Marine Nitrogen Cycle Research Articles

Marine nitrogen cycling dynamics under altering redox conditions: Insights from deposition of sapropels S1 and the ambiguous S2 in the Eastern Mediterranean Sea

The eastern Mediterranean Sea (EMS) sedimentary record is periodically interspersed with organic-rich ‘sapropel’ layers. Sapropels are characteristic of basin-wide anoxic events, triggered by precession-forced insolation maxima. Relatively subdued insolation maxima, however, are not always expressed as distinct sapropel events. The EMS sedimentary record is thus useful to investigate feedbacks between marine anoxia and the nitrogen (N) cycle and offers an analogue for modern deoxygenation and past oceanic anoxic events. To this end, we investigated a ∼68 kyr sedimentary record from the EMS containing the well-established sapropel S1 (deposited in two phases: S1a [∼10.5–8.5 ka BP] and S1b [∼7.8–6.1 ka BP]) and sediments timed to the ambiguous S2 sapropel (∼53 ka BP). We used lipid biomarkers of microorganisms to reconstruct key N-cycle components: (1) anaerobic ammonium oxidation (anammox) using ladderanes and a stereoisomer of bacteriohopanetetrol (BHT-x), (2) dinitrogen gas (N2) fixation using heterocyte glycolipids, and (3) nitrification by Thaumarchaeota using crenarchaeol. Additionally, benthic foraminifera and trace metals (U, Mo, Mn) were used to reconstruct redox conditions. During S1a, abundances of Thaumarchaeota increased, likely promoted by elevated high-nutrient freshwater discharge. At this time, a combination of phosphorus supply and intensified loss of bioavailable N via water column anammox, may have reinforced anoxia by favoring diatom-diazotroph associations. During S1b, anammox is equally intense. Yet, no positive feedback on N2-fixation is observed, likely because diazotrophs were phosphorus limited. Instead, anammox may have provided negative feedback on anoxia by quenching primary production. Ladderanes suggest additional episodes of anammox between ∼69 to 39 cal ka BP, corresponding to brief periods of water column deoxygenation. Anoxia likely occurred at the sediment–water interface in S2-timed sediments (53–51 cal ka BP). During these episodes, ladderanes co-occur with the later eluting BHT-34R stereoisomer. δ13CBHT-34R indicate an anammox source, potentially synthesized by marine sedimentary anammox bacteria. No corresponding increase in diatom-diazotroph associations is observed, likely due to the oligotrophic conditions and the limited effect of sedimentary anammox on N-availability in the euphotic zone. Our results highlight various modes of operation of the N-cycle at different degrees of deoxygenation, which depend amongst others on nutrient-availability and the niche-segregation of N-loss and N2-fixating microorganisms.

An example of the Middle to Late Devonian marine nitrogen cycle from mudstones of the Horn River Group, Northwest Territories, Canada

The Middle to Late Devonian was characterized by the widespread deposition of organic-rich mudstone units and successive biotic crises and anoxic events in the marine realm, the cause of which remains debated and requires constraints from associated marine conditions. This study provides an example of the marine nitrogen cycle throughout the late Eifelian to middle Frasnian anoxic pulses. We present new and previously published organic whole-rock N (δ15Nbulk) and carbon (δ13Corg) isotopic datasets from organic-rich mudstone units of the Horn River Group (Canol and Hare Indian Formations) and overlying Imperial Formation in the Central Mackenzie Valley, Northwest Territories, Canada. In the ConocoPhillips Mirror Lake N-20 core, δ13Corg ranges from −31.0 ‰ to −24.3 ‰ with δ15Nbulk from −3.8 ‰ to +1.9 ‰, whereas the Husky Little Bear N-09 core is characterized by δ13Corg from −31.0 ‰ to −27.2 ‰ and δ15Nbulk from −2.0 ‰ to +5.9 ‰. The N isotopic signatures near 0 ‰ and a lack of δ15Nbulk – δ13Corg relationship are characteristic of N2 fixation by primary producers. Regular oscillations in δ15Nbulk are interpreted as the product of episodic, mild oxygenation events. Together, our δ13Corg and δ15Nbulk results suggest that locally, N2 fixation was the dominant source of N for primary producers in the late Eifelian to middle Frasnian, despite fluctuations in δ13Corg and global marine paleoredox. These findings contribute to our understanding of the nitrogen speciation and bioavailability associated with anoxic events, biotic crises, and widespread organic carbon burial in the Eifelian to Frasnian oceans.

