Biological CO2 Sequestration Research Articles

Electrical-biological hybrid system for carbon efficient isobutanol production

We have developed an electrical-biological hybrid system wherein an engineered microorganism consumes electrocatalytically produced formate from CO2 to supplement the bioproduction of isobutanol, a valuable fuel chemical. Biological CO2 sequestration is notoriously slow compared to electrochemical CO2 reduction, while electrochemical methods struggle to generate carbon-carbon bonds which readily form in biological systems. A hybrid system provides a promising method for combining the benefits of both biology and electrochemistry. Previously, Escherichia coli was engineered to assimilate formate and CO2 in central metabolism using the reductive glycine pathway. In this work, we have shown that chemical production in E. coli can benefit from single carbon substrates when equipped with the RGP. By installing the RGP and the isobutanol biosynthetic pathway into E. coli and by further genetic modifications, we have generated a strain of E. coli that can consume formate and produce isobutanol at a yield of >100% of theoretical maximum from glucose. Our results demonstrate that carbon produced from electrocatalytically reduced CO2 can bolster chemical production in E. coli. This study shows that E. coli can be engineered towards carbon efficient methods of chemical production.

Review and Perspectives of Emerging Green Technology for the Sequestration of Carbon Dioxide into Value-Added Products: An Intensifying Development

Global warming has emerged as a major problem for humanity in this anthropocentric period. The constantly rising amount of carbon dioxide (CO2) in the atmosphere has caused global warming, which has caused a series of changes in the Earth’s climate and weather systems. Rapid anthropogenic activity such as industrialization and urbanization has led to increasing the CO2 level in the environment. CO2 is captured and utilized as the carbon source for synthesizing various value-added products through physical and chemical methods. However, the physical and chemical techniques do not provide an ecologically friendly or cost-effective procedure. Some advanced innovative, environmentally friendly green methods based on photosynthesis, microbial electrosynthesis, and gas fermentation processes can help to clean the atmosphere by sequestering CO2 and producing value-added products from CO2. This approach has the potential to reduce greenhouse gas emissions, increase productivity, and create new economic opportunities. In this review, we found that the photosynthetic process has a high capacity for CO2 sequestration. In contrast, the microbial electrosynthesis and gas fermentation processes have a high efficiency of value-added product synthesis. These biological CO2 sequestration technologies contribute to the sustainability of the global carbon process. However, each method has its own strengths and weaknesses, and the choice of method will depend on the specific goals and resources of a sequestration project. This report also defined the various influencing factors of photosynthetic, microbial electrosynthesis, and gas fermentation processes for optimized development of these technologies, while challenges and future directions are also discussed.

Integrated Approach for Carbon Sequestration and Wastewater Treatment Using Algal–Bacterial Consortia: Opportunities and Challenges

Increasing concentrations of carbon dioxide (CO2), one of the important greenhouse gases, due to combustion of fossil fuels, particularly burning coal, have become the major cause for global warming. As a consequence, many research programs on CO2 management (capture, storage, and sequestration) are being highlighted. Biological sequestration of CO2 by algae is gaining importance, as it makes use of the photosynthetic capability of these aquatic species to efficiently capture CO2 emitted from various industries and converting it into algal biomass as well as a wide range of metabolites such as polysaccharides, amino acids, fatty acids, pigments, and vitamins. In addition, their ability to thrive in rugged conditions such as seawater, contaminated lakes, and even in certain industrial wastewaters containing high organic and inorganic nutrients loads, has attracted the attention of researchers to integrate carbon capture and wastewater treatment. Algae offer a simple solution to tertiary treatments due to their nutrient removal efficiency, particularly inorganic nitrogen and phosphorus uptake. The algal–bacterial energy nexus is an important strategy capable of removing pollutants from wastewater in a synergistic manner. This review article highlights the mechanism involved in biological fixation of CO2 by microalgae, their cultivation systems, factors influencing algal cultivation in wastewater and CO2 uptake, the effect of co-cultivation of algae and bacteria in wastewater treatment systems, and challenges and opportunities.

