Contributions Of Different Carbon Sources Research Articles

Assessing biogeochemical controls on porewater dissolved inorganic carbon cycling in the gas hydrate-bearing sediments of the Makran accretionary wedge, Northeastern Arabian Sea off Pakistan

Quantitatively assessing the porewater dissolved inorganic carbon (DIC) cycling in methane-enriched marine sediments is crucial to understanding the contributions of different carbon sources to the global marine carbon pool. In this study, Makran accretionary wedge was divided into Zone 1 (high methane flux area) and Zone 2 (background area). Porewater geochemical compositions (Cl–, SO42–, NH4+, Mg2+, Ca2+, Ba2+, DIC and δ13C-DIC) and a reaction-transport model were used to determine the DIC source and calculate the DIC flux through carbonate precipitation and releasing into overlying seawater in sediments. Zone 1 is characterized by the shallower depth of sulfate-methane transition (SMT), where most of porewater sulfate was consumed by anaerobic oxidation of methane (AOM). In contrast, a relatively low flux of methane diffusion in Zone 2 results in a deeper SMT depth and shallow sulfate is predominantly consumed by organoclastic sulfate reduction (OSR). Based on the porewater geochemical profiles and δ13C mass balance, the proportions of porewater DIC originating from methane were calculated as 51% in Zone 1 and nearly 0% in Zone 2. An increase of porewater DIC concentration leads to authigenic carbonate precipitation. Solid total inorganic carbon (TIC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM) analysis display that carbonate content increases with depth and aragonite appears at or below the depths of SMT. Meanwhile, the flux of DIC released from sediments calculated by the reaction-transport model is 51.3 ~ 90.4 mmol/m2·yr in Zone 1, which is significantly higher than that in Zone 2 (22.4 mmol/m2·yr). This study demonstrates that AOM serves as the dominant biogeochemical process regulating the porewater DIC cycle, which has an important impact on the authigenic carbonate burial and the seawater carbonate chemistry.

Quantitative assessment of dissolved inorganic carbon cycling in marine sediments from gas hydrate-bearing areas in the South China Sea

Quantitatively assessing dissolved inorganic carbon (DIC) cycling in methane-rich marine sediments is essential for understanding the contributions of different carbon sources to the global carbon cycle. Here, the data of δ13C-DIC, SO42−, DIC, Ca2+, Mg2+, PO43−, and a reactive transport model were used to analyze the DIC and methane source and calculate the DIC budget in the shallow sediments at two methane seep sites in the Xisha Trough (CL27A) and to the west of the Haima cold seeps in the Qiongdongnan Basin (CL44). Model results show that methane contributed to 92.75% and 79.95% of DIC sources through anaerobic oxidation of methane (AOM) versus only 7.25% and 12.90% by organic matter via organoclastic sulfate reduction (OSR) and methanogenesis at both sites. However, the methane sources vary between the two sites. External thermogenic methane that is decoupled from contemporaneous organic matter deposition drives the AOM at CL27A. A combination of upwardly diffusing external biogenic methane and internal methane derived from in situ methanogenesis drives the AOM at CL44. Internal methane flux from depth reveals the deep methanogenic zone influences the DIC cycling in the shallow sediments at CL44. It is accompanied by the upward diffusive DIC flux that can be identified by the δ13C-DIC data when modeling, which contributes to the remaining 7.15% of the DIC source. Neglecting this DIC source that is set as a fixed value at the lower boundary will misunderstand the DIC cycling in the shallow sediments. This study identified and highlighted the influence of external thermogenic methane and the methanogenic zone on the DIC cycling in marine sediments, and demonstrated assessing DIC cycling accurately in methane-rich marine sediments is important to estimate its contribution to oceanic carbon cycling.

Sources, transportations and variation characteristics of dissolved inorganic carbon in Lake Poyang, China