Emissions of Nitric Oxide from Photochemical and Microbial Processes in Coastal Waters of the Yellow and East China Seas.

Nitric oxide (NO) is an atmospheric pollutant and climate forcer as well as a key intermediary in the marine nitrogen cycle, but the ocean's NO contribution and production mechanisms remain unclear. Here, high-resolution NO observations were conducted simultaneously in the surface ocean and the lower atmosphere of the Yellow Sea and the East China Sea; moreover, NO production from photolysis and microbial processes was analyzed. The NO sea-air exchange showed uneven distributions (RSD = 349.1%) with an average flux of 5.3 ± 18.5 × 10-17 mol cm-2 s-1. In coastal waters where nitrite photolysis was the predominant source (89.0%), NO concentrations were remarkably higher (84.7%) than the overall average of the study area. The NO from archaeal nitrification accounted for 52.8% of all microbial production (11.0%). We also examined the relationship between gaseous NO and ozone which helped identify sources of atmospheric NO. The sea-to-air flux of NO in coastal waters was narrowed by contaminated air with elevated NO concentrations. These findings indicate that the emissions of NO from coastal waters, mainly controlled by reactive nitrogen inputs, will increase with the reduced terrestrial NO discharge.

Microplastics trigger the Matthew effect on nitrogen assimilation in marine diatoms at an environmentally relevant concentration

Microplastics (MPs, diameter <5 mm) are widely distributed on Earth, especially in the oceans. Diatoms account for ∼40% of marine primary productivity and affect the global biogeochemical cycles of macroelements. However, the effects of MPs on marine nitrogen cycling remain poorly understood, particularly comparisons between nitrogen-replete and nitrogen-limited conditions. We found that MPs trigger the Matthew effect on nitrogen assimilation in diatoms, where MPs inhibited nitrogen assimilation under nitrogen-limited conditions while enhancing nitrogen metabolism under nitrogen-replete conditions in Phaeodactylum tricornutum. Nitrate reductase (NR) and nitrite reductase (NIR) are upregulated, but nitrate transporter (NRT) and glutamine synthetase (GS) are downregulated by MPs under nitrogen-limited conditions. In contrast, NR, NIR, and GS are all upregulated by MPs under nitrogen-replete conditions. MPs accelerate nitrogen anabolic processes with an increase in the accumulation of carbohydrates by 80.7 ± 7.9% and enhance the activities of key nitrogen-metabolizing enzymes (8.20–44.90%) under nitrogen-replete conditions. In contrast, the abundance of carbohydrates decreases by 22.0–34.4%, and NRT activity is inhibited by 79.0–86.5% in nitrogen-limited algae exposed to MPs. Metabolomic and transcriptomic analyses were performed to further explore the molecular mechanisms of reprogrammed nitrogen assimilation, including carbon metabolism, nitrogen transport and ammonia assimilation. The aforementioned spatial redistribution (e.g., the Matthew effect between nitrogen-replete and -limited conditions) of nitrogen assimilation highlights the potential risks of MP contamination in the ocean.

North-south differences in hypoxia and nitrogen cycle of the East China Sea over the last century indicated by sedimentary bacteriohopanepolyols