Analyzing and optimizing the carbon utilization and lipid yield at different light intensities for Scenedesmus arcuatus

Microalgae, the photosynthetic microorganism growing abundantly in marine and aquatic ecosystems, are potential source for biological sequestration of CO2. The carbon uptake differs in the presence of other nutrients, light intensity etc. The biomass yield of Scenedesmus arcuatus var capitatus was studied based on the Face Centred Central Composite design (FCCD) of Response Surface Methodology (RSM) for nitrate, phosphate and carbonate under different conditions (laboratory, room and sunlight conditions). Various pre-treatments (osmotic shock, autoclaving, microwave and ultrasonication) were employed to find the best method for maximum lipid yield. The biomass yield reached a maximum of 1 g/L under sunlight conditions of nitrate concentration 500 ppm and carbonate 2000 ppm. The laboratory conditions resulted in a biomass yield of 0.59 g/L at 500 ppm nitrate, 1000 ppm carbonate and 250 ppm phosphate. Under room conditions, the yield was very low (0.11 g/L). Osmotic shock resulted in higher lipid yield than the other pre-treatment methods. The ability of Scenedesmus arcuatus to uptake high carbon under sunlight conditions and to adapt to high light intensity and fluctuations in light intensity concludes that this species is suitable for large-scale open pond cultivation for CO2 sequestration and production of metabolites.

Nitrogen-limited cultivation of locally isolated Desmodesmus sp. for sequestration of CO2 from simulated cement flue gas and generation of feedstock for biofuel production

Biological CO2 sequestration from flue gas using microalgae has emerged as a promising alternative to conventional carbon capture technologies, since it concurrently generates valuable biomass which can be utilized to produce biofuels. In the current study, the locally isolated microalga Desmodesmus sp. was utilized for sequestration of CO2 from gas mixtures simulating undiluted cement flue gas (1x), and cement flue gas diluted by two-fold (1/2x), four-fold (1/4x) and eight-fold (1/8x). The current study aimed to assess the feasibility of maintaining high CO2 sequestration rates in nitrogen-limited culture media while simultaneously producing biomass rich in target metabolites for biofuel production. Accordingly, Bold’s Basal Medium (BBM) and BBM with three times the standard nitrate concentration (3N-BBM) were employed as culture media for nitrogen limited and nitrogen replete experiments respectively. The highest CO2 fixation rates were demonstrated in undiluted flue gas containing 15.50% CO2. Moreover, the average CO2 fixation rate of Desmodesmus sp. over the 8-day cultivation period (0.21 ± 0.02 g/L/d) was not significantly reduced in nitrate limited media (BBM). Nitrate limited cultivation in undiluted flue gas enhanced the accumulation of carbohydrates in microalgae (32.44 ± 0.45% and 327.65 ± 23.39 mg/L), although the increment in lipid content was not as significant as expected (41.54 ± 1.13% and 419.57 ± 31.52 mg/L). Biodiesel properties of microalgal lipids were within the limits of the ASTM D6751–12 standard. Higher heating values of microalgae biomass were in the range of 21.97–23.17 MJ/kg. Nonetheless, pilot scale studies using actual flue gas are essential prior to microalgal biofuel production and simultaneous CO2 sequestration.

Covalent immobilization of carbonic anhydrase on amine functionalized alumino-Siloxane aerogel beads for biomimetic sequestration of CO2