为深入理解鄱阳湖水体溶解性无机碳（DIC）、碳同位素时空分布特征及其影响因素，继而了解鄱阳湖碳通量及其主要碳源贡献率，于2019—2020年典型水文季节对鄱阳湖湖区及“五河”入湖口进行样品采集分析，采用统计学方法初步分析鄱阳湖及入湖口水体中DIC及其同位素（δ¹³C_DIC）分布特征.结果表明：（1） DIC浓度丰水期大于枯水期，而δ¹³C_DIC值则相反，枯水期偏正，丰水期相对偏负；（2）鄱阳湖丰水期DIC月通量为266.1~268.4 t，平均值为267.3 t，枯水期DIC月通量为2.88~5.94 t，平均值为4.41 t，丰水期DIC通量是枯水期的60.6倍.CO₂水-气交换对鄱阳湖碳通量贡献率最大，为73.07%，其次为河流输入，为15.53%；（3）溶解碳酸盐岩风化是控制流域DIC来源及其δ¹³C_DIC组成的主要机制，季节性降雨形成的径流量变化是不同碳源对鄱阳湖碳通量贡献的主要控制因素.;Water samples of Lake Poyang and its five inflow river were collected in typical hydrological seasons from 2019 to 2020, to reveal the spatial variations of dissolved inorganic carbon (DIC) and its isotopic composition, as well as to estimate the carbon flux and the sources of the three main carbons in Lake Poyang. The results indicated that DIC concentrations in wet seasons were higher than those in dry seasons whereas both higher δ¹³C_DIC and the δ¹³C_DIC in dry seasons. The DIC flux in the surface water of Lake Poyang ranged from 266.1 to 268.4 t/month (the average was 267.3 t/month) in wet seasons, and ranged from 2.88 to 5.94 t/month (the average was 4.41 t/month) in dry seasons, exhibiting a difference nearly 60.6 times in different seasons. The contribution of gas CO₂ exchange between lake water and atmosphere to the DIC flux in Lake Poyang was the largest, accounting for 73.07%, followed by river input with 15.53%. The weathering of carbonate rock by H₂CO₃ was the dominating reaction controlling both DIC sources and δ¹³C_DIC compositions, and the variation of seasonal rainfall runoff was the main controlling factor of the contribution of different carbon sources to the carbon flux in Lake Poyang.

Stable isotopes reveal food web reliance on different carbon sources in a subtropical watershed in South China

Fish species in the Pearl River Watershed (PRW), the largest subtropical watershed in southern China, have declined dramatically in recent decades. To protect river habitats and maintain the integrity of river ecosystems, we need to know how different carbon sources contribute to aquatic food webs. We used information about stable carbon and nitrogen isotopes and IsoSource software to investigate the relative contributions of different carbon sources to the food web of the PRW. We found that the stable signatures of C3 plants (C3P), particulate organic matter (POM), submersed water grasses (SWG), and C4 plants (C4P) were significantly different and fell into four distinct groups. Of these, C3P was a high and stable source of carbon to certain consumers, while C4P was a low but stable carbon source. In comparison, the contributions from the POM and SWG groups were unstable and varied widely, between the 1st and 99th confidence intervals. The results suggest that the C3P, POM, and SWG groups might be important carbon sources for the PRW food web. Analysis of the 99th confidence interval contribution of each group that provided more than 50% showed that consumers in the PRW food web might be divided into those that mainly relied on C3P, those that mainly relied on SWG, those that potentially relied on C3P-SWG, and those that potentially relied on POM-SWG. This study provides basic information that will help support the protection and management of the PRW ecosystem, and will be especially useful to help restore submersed macrophyte beds and riparian buffer strips in this watershed.

Isotopic variation among Amazonian floodplain woody plants and implications for food-web research

Isotopic variation within food sources adds uncertainty to models intended to reconstruct trophic pathways. Understanding this variation is pivotal for planning sampling protocols for food-web research. This study investigates natural variation in C and N stable isotopes among plant species in two western Amazon flooded forests with contrasting watershed biogeochemistry (white-water várzea-forest and black-water igapó-forest). Our objectives were to compare δ13C and δ15N of leaves and fruits between sites; assess the magnitude of within-site variation in δ13C and δ15N of leaves (várzea: 28 spp., igapó: 10 spp.) and fruits (várzea: 22 spp., igapó: 22 spp.); determine within-plant variation in δ13C and δ15N of leaf, wood and fruit tissues; and test whether inter-specific variation in δ13C and δ15N influence the results of a mixing model predicting the contribution of terrestrial C sources to an aquatic consumer. Mean δ13C values of leaves and fruits were not statistically different between the two sites despite regional differences in biogeochemistry and floristic composition. In contrast, mean δ15N of leaves and fruits were significantly lower at the várzea than at the igapó site. The high floristic diversity of both forests was reflected in large within-site interspecific variation in both δ13C and δ15N. Paired comparisons revealed that δ13C of wood and fruits and δ15N of fruits were generally greater than values obtained for leaves from the same plant. The predicted contribution of different carbon sources to the consumer biomass changed between models as a function of source variability. We discuss implications of source variation for designing sampling protocols, interpreting isotopic signatures, and establishing trophic links between plants and consumers. Our findings highlight the importance of in situ sampling to establish reliable primary production baselines for local food webs.