Coastal hypoxic zones are expanding due to anthropogenic activities and climate changes. Increased hypoxia has greatly affected the marine nitrogen cycle. There are two typical seasonal hypoxic core areas in the north and south of the East China Sea (ECS). However, the interannual variations, north-south differences in hypoxia and their effects on the nitrogen cycle are still poorly understood. This study investigated bacteriohopanepolyols (BHPs), especially newly developed marine anammox bacteria and hypoxic biomarker bacteriohopanetetrol stereoisomer (BHT-x) and cyanobacterial nitrogen fixation biomarker 2-methyl BHT in two sediment cores, aiming to reconstruct hypoxia and nitrogen cycle records in the northern and southern hypoxic zones of the ECS over the last century. The results showed that BHT-x ratio in northern core 3100–2 presented higher values since 1960s (>0.2) and increased after 2000 (0.25–0.57), while it decreased in southern core DH6–1 after 2000 (0.03–0.11), indicating the continuously intensified hypoxia in the northern area and significantly weakened hypoxia in the southern area. Hypoxia is mainly associated with organic matter degradation and seawater stratification. The similar temporal increases of BHT-x ratios and marine-derived organic matter (MOM) contents in core 3100–2 suggest that eutrophication-driven MOM degradation is the main reason for hypoxia formation, which is exacerbated by strong water stratification there. Differently, although MOM contents still increased in core DH6–1 after 2000, the hypoxia weakened and even disappeared due to lessened stratification. In addition, the temporal variations of BHT-x and 2-methyl BHT in both cores indicated that anammox and nitrogen fixation simultaneously increased during periods of hypoxia. The co-variation suggests that hypoxia promotes anammox, and nitrogen loss may induce an increase in nitrogen fixation to compensate for nitrogen availability.

Cyanate dynamics under algal blooms and sediment resuspension events in a shallow micro-tidal estuary in the lower Chesapeake Bay

Although an emerging component in the marine nitrogen (N) cycle, cyanate concentrations and cycling have not been examined in estuarine systems to date. To better understand controls on cyanate concentrations in estuaries, time series data of cyanate and nutrient concentrations in the Lafayette River, a micro-tidal, sub-tributary of the lower Chesapeake Bay, were examined between June and September 2018, and May to September 2019. Cyanate concentrations ranged from near the detection limit (0.4 nmol L−1) to 82.9 nmol L−1 in 2018, and 6.8–207.4 nmol L−1 in 2019. Variations in cyanate concentrations were highly correlated with chlorophyll biomass in the summer, biomass degradation in early fall, and sediment resuspension that occurred in response to meteorological forcing. Cyanate concentrations increased after Chl a concentrations decreased suggesting algal decomposition as a source of cyanate. High cyanate concentrations in bottom waters, corresponded to wind-induced sediment resuspension events in the Lafayette River, again suggesting organic matter decomposition as a source of cyanate. Cyanate concentrations in sediment pore water varied between years; in summer 2018, cyanate concentrations were up to 150 nmol L−1, while in 2019, they were three times lower. To confirm an algal source for cyanate, a degradation experiment was conducted using Lafayette River water collected during a bloom of Margalefidinium polykrikoides in 2018. In dark incubation bottles, cyanate was one of the first labile organic nitrogen products produced, suggesting the contention that high concentrations of cyanate in late summer and fall were the result of organic matter decomposition. Neither cyanate nor ammonium accumulated in light bottles suggesting production and uptake are tightly coupled and microbes have a high affinity for cyanate in the light. In dark bottles, cyanate production rates were 6.8 nmol L−1 d−1, while microbial removal rates during the late phase of degradation were 1.5 nmol L−1 d−1, suggesting that cyanate may not be a preferred nitrogen substrate for microbes (including nitrifiers) in dark bottles or that microbes have a lower affinity for cyanate in the dark, allowing cyanate to reach steady state at concentrations greater than the detection limit.