Biological CO2 sequestration using carbonic anhydrase is the most promising and eco-friendly technique in CO2 capture technology. The present study entails biomimetic CO2 capture using Carbonic Anhydrase from bovine erythrocytes (BCA) immobilized on versatile porous alumino-siloxane hybrid aerogel beads through covalent attachment. Alumino-siloxane hybrid aerogel beads (Al/Si) were functionalized with (3-Aminopropyl)- trimethoxysilane to achieve effective loading of the enzyme. Amine functionalized hybrid beads (Al/Si-NH2) were characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared (FTIR), Field Emission-Scanning Electron Microscope (FE-SEM), and Brunauer–Emmett–Teller (BET). The biocatalytic activity of BCA immobilized on amine-functionalized alumino-siloxane aerogel beads (BCA-Al/Si-NH2) was estimated by hydrolysis of p-nitrophenyl acetate (p-NPA). Kinetic parameter, km and kcat/ km values of BCA-Al/Si-NH2 hybrid beads were found to be comparable with the free enzyme. BCA immobilized Al/Si-NH2 hybrid beads possess thermal stability, pH stability, and storage stability and retained almost 90% of activity over ten consecutive reaction cycles. The carbonation capacity of BCA-Al/Si-NH2 hybrid beads is found to be 12 mg of CaCO3/17 μg of BCA. Thus, BCA immobilized on amine-functionalized alumino-siloxane aerogel beads has been found to directly promote the absorption of CO2 and store the captured CO2 as inert solid carbonates.

Homoacetogenesis and microbial community composition are shaped by pH and total sulfide concentration.

SummaryBiological CO2 sequestration through acetogenesis with H2 as electron donor is a promising technology to reduce greenhouse gas emissions. Today, a major issue is the presence of impurities such as hydrogen sulfide (H2S) in CO2 containing gases, as they are known to inhibit acetogenesis in CO2‐based fermentations. However, exact values of toxicity and inhibition are not well‐defined. To tackle this uncertainty, a series of toxicity experiments were conducted, with a mixed homoacetogenic culture, total dissolved sulfide concentrations ([TDS]) varied between 0 and 5 mM and pH between 5 and 7. The extent of inhibition was evaluated based on acetate production rates and microbial growth. Maximum acetate production rates of 0.12, 0.09 and 0.04 mM h‐1 were achieved in the controls without sulfide at pH 7, pH 6 and pH 5. The half‐maximal inhibitory concentration (IC50 qAc) was 0.86, 1.16 and 1.36 mM [TDS] for pH 7, pH 6 and pH 5. At [TDS] above 3.33 mM, acetate production and microbial growth were completely inhibited at all pHs. 16S rRNA gene amplicon sequencing revealed major community composition transitions that could be attributed to both pH and [TDS]. Based on the observed toxicity levels, treatment approaches for incoming industrial CO2 streams can be determined.

Optimization of Nutrients for the Growth of Scenedesmus Sp. By Response Surface Methodology

Microalgae, the photosynthetic microorganisms growing abundantly in various ecosystems are not just weeds but a potential source for sequestration of CO2. Biological sequestration of CO2 by microalgae is much more significant from the physical and chemical methods as it produces biomass (source of biofuels) apart from sequestering CO2.The growth of microalgae depends upon and requires optimum concentration of light intensity, temperature, nutrients, concentration of inoculum to produce higher yield of biomass as well its byproducts. The present study aims to optimize the nutrients using RSM for growth of microalgae, Scenedesmus sp, which was isolated locally. The other parameters such as light intensity (160 µE/m2 /s), temperature (250 C) and light/dark cycle (12/12) were maintained constant to reveal the nutrient effect on the biomass growth. The nutrients studied were nitrate (urea (0-500 ppm)), phosphate (potassium dihydrogen phosphate (0-500 ppm)), and bicarbonate ((potassium hydrogen carbonate (0-2000 ppm)). Face Centered Central Composite Design (FCCD) of 15 runs was obtained using Design Expert 8.0.7.1 (Trail version). The higher yield of biomass was obtained at optimized nutrient concentrations of 0 ppm nitrate, 250 ppm phosphate, and 1000 ppm bicarbonate. The response equation (biomass yield) as a function of nitrate, phosphate and carbonate concentrations was obtained which helps to identify the effect of various composition. The RSM technique will help to identify the best combination of nutrients for the growth of Scenedesmus sp.