Factors controlling shell carbon isotopic composition of land snail <i>Acusta despecta sieboldiana</i> estimated from laboratory culturing experiment

Abstract. The carbon isotopic composition (δ13C) of land snail shell carbonate derives from three potential sources: diet, atmospheric CO2, and ingested carbonate (limestone). However, their relative contributions remain unclear. Under various environmental conditions, we cultured one land snail subspecies, Acusta despecta sieboldiana, collected from Yokohama, Japan, and confirmed that all of these sources affect shell carbonate δ13C values. Herein, we consider the influences of metabolic rates and temperature on the carbon isotopic composition of the shell carbonate. Based on results obtained from previous works and this study, a simple but credible framework is presented to illustrate how each source and environmental parameter affects shell carbonate δ13C values. According to this framework and some reasonable assumptions, we estimated the contributions of different carbon sources for each snail individual: for cabbage-fed (C3 plant) groups, the contributions of diet, atmospheric CO2, and ingested limestone vary in the ranges of 66–80, 16–24, and 0–13%, respectively. For corn-fed (C4 plant) groups, because of the possible food stress (less ability to consume C4 plants), the values vary in the ranges of 56–64, 18–20, and 16–26%, respectively. Moreover, according to the literature and our observations, the subspecies we cultured in this study show preferences towards different plant species for food. Therefore, we suggest that the potential food preference should be considered adequately for some species in paleoenvironment studies. Finally, we inferred that only the isotopic exchange of the calcite-HCO3−-aragonite equilibrium during egg laying and hatching of our cultured snails controls carbon isotope fractionation.

Effect of corm size, water stress and cultivation conditions on photosynthesis and biomass partitioning during the vegetative growth of saffron (Crocus sativus L.)

A major constraint limiting saffron cultivation is the difficulty of obtaining high quality corms for propagation. There is limited knowledge about the physiology of vegetative development of saffron. The aim of this work was to analyse the relative contribution of different carbon sources (leaf photosynthetic activity and mother corm reserves) to the growth of vegetative organs and to analyse the influence of internal and external factors on the photosynthetic processes sustaining corm growth. The photosynthetic rate and changes in biomass were measured over the course of the vegetative growing season. The influence of the size of the mother corm, the cultivation method used (greenhouse versus field) and water availability on saffron production was assessed.Most of the remaining mother corm reserves after flowering were depleted during root and leaf development. After these organs reached their maximum size, the growth of replacement corms was initiated, and this development primarily relied on photosynthesis. Saffron mother corm reserves only contributed 10% to the biomass during vegetative development. The photosynthetic rate was consistently very high (26μmolm−2s−1) throughout the year but was reduced in plants from the largest corms. A sink capacity limitation arose during growth of replacement corms since the increase of biomass was limited before total leaf senescence. Saffron grown under greenhouse conditions exhibited traits characteristic of plants grown under low-irradiance, even after the saffron plants were transferred to the field. This fact limits the production of replacement corms in this environment. A decrease in the photosynthetic rate was observed under water stress, but the supply of water to the leaves from the corms and roots was found to maintain a still high photosynthetic rate (12μmolm−2s−1) even at highly negative soil water potentials.

Relationship between δ13C of chironomid remains and methane flux in Swedish lakes

Summary1. Methanogenic carbon can be incorporated by methane‐oxidising bacteria, leading to a 13C‐depleted stable carbon isotopic composition (δ13C) of chironomids that feed on these microorganisms. This has been shown for the chironomid tribe Chironomini, but very little information is available about the δ13C of other abundant chironomid groups and the relationship between chironomid δ13C and methane production in lakes.2. Methane flux was measured at the water surface of seven lakes in Sweden. Furthermore, fluxes from the sediments to the water column were measured in transects in two of the lakes. Methane fluxes were then compared with δ13C of chitinous chironomid remains isolated from the lake surface sediments. Several different chironomid groups were examined (Chironomini, Orthocladiinae, Tanypodinae and Tanytarsini).3. Remains of Orthocladiinae in the seven study lakes had the highest δ13C values (−31.3 to −27.0‰), most likely reflecting δ13C of algae and other plant‐derived organic matter. Remains of Chironomini and Tanypodinae had lower δ13C values (−33.2 to −27.6‰ and −33.6 to −28.0‰, respectively). A significant negative correlation was observed between methane fluxes at the lake surface and δ13C of Chironomini (r = −0.90, P = 0.006). Methane release from the sediments was also negatively correlated with δ13C of Chironomini (r = −0.67, P = 0.025) in the transect samples obtained from two of the lakes. The remains of other chironomid taxa were only weakly or not correlated with methane fluxes measured in our study lakes (P > 0.05).4. Selective incorporation of methane‐derived carbon can explain the observed correlations between methane fluxes and δ13C values of Chironomini. Remains of this group might therefore have the potential to provide information about past changes in methane availability in lakes using sediment records. However, differences in productivity, algal δ13C composition and the importance of allochthonous organic matter input between the studied lakes may also have influenced Chironomini δ13C. More detailed studies with a higher number of analysed samples and detailed measurement of δ13C of different ecosystem components (e.g. methane, dissolved inorganic carbon) will be necessary to further resolve the relative contribution of different carbon sources to δ13C of chironomid remains.