Nitrogen cycling during the peak Cambrian explosion

Previous studies on early-Cambrian nitrogen (N) cycle suggested a modern-type nitrogen reservoir with elevated nitrate (NO3−) availability in a globally oxygenated ocean for the Cambrian explosion. However, recent marine redox studies on the Chengjiang Biota revealed a stratified ocean with anoxic deeper waters for the peak Cambrian explosion, asking for further works on the N cycling during this critical period. Chengjiang and Qingjiang biotas are the two top-rank Burgess Shale-type fossil Lagerstätten that recorded peaked metazoan diversification of the Cambrian explosion at different paleoenvironmental regimes during the Cambrian Age 3. In this study, we conducted a detailed geochemical investigation of the early-Cambrian Shuijingtuo Formation from two drill cores containing the Qingjiang Biota in Hubei Province, South China. The results suggest progressive oxygenation of the depositional environment from euxinic to fluctuating, suboxic conditions with a low chemocline in the water column, accompanied by an increase of δ15N values from <−4‰ to between −2‰ and +1‰. Especially, both event and background beds of the Qingjiang Biota are characterized by δ15N values ranging from −1‰ to +1‰. Comparative analysis reveals consistent variations of δ15N signatures and marine redox states at the Qingjiang and Chengjiang areas during the Cambrian Age 3, with the latter exhibiting systematically higher δ15N values ranging mainly from +1‰ to +2‰ at the biota interval. Taken together, the δ15N signatures of the Qingjiang and Chengjiang biotas can be most parsimoniously explained by prevailed aerobic marine nitrogen cycling featured by active biological N2 fixation and a limited nitrate reservoir in the shallow ocean. The nitrogen isotope gradient between the Qingjiang and Chengjiang areas may be reflective of varied NO3− availability in seawater. These findings contribute to a clearer biogeochemical context that supported prosperous marine ecosystems during the peak Cambrian explosion, and provide new insights on spatiotemporal evolution of the marine nitrogen cycle during this critical period.

Insights from Fossil-Bound Nitrogen Isotopes in Diatoms, Foraminifera, and Corals.

Nitrogen is a major limiting element for biological productivity, and thus understanding past variations in nitrogen cycling is central to understanding past and future ocean biogeochemical cycling, global climate cycles, and biodiversity. Organic nitrogen encapsulated in fossil biominerals is generally protected from alteration, making it an important archive of the marine nitrogen cycle on seasonal to million-year timescales. The isotopic composition of fossil-bound nitrogen reflects variations in the large-scale nitrogen inventory, local sources and processing, and ecological and physiological traits of organisms. The ability to measure trace amounts of fossil-bound nitrogen has expanded with recent method developments. In this article, we review the foundations and ground truthing for three important fossil-bound proxy types: diatoms, foraminifera, and corals. We highlight their utility with examples of high-resolution evidence for anthropogenic inputs of nitrogen to the oceans, glacial-interglacial-scale assessments of nitrogen inventory change, and evidence for enhanced CO2 drawdown in the high-latitude ocean. Future directions include expanded method development, characterization of ecological and physiological variation, and exploration of extended timescales to push reconstructions further back in Earth's history.

A weak cold eddy influencing nitrogen form in the western part of the South China Sea

Nitrogen is an essential element for all life and it exhibits in three major dissolved inorganic chemical forms to participate in the marine nitrogen cycling. In this study, a transect through the weak cold eddy was undertaken near Xisha Islands of the South China Sea (SCS) in summer of 2009. There was a trend of decreased temperature inside the eddy, while PO4 and NO3 increased in comparison to outside the eddy The primary nitrite maximum (PNM) was found in the depth of about 50 m in the in-situ samples. Phytoplankton excretion has advantage of PNM formation and maintenance in the euphotic layer. Additionally, nitrate anomaly (N*) were positive, which implies that nitrogen fixation occurs in the surface water in the studying period. These results indicate that cold eddy has influence on regulating inorganic nitrogen biogeochemistry in the studying area during sampling period.

Dual isotopes of nitrite in the Amundsen Sea in summer

Nitrite (NO2−) is a key intermediate in the nitrogen (N) cycle, and its transformation is accomplished by microbial communities. However, due to few studies on the nitrite cycle, a clear assessment of the contribution to the marine biogeochemical cycle is missing. Here, we present data on nitrogen and oxygen isotopic composition of NO2− in the Amundsen Sea in summer, and explore the biogeochemical processes that influence the NO2− cycle. Extremely low δ15NNO2 and abnormally high δ18ONO2 were found in the upper waters of the Amundsen Sea, with δ15NNO2 as low as −58.4 ‰ and δ18ONO2 as high as 44.4 ‰. Enzymatic isotopic exchange reactions between nitrate and nitrite have been proposed to be responsible for these isotopic anomalies. The mirror-symmetrical variation between δ15NNO2 and δ18ONO2 suggests that the isotopic fractionation effects of nitrogen and oxygen are opposite in isotope exchange reactions. Dual isotopes of nitrite indicate that ammonia oxidation is the main source of nitrite, thus nitrification plays an important role in the formation of primary nitrite maximum in the upper Amundsen Sea. The nitrogen and oxygen isotopic compositions of nitrite provide support for clarifying multiple processes of marine nitrogen cycle.