Fixation of CO2, electron donor and redox microenvironment regulate succinic acid production in Citrobacter amalonaticus

Biological sequestration of CO2 for generating value added products is an emerging strategy. Succinic acid (SA) is an important C4 building block chemical, and its biological production via CO2 sequestration, holds many practical applications. This study presents an in-depth insight on SA production using isolated strain belonging to genus Citrobacter, more closely related to Citrobacter amalonaticus by considering critical process parameters such as different carbon sources at various initial concentrations, buffering agent (NaHCO3) concentrations and different pH conditions. The effect of H2 gas as an electron donor and availability of CO2 during SA production was also evaluated. The results from this work demonstrated that the isolated strain depicted the ability to utilize diverse carbon sources and highest SA production was achieved with sucrose as a substrate, indicating that reduced carbon substrates help in maximizing the redox potential. Incorporation of CO2 and H2 not only enhanced the production of SA but also affected the total acids profile favoring the production of SA over lactic, formic and acetic acids. Additional supply of CO2 and H2 led to maximum SA production of 12.07 gL−1, productivity of 0.36 gL−1 h−1 and SA yield of 48.5%. In control operation when no gases were supplied and in other test conditions where either of the gases were supplied, lactic acid was the major end product followed by acetic acid. The positive effect of CO2 for SA production provides scope for sustainable integration of SA and the CO2-generating biofuel industries or industrial side streams.

Impact of ocean acidification and high solar radiation on productivity and species composition of a late summer phytoplankton community of the coastal Western Antarctic Peninsula

AbstractThe Western Antarctic Peninsula (WAP), one of the most productive regions of the Southern Ocean, is currently undergoing rapid environmental changes such as ocean acidification (OA) and increased daily irradiances from enhanced surface‐water stratification. To assess the potential for future biological CO2 sequestration of this region, we incubated a natural phytoplankton assemblage from Ryder Bay, WAP, under a range of pCO2 levels (180 μatm, 450 μatm, and 1000 μatm) combined with either moderate or high natural solar radiation (MSR: 124 μmol photons m−2 s−1 and HSR: 435 μmol photons m−2 s−1, respectively). The initial and final phytoplankton communities were numerically dominated by the prymnesiophyte Phaeocystis antarctica, with the single cells initially being predominant and solitary and colonial cells reaching similar high abundances by the end. Only when communities were grown under ambient pCO2 in conjunction with HSR did the small diatom Fragilariopsis pseudonana outcompete P. antarctica at the end of the experiment. Such positive light‐dependent growth response of the diatom was, however, dampened by OA. These changes in community composition were caused by an enhanced photosensitivity of diatoms, especially F. pseudonana, under OA and HSR, reducing thereby their competitiveness toward P. antarctica. Moreover, community primary production (PP) of all treatments yielded similar high rates at the start and the end of the experiment, but with the main contributors shifting from initially large to small cells toward the end. Even though community PP of Ryder Bay phytoplankton was insensitive to the changes in light and CO2 availability, the observed size‐dependent shift in productivity could, however, weaken the biological CO2 sequestration potential of this region in the future.

Significant Biologically Mediated CO2 Uptake in the Pacific Arctic During the Late Open Water Season

AbstractShifting baselines in the Arctic atmosphere‐sea ice‐ocean system have significant potential to alter biogeochemical cycling and ecosystem dynamics. In particular, the impact of increased open water duration on lower trophic level productivity and biological CO2 sequestration is poorly understood. Using high‐resolution observations of surface seawater dissolved O2/Ar and pCO2 collected in the Pacific Arctic in October 2011 and 2012, we evaluate spatial variability in biological metabolic status (autotrophy vs heterotrophy) as constrained by O2/Ar saturation (∆O2/Ar) as well as the relationship between net biological production and the sea‐air gradient of pCO2 (∆pCO2). We find a robust relationship between ∆pCO2 and ∆O2/Ar (correlation coefficient of −0.74 and −0.61 for 2011 and 2012, respectively), which suggests that biological production in the late open water season is an important determinant of the air‐sea CO2 gradient at a timeframe of maximal ocean uptake for CO2 in this region. Patchiness in biological production as indicated by ∆O2/Ar suggests spatially variable nutrient supply mechanisms supporting late season growth amidst a generally strongly stratified and nutrient‐limited condition.

