dissolved organic matter by microbes

Abstract. With the increased occurrence of forest fires around the world, interest in the chemistry of pyrogenic organic matter (pyOM) and its fate in the environment has increased. Upon leaching from soils by rain events, significant amounts of dissolved pyOM (pyDOM) enter the aquatic environment and interact with microbial communities that are essential for cycling organic matter within the different biogeochemical cycles. To evaluate the bio-reactivity of pyDOM, aqueous extracts of laboratory-produced chars were incubated with soil microbes and the molecular changes to the composition of pyDOM were probed using ultrahigh resolution mass spectrometry (Fourier transform – ion cyclotron resonance – mass spectrometry). Given that photo-degradation also affects the composition and reactivity of pyDOM during terrigenous-to-marine export, the effects of photochemistry were also evaluated in the context of the bio-reactivity of pyDOM. Ultrahigh resolution mass spectrometry revealed that, after incubation, many different (both aromatic and aliphatic) compounds were degraded, and new labile compounds, 22–40 % of which were peptide-like, were produced. This indicated that a portion of pyDOM has been labilized into microbial biomass during the incubations. Fluorescence excitation-emission matrix spectra revealed that some fraction of these new molecules is associated with fluorophores from proteinaceous and/or autochthonous/microbial biomass origin. Two-dimensional 1H-1H total correlation NMR spectroscopy identified a peptidoglycan-like backbone within the microbially produced compounds. These results are consistent with previous observations of nitrogen from peptidoglycans within the soil and ocean nitrogen cycles. Interestingly, the exact nature of the bio-produced organic matter was found to vary drastically among samples indicating that the used microbial consortium may produce different exudates based on the composition of the initial pyDOM. Another potential explanation for the vast diversity of molecules is that microbes only consume low molecular weight compounds, but they also produce reactive oxygen species (ROS), which initiate oxidative and recombination reactions that produce new molecules. The observed microbially-mediated diversification of pyDOM suggests that pyDOM contributes to the observed large complexity of natural organic matter. More broadly, pyDOM can be substrate for microbial growth and be incorporated in environmental food webs.

Air‐Sea Ammonia Fluxes Calculated From High‐Resolution Summertime Observations Across the Atlantic Southern Ocean

AbstractOceanic ammonia emissions are the largest natural source of ammonia globally, but the magnitude of the air‐sea flux in remote regions with minimal human influence remains uncertain. Here, we measured the concentration of surface ocean ammonium and atmospheric ammonia gas every two hours across a latitudinal transect (34.5°S to 61°S) of the Atlantic Southern Ocean during summer. Surface ocean ammonium concentrations ranged from undetectable to 0.36 µM and ammonia gas concentrations ranged from 0.6 to 5.1 nmol m−3. Calculated ammonia fluxes ranged from −2.8 to −75 pmol m−2 s−1, and were consistently from the atmosphere into the ocean, even in regions where surface ocean ammonium concentrations were relatively high. As expected, temperature was the dominant control on the air‐sea ammonia flux across the latitudinal transect. However, a sensitivity analysis suggests that seasonality in the surface Southern Ocean nitrogen cycle may have a major influence on the direction of the ammonia flux.

Continuous Chemiluminescence Measurements of Dissolved Nitric Oxide (NO) and Nitrogen Dioxide (NO2) in the Ocean Surface Layer of the East China Sea

Nitric oxide (NO) is a short-lived intermediate of the oceanic nitrogen cycle, and it is produced by biological and photochemical processes in the ocean. Nitrogen dioxide (NO2) is a reactive atmospheric compound which has not been determined in the ocean so far. Here, we present the setup and validation of a novel continuous underway measurement system to measure dissolved NO and NO2 in the surface ocean. The system consists of a seawater/gas equilibration component coupled to a chemiluminescence detector. It was successfully deployed during a 12 day cruise to the East China Sea in May 2018. Dissolved NO and NO2 surface concentrations ranged from <limit of detection (LOD) to 98 × 10-12 mol L-1 and <LOD to 83 × 10-12 mol L-1, respectively. The ECS was supersaturated with NO but significantly undersaturated with NO2, indicating that the surface waters were a source for atmospheric NO but a sink for atmospheric NO2 at the time of our measurements.

Development of the MIROC-ES2L Earth system model and the evaluation of biogeochemical processes and feedbacks

Abstract. This article describes the new Earth system model (ESM), the Model for Interdisciplinary Research on Climate, Earth System version 2 for Long-term simulations (MIROC-ES2L), using a state-of-the-art climate model as the physical core. This model embeds a terrestrial biogeochemical component with explicit carbon–nitrogen interaction to account for soil nutrient control on plant growth and the land carbon sink. The model's ocean biogeochemical component is largely updated to simulate the biogeochemical cycles of carbon, nitrogen, phosphorus, iron, and oxygen such that oceanic primary productivity can be controlled by multiple nutrient limitations. The ocean nitrogen cycle is coupled with the land component via river discharge processes, and external inputs of iron from pyrogenic and lithogenic sources are considered. Comparison of a historical simulation with observation studies showed that the model could reproduce the transient global climate change and carbon cycle as well as the observed large-scale spatial patterns of the land carbon cycle and upper-ocean biogeochemistry. The model demonstrated historical human perturbation of the nitrogen cycle through land use and agriculture and simulated the resultant impact on the terrestrial carbon cycle. Sensitivity analyses under preindustrial conditions revealed that the simulated ocean biogeochemistry could be altered regionally (and substantially) by nutrient input from the atmosphere and rivers. Based on an idealized experiment in which CO2 was prescribed to increase at a rate of 1 % yr−1, the transient climate response (TCR) is estimated to be 1.5 K, i.e., approximately 70 % of that from our previous ESM used in the Coupled Model Intercomparison Project Phase 5 (CMIP5). The cumulative airborne fraction (AF) is also reduced by 15 % because of the intensified land carbon sink, which results in an airborne fraction close to the multimodel mean of the CMIP5 ESMs. The transient climate response to cumulative carbon emissions (TCRE) is 1.3 K EgC−1, i.e., slightly smaller than the average of the CMIP5 ESMs, which suggests that “optimistic” future climate projections will be made by the model. This model and the simulation results contribute to CMIP6. The MIROC-ES2L could further improve our understanding of climate–biogeochemical interaction mechanisms, projections of future environmental changes, and exploration of our future options regarding sustainable development by evolving the processes of climate, biogeochemistry, and human activities in a holistic and interactive manner.

Bottom trawling reduces benthic denitrification and has the potential to influence the global nitrogen cycle

AbstractBottom trawling and eutrophication are large stressors that are critically coupled. Here we show, using a before‐after control‐effect design, the significant reduction in denitrification as a result of experimental bottom trawling in a shallow coastal system. Trawl disturbance destroys the complex three‐dimensional redox structures in surface sediments that maximize denitrification potential, resulting in up to a 50% reduction in net denitrification. The decrease in net denitrification also increased after each trawling event suggesting a declining resilience to trawling and eutrophication. Bottom trawling occurs at such a large scale that it could result in significant amounts of nitrogen being retained on the continental shelf. As such, impacts on the global ocean nitrogen cycle and associated eutrophication should be counted among the many negative consequences of extensive seafloor trawling.