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.

A Synthesis of Viral Contribution to Marine Nitrogen Cycling.

Nitrogen is an essential component of major cellular macromolecules, such as DNA and proteins. Its bioavailability has a fundamental influence on the primary production of both terrestrial and oceanic ecosystems. Diverse marine microbes consume nitrogen, while only a limited taxon could replenish it, leaving nitrogen one of the most deficient nutrients in the ocean. A variety of microbes are involved in complex biogeochemical transformations of nitrogen compounds, and their ecological functions might be regulated by viruses in different manners. First and foremost, viruses drive marine nitrogen flow via host cell lysis, releasing abundant organic nitrogen into the surrounding environment. Secondly, viruses can also participate in the marine nitrogen cycle by expressing auxiliary metabolic genes (AMGs) to modulate host nitrogen metabolic pathways, such as nitrification, denitrification, anammox, and nitrogen transmembrane transport. Additionally, viruses also serve as a considerable reservoir of nitrogen element. The efficient turnover of viruses fundamentally promotes nitrogen flow in the oceans. In this review, we summarize viral contributions in the marine nitrogen cycling in different aspects and discuss challenges and issues based on recent discoveries of novel viruses involved in different processes of nitrogen biotransformation.

Simulated Future Trends in Marine Nitrogen Fixation Are Sensitive to Model Iron Implementation

AbstractBiological nitrogen fixation is an important oceanic nitrogen source, potentially stabilizing marine fertility in an increasingly stratified and nutrient‐depleted ocean. Iron limitation of low latitude primary producers has been previously demonstrated to affect simulated regional ecosystem responses to climate warming or nitrogen cycle perturbation. Here we use three biogeochemical models that vary in their representation of the iron cycle to estimate change in the marine nitrogen cycle under a high CO2 emissions future scenario (RCP8.5). The first model neglects explicit iron effects on biology (NoFe), the second utilizes prescribed, seasonally cyclic iron concentrations and associated limitation factors (FeMask), and the third contains a fully dynamic iron cycle (FeDyn). Models were calibrated using observed fields to produce near‐equivalent nutrient and oxygen fits, with productivity ranging from 49 to 75 Pg C yr−1. Global marine nitrogen fixation increases by 71.1% with respect to the preindustrial value by the year 2100 in NoFe, while it remains stable (0.7% decrease in FeMask and 0.3% increase in FeDyn) in explicit iron models. The mitigation of global nitrogen fixation trend in the models that include a representation of iron originates in the Eastern boundary upwelling zones, where the bottom‐up control of iron limitation reduces export production with warming, which shrinks the oxygen deficient volume, and reduces denitrification. Warming‐induced trends in the oxygen deficient volume in the upwelling zones have a cascading effect on the global nitrogen cycle, just as they have previously been shown to affect tropical net primary production.

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.

A High‐Resolution Atlas of the Eastern Tropical Pacific Oxygen Deficient Zones

AbstractOxygen deficient zones (ODZs) are important biogeochemical provinces of the global oceans wherein standing dissolved oxygen concentrations decrease to nanomolar levels. Despite their confinement, these regions are disproportionally important to the ocean's role in modulating Earth's climate through the interactions between the marine nitrogen cycle and that of carbon. Moreover, the spatial domain of low oxygen regions of the ocean is predicted to change as a consequence of ocean warming, increased stratification, and changes in circulation and productivity. However, the expanse of the modern ODZs is poorly resolved due to a dearth of direct sampling compounded with errors that arise in the processing and gridding of the sparse measurements that do exist. Here, we take a novel approach to map the horizontal and vertical extent of the two major ODZs of the eastern tropical Pacific via analysis of meter‐scale resolution electrode sensors from both ship casts and Argo profiles, rather than from discretized bottle measurements. The resulting three‐dimensional data product is based on a compendium of nearly 15 million measurements taken across three decades and provides the precise locations of low oxygen water, elucidating the ODZs' three‐dimensional structures. It can be utilized by researchers to validate models, plan cruise occupations, and as a comparison for future change. Calculations made with this high‐resolution atlas also provide the volumes, layers of maximal areal extent, and other descriptive statistics for both Pacific ODZs. Finally, the atlas reveals fine‐scale features of oxygenated water mass intrusions and regional differences across these anoxic zones.