Die Kompensationspflicht für Treibstoffimporteure: eine zentrale Massnahme der Schweizer Klimapolitik

Obligation to compensate for fossil fuel importers: a central measure of Swiss climate policy The Federal Act on the Reduction of CO2 Emissions (CO2 Act; CC 641.71) requires fossil fuel producers and importers as well as operators of fossil fuel power plants to use domestic measures to compensate for a part of their CO2 emissions. As a result, they carry out domestic emission reduction projects or programs to meet this requirement. Demonstrated emission reductions are documented with an attestation or counted directly towards the compliance. Eligible projects or programs include all greenhouse gases listed in Article 1 of the Ordinance on the Reduction of CO2 Emissions (CO2 Ordinance; CC 641.711) or involve biological CO2 sequestration (capture and storage of CO2) in wood products. The CO2 Ordinance sets out the requirements for the respective projects and programs: just like international projects under the Clean Development Mechanism (CDM) of the Kyoto Protocol, compensation projects and programs in Switzerland must follow a specific procedure. The critical step in this procedure is to demonstrate that the reductions are additional, i.e. they would not have been achieved without the project/program. The instrument of compensation for fossil fuel importers contributes significantly to achieving national climate targets. The administrative office for compensation operated jointly by the Federal Office for the Environment (FOEN) and the Swiss Federal Office of Energy (SFOE) is the contact point for project developers, validators and those with compensation obligations.

Lipid accumulation and CO2 utilization of two marine oil-rich microalgal strains in response to CO2 aeration

Biological CO2 sequestration by microalgae is a promising and environmentally friendly technology applied to sequester CO2. The characteristics of neutral lipid accumulation by two marine oil-rich microalgal strains, namely, Isochrysis galbana and Nannochloropsis sp., through CO2 enrichment cultivation were investigated in this study. The optimum culture conditions of the two microalgal strains are 10% CO2 and f medium. The maximum biomass productivity, total lipid content, maximum lipid productivity, carbon content, and CO2 fixation ability of the two microalgal strains were obtained. The corresponding parameters of the two strains were as follows: ((142.42±4.58) g/(m2·d), (149.92±1.80) g/(m2·d)), ((39.95±0.77)%, (37.91±0.58)%), ((84.47±1.56) g/(m2·d), (89.90±1.98) g/(m2·d)), ((45.98±1.75)%, (46.88±2.01)%), and ((33.74±1.65) g/(m2·d), (34.08±1.32) g/(m2·d)). Results indicated that the two marine microalgal strains with high CO2 fixation ability are potential strains for marine biodiesel development coupled with CO2 emission reduction.

Application of a new ceramic hydrophobic membrane for providing CO2 in algal photobioreactor during cultivation of Arthrospira sp.

Biological sequestration of CO2 using microalgal route emerges as a promising option due to its cleaner approach which can be used to produce various forms of bioenergy. The current study reports on development of a new ceramic hydrophobic membrane and its application in an algal photobioreactor towards an efficient dissolution of CO2 during cultivation of Arthrospira sp. Clay-alumina based ceramic capillary support elements were coated with bentonite clay and surface modified using polydimethyl siloxane (PDMS) to impart hydrophobicity. The prepared membrane indicated pore size of 5.0nm with around 35.3% porosity and contact angle of 147°. A two-element membrane module having 150mm length, 3.16 outer diameter and 2.16mm inner diameter was used as a nano bubble sparger to provide CO2 in 1L of the culture medium and an enhanced overall mass transfer coefficient of about 38×10−4m/s was obtained using 19.5% CO2 gas mixture and 0.23m/s gas velocity. The membrane was efficient in removal of dissolved oxygen produced during photosynthesis, resulting in higher algal growth and higher CO2 sequestration. Algal growth of about 178.2g(dwt)/m3/day were obtained at 24h interval of CO2 supply in the photobioreactor and subsequent CO2 sequestration obtained was 13.99g/m3/h. Under optimized growth conditions the produced biomass yielded biodiesel content of about 5.82% of dry weight of algae. Application of the ceramic hydrophobic membrane thus enables enhanced mass transfer coefficient and dissolved oxygen removal efficiency making it suitable in photobioreactor application towards algal bio-sequestration of CO2 and subsequent biomass and biofuel production.