A case for addressing the unresolved role of permeable shelf sediments in ocean denitrification

AbstractThe marine nitrogen cycle is a main driver of ocean productivity and affects global climate. Despite decades of study, we still have an incomplete understanding of the role of the marine nitrogen cycle in the Earth system. While marine sediments play a major role in nitrogen cycling in the ocean, magnitudes and mechanisms are largely unconstrained. Recent research suggests that permeable sandy sediments on the highly energetic and dynamic continental shelf are key components of the marine nitrogen cycle, but data to quantify their contribution are lacking. Here, we use insights from measurements and modeling studies to substantiate the hypothesis that shelf sediments are an overlooked driver of the marine nitrogen cycle. Specifically, we propose that permeable shelf sediments are sites for intense nitrogen conversions and suggest a three‐pronged approach to address unresolved controversies.

Riverine nitrogen supply to the global ocean and its limited impact on global marine primary production: a feedback study using an Earth system model

Abstract. A common notion is that negative feedbacks stabilize the natural marine nitrogen inventory. Recent modeling studies have shown, however, some potential for localized positive feedbacks leading to substantial nitrogen losses in regions where nitrogen fixation and denitrification occur in proximity to each other. Here we include dissolved nitrogen from river discharge in a global 3-D ocean biogeochemistry model and study the effects on near-coastal and remote-open-ocean biogeochemistry. We find that at a steady state the biogeochemical feedbacks in the marine nitrogen cycle, nitrogen input from biological N2 fixation, and nitrogen loss via denitrification mostly compensate for the imposed yearly addition of 22.8 to 45.6 Tg of riverine nitrogen and limit the impact on global marine productivity to &lt; 2 %. Global experiments that regionally isolate river nutrient input show that the sign and strength of the feedbacks depend on the location of the river discharge and the oxygen status of the receiving marine environment. Marine productivity generally increases in proximity to the nitrogen input, but we also find a decline in productivity in the modeled Bay of Bengal and near the mouth of the Amazon River. While most of the changes are located in shelf and near-coastal oceans, nitrogen supply from the rivers can impact the open ocean, due to feedbacks or knock-on effects.

Diversity, prevalence, and expression of cyanase genes (cynS) in planktonic marine microorganisms

Cyanate is utilized by many microbes as an organic nitrogen source. The key enzyme for cyanate metabolism is cyanase, converting cyanate to ammonium and carbon dioxide. Although the cyanase gene cynS has been identified in many species, the diversity, prevalence, and expression of cynS in marine microbial communities remains poorly understood. Here, based on the full-length cDNA sequence of a dinoflagellate cynS and 260 homologs across the tree of life, we extend the conserved nature of cyanases by the identification of additional ultra-conserved residues as part of the modeled holoenzyme structure. Our phylogenetic analysis showed that horizontal gene transfer of cynS appears to be more prominent than previously reported for bacteria, archaea, chlorophytes, and metazoans. Quantitative analyses of marine planktonic metagenomes revealed that cynS is as prevalent as ureC (urease subunit alpha), suggesting that cyanate plays an important role in nitrogen metabolism of marine microbes. Highly abundant cynS transcripts from phytoplankton and nitrite-oxidizing bacteria identified in global ocean metatranscriptomes indicate that cyanases potentially occupy a key position in the marine nitrogen cycle by facilitating photosynthetic assimilation of organic N and its remineralisation to NO3 by the activity of nitrifying bacteria.

Exploring changes in bacterioplankton community structure in response to tannic acid, a major component of mangrove litterfall of Sundarbans mangrove ecosystem: a laboratory mesocosm approach.