Deep-sea coral evidence for lower Southern Ocean surface nitrate concentrations during the last ice age

The Southern Ocean regulates the ocean's biological sequestration of CO2 and is widely suspected to underpin much of the ice age decline in atmospheric CO2 concentration, but the specific changes in the region are debated. Although more complete drawdown of surface nutrients by phytoplankton during the ice ages is supported by some sediment core-based measurements, the use of different proxies in different regions has precluded a unified view of Southern Ocean biogeochemical change. Here, we report measurements of the 15N/14N of fossil-bound organic matter in the stony deep-sea coral Desmophyllum dianthus, a tool for reconstructing surface ocean nutrient conditions. The central robust observation is of higher 15N/14N across the Southern Ocean during the Last Glacial Maximum (LGM), 18-25 thousand years ago. These data suggest a reduced summer surface nitrate concentration in both the Antarctic and Subantarctic Zones during the LGM, with little surface nitrate transport between them. After the ice age, the increase in Antarctic surface nitrate occurred through the deglaciation and continued in the Holocene. The rise in Subantarctic surface nitrate appears to have had both early deglacial and late deglacial/Holocene components, preliminarily attributed to the end of Subantarctic iron fertilization and increasing nitrate input from the surface Antarctic Zone, respectively.

CO2 sequestration by methanogens in activated sludge for methane production

Carbon dioxide (CO2) is the main greenhouse gas; hence, processes are needed to remove it from the environment. Here, CO2 was used as the substrate to generate methane (CH4) by using enriched methanogens after anaerobic enrichment of waste activated sludge (WAS); therefore, we demonstrate that methanogens from WAS have significant potential for converting the greenhouse gas CO2 into the fuel methane. Methane production was found to increase 70 fold by active methanogens in the enriched methanogens culture after 3days in the presence of H2 and CO2. Throughout the process, CO2 was completely consumed after 4days of incubation in the vials after sparging with a mixture of H2 and CO2, resulting in significant biological CO2 sequestration by methanogens. Using a mixture of H2 and 13CO2, we also demonstrated that the methane produced is due to the utilization of CO2. Microbial community studies via by quantitative real time PCR (qRT-PCR) indicate the dominance of archaea in the enriched methanogens culture of WAS. Archaeal community studies of the enriched methanogens via high-throughput 16S rRNA sequencing also showed that the archaea consist mainly of hydrogenotrophic and aceticlastic methanogens such as Methanobacteriaceae, Methanospirillaceae and Methanosarcinaceae spp. which are actively grown in H2 and CO2. We envision that CO2 gas from power plants can be directed to enriched methanogens of WAS to prevent release of this greenhouse gas while generating a useful biofuel (methane) or other valuable products using this single carbon atom.

Algae as biofuel

The combustion of fossil fuels contributes to climate change by introducing greenhouse gases to the atmosphere, and the need for an alternative fuel source is increasingly important. Biomass, either terrestrial or aquatic is considered a potential renewable energy source for biofuel production. Microalgae or seaweed (Macroalgae) are considered superior compared with terrestrial plants – in terms of solar energy storage, nutrient assimilation and potential for biofuel production – due to their higher photosynthetic efficiency, higher biomass yield and rates. The use of algae for biofuel production is expected to play an important role in securing energy supply in the next decades. For example, not only in the processing of those algae directly related to biofuels production but also in the possibility of combining this processing with pollution control, in particular with biological sequestration of CO2 emissions and other greenhouse gases, or wastewater treatment. Energy from biomass (biological, including biofuels and non-biological processes including gasification, combustion, and so on) is expected to provide more than a half share of the total energy to achieve the desired target. The various aspects associated with the design of algae production units are described in this paper, giving an overview of the current state of development of algae cultivation systems (photo-bioreactors and open ponds). Other potential applications and products from microalgae are also presented in this paper – for example applications in teh biological sequestration of CO2 and in wastewater treatment. In addition, the use of microalgae as food additives and for aquaculture is also discussed.