Tannic acid is a secondary compound produced by vascular plants and is a major component of mangrove litterfall. Tannic acid is water soluble, leaches out from mangrove litterfall and contributes to DOC and DON pools in adjacent estuaries. About 50% of the litterfall may be degraded and channelized into the marine microbial loop. The influence of tannic acid on bacterioplankton community structure was tested by setting up laboratory-based barrel experiments. Estuarine water from Stn3 of Sundarbans Biological Observatory Time Series(SBOTS) was enriched with tannic acid, and the change in concentration of dissolved nutrients was determined on a daily basis over a span of 15 days. Concentrations of tannic acid, gallic acid and other dissolved nutrients such as nitrate and ortho-phosphate were determined using a UV-Vis spectrophotometer. Tannic acid significantly affected the concentrations of gallic acid and dissolved nitrate in the barrels. Degradation of tannic acid was tracked by a decrease in concentration of tannic acid and generation of gallic acid. The influence of tannic acid on bacterioplankton community structure was analysed on the start (day 0), intermediate (day 3, day 5, day 7 and day 9) and end (day 15) of the experiment. Bacterioplankton community structure was elucidated by sequencing the V3-V4 region of 16S ribosomal RNA on an Illumina MiSeq platform. Proteobacteria was found to be the most dominant bacterial phylum in control and tannic acid-enriched barrels (barrels 1 and 2) on day 0. With the progression of experiment, the abundance of Proteobacteria altered significantly in the control barrel indicating the possible role of this phylum in the breakdown of tannic acid within estuarine mangroves. The abundance of Proteobacteria in the tannic acid-enriched barrels remained high, indicating that members of Proteobacteria may be capable of using tannic acid as a source of carbon and nitrogen. Tannic acid appeared to inhibit most of the other bacterioplankton phyla including Actinobacteria, Acidobacteria and Verrucomicrobia that existed in large abundance in the control barrel on day 15 but were almost absent in the tannic acid-enriched barrels. At class level, Bacteroides was found to be present in highest abundance in the tannic acid-enriched barrels. Tannic acid appeared to strongly influence the abundant bacterioplankton phyla and families as indicated by Pearson's correlation coefficient and non-metric multidimensional scaling ordination plots. Gallic acid is one of the final products of tannic acid degradation. Breakdown of tannic acid could influence the marine nitrogen and carbon cycling by releasing DON and DOC, respectively, into the adjacent estuaries. Information of breakdown and remineralization of components of litterfall such as tannic acid would also be important for calculation of carbon and nitrogen budgets of coastal ecosystems including in mangroves.

The occurrence of 2‐methylhopanoids in modern bacteria and the geological record

The 2-methylhopanes (2-MeHops) are molecular fossils of 2-methylbacteriohopanepolyols (2-MeBHPs) and among the oldest biomarkers on Earth. However, these biomarkers' specific sources are currently unexplained, including whether they reflect an expansion of marine cyanobacteria. Here, we study the occurrence of 2-MeBHPs and the genes involved in their synthesis in modern bacteria and explore the occurrence of 2-MeHops in the geological record. We find that the gene responsible for 2-MeBHP synthesis (hpnP) is widespread in cyano- and ⍺-proteobacteria, but absent or very limited in other classes/phyla of bacteria. This result is consistent with the dominance of 2-MeBHP in cyano- and ⍺-proteobacterial cultures. The review of their geological occurrence indicates that 2-MeHops are found from the Paleoproterozoic onwards, although some Precambrian samples might be biased by drilling contamination. During the Phanerozoic, high 2-MeHops' relative abundances (index >15%) are associated with climatic and biogeochemical perturbations such as the Permo/Triassic boundary and the Oceanic Anoxic Events. We analyzed the modern habitat of all hpnP-containing bacteria and find that the only one species coming from an undisputed open marine habitat is an ⍺-proteobacterium acting upon the marine nitrogen cycle. Although organisms can change their habitat in response to environmental stress and evolutionary pressure, we speculate that the high sedimentary 2-MeHops' occurrence observed during the Phanerozoic reflect ⍺-proteobacteria expansion and marine N-cycle perturbations in response to climatic and environmental change.