Synthesis of observed air–sea CO<sub>2</sub> exchange fluxes in the river-dominated East China Sea and improved estimates of annual and seasonal net mean fluxes

Abstract. Limited observations exist for a reliable assessment of annual CO2 uptake that takes into consideration the strong seasonal variation in the river-dominated East China Sea (ECS). Here we explore seasonally representative CO2 uptakes by the whole East China Sea derived from observations over a 14-year period. We firstly identified the biological sequestration of CO2 taking place in the highly productive, nutrient-enriched Changjiang River plume, dictated by the Changjiang River discharge in warm seasons. We have therefore established an empirical algorithm as a function of sea surface temperature (SST) and Changjiang River discharge (CRD) for predicting sea surface pCO2. Syntheses based on both observations and models show that the annually averaged CO2 uptake from atmosphere during the period 1998–2011 was constrained to about 1.8 ± 0.5 mol C m−2 yr−1. This assessment of annual CO2 uptake is more reliable and representative, compared to previous estimates, in terms of temporal and spatial coverage. Additionally, the CO2 time series, exhibiting distinct seasonal pattern, gives mean fluxes of −3.7 ± 0.5, −1.1 ± 1.3, −0.3 ± 0.8 and −2.5 ± 0.7 mol C m−2 yr−1 in spring, summer, fall and winter, respectively, and also reveals apparent interannual variations. The flux seasonality shows a strong sink in spring and a weak source in late summer–mid-fall. The weak sink status during warm periods in summer–fall is fairly sensitive to changes of pCO2 and may easily shift from a sink to a source altered by environmental changes under climate change and anthropogenic forcing.

CO2uptake in the East China Sea relying on Changjiang runoff is prone to change

[1] Limited knowledge exists concerning the unusually large CO2 uptake capacity in the East China Sea (ECS), which is the eminent continental shelf pump for efficient transfer of atmospheric CO2 to the deep ocean. Here we show evidence of strong control of river runoff on the CO2uptake capacity of the ECS. From 8-years of observations in the productive ECS shelf, we present the first dataset to show the complete seasonal cycle of CO2 flux, which gives an annual flux of 2.3 ± 0.4 mol C m−2 y−1 as a net sink of atmospheric CO2. Further, we found biological sequestration of CO2 taking place in the highly productive Changjiang river plume in warm seasons due to the riverine nutrient enrichment. Consequently, changes in the plume area due to changes in the Changjiang River Discharge (referred to as the Discharge hereafter) strongly affect the CO2 uptake capacity. As the Discharge may decrease due to the Three Gorges Dam operation, the Changjiang plume will probably also decrease, resulting in reduction in CO2 uptake capacity and even a shift from a CO2 sink to a source.

Surface water productivity and paleoceanographic implications in the Cenozoic Arctic Ocean

Study of bulk nitrogen contents and isotopic composition in Arctic Ocean sequences (Integrated Ocean Drilling Program Expedition 302) over the past 60 Ma revealed changes in the export flux and sources of sedimentary nitrogen. The paleoproductivity calculated from the fraction of organic nitrogen to total nitrogen is distinctly lower (<20 g C m−2 a−1) during the ice‐covered Neogene compared to the ice‐free, warm, and biologically active early Paleogene (∼50–100 g C m−2 a−1). Nitrogen isotope measurements from late Paleocene to early Eocene sediments provide evidence for a stepwise stratification and nutrient depletion in surface water masses. Cyanobacterial nitrogen fixation appeared to be the main source of nutrient N in the Arctic Ocean during the early‐middle Eocene characterized by conditions of strong oxygen depletion, high nutrient N losses, and high organic carbon accumulation rates. We speculate that biological CO2 sequestration in the Arctic Ocean and enhanced organic carbon burial rates may have contributed to lower atmospheric CO2 subsequent to the early Eocene climate optimum.