Contributions of Different Organic Carbon Sources to Daphnia in the Pelagic Foodweb of a Small Polyhumic Lake: Results from Mesocosm DI13C-Additions

Freshwater ecosystems derive organic carbon from both allochthonous and autochthonous sources. We studied the relative contributions of different carbon sources to zooplankton in a small, polyhumic, steeply stratified lake, using six replicate surface-to-sediment enclosures established during summer and autumn 2004. We added 13C-enriched bicarbonate to the epilimnion of half the enclosures for three weeks during each season and monitored carbon stable isotope ratios of DIC, DOC, POC and Daphnia, along with physical, chemical and biological variables. During summer, 13C-enriched DIC (δ13C up to 44 ± 7.2‰) was soon taken up by phytoplankton (δ13C up to −5.1 ± 13.6‰) and was transmitted to Daphnia (δ13C up to −1.7 ± 7.2‰), demonstrating consumption of phytoplankton. In contrast, during autumn, 13C-enriched DIC (δ13C up to 56.3 ± 9.8‰) was not transmitted to Daphnia, whose δ13C became progressively lower (δ13C down to −45.6 ± 3.3‰) concomitant with decreasing methane concentration. Outputs from a model suggested phytoplankton contributed 64–84% of Daphnia diet during summer, whereas a calculated pelagic carbon mass balance indicated only 30–40% could have come from phytoplankton. Although autumn primary production was negligible, zooplankton biomass persisted at the summer level. The model suggested methanotrophic bacteria contributed 64–87% of Daphnia diet during autumn, although the calculated carbon mass balance indicated a contribution of 37–112%. Thus methanotrophic bacteria could supply virtually all the carbon requirement of Daphnia during autumn in this lake. The strongly 13C-depleted Daphnia values, together with the outputs from the models and the calculated carbon mass balance showed that methanotrophic bacteria can be a greater carbon source for Daphnia in lakes than previously suspected.

Contribution of different carbon sources to isoprene biosynthesis in poplar leaves.

This study was performed to test if alternative carbon sources besides recently photosynthetically fixed CO2 are used for isoprene formation in the leaves of young poplar (Populus x canescens) trees. In a 13CO2 atmosphere under steady state conditions, only about 75% of isoprene became 13C labeled within minutes. A considerable part of the unlabeled carbon may be derived from xylem transported carbohydrates, as may be shown by feeding leaves with [U-13C]Glc. As a consequence of this treatment approximately 8% to 10% of the carbon emitted as isoprene was 13C labeled. In order to identify further carbon sources, poplar leaves were depleted of leaf internal carbon pools and the carbon pools were refilled with 13C labeled carbon by exposure to 13CO2. Results from this treatment showed that about 30% of isoprene carbon became 13C labeled, clearly suggesting that, in addition to xylem transported carbon and CO2, leaf internal carbon pools, e.g. starch, are used for isoprene formation. This use was even increased when net assimilation was reduced, for example by abscisic acid application. The data provide clear evidence of a dynamic exchange of carbon between different cellular precursors for isoprene biosynthesis, and an increasing importance of these alternative carbon pools under conditions of limited photosynthesis. Feeding [1,2-13C]Glc and [3-13C]Glc to leaves via the xylem suggested that alternative carbon sources are probably derived from cytosolic pyruvate/phosphoenolpyruvate equivalents and incorporated into isoprene according to the predicted cleavage of the 3-C position of pyruvate during the initial step of the plastidic deoxyxylulose-5-phosphate pathway.

Metabolism of [1- 13C]glucose and [2- 13C]acetate in the hypoxic rat brain

The effects of hypoxia on the metabolism of the central nervous system were investigated in rats submitted to a low oxygen atmosphere (8% O 2; 92% N 2). [1- 13C]glucose and [2- 13C]acetate were used as substrates, this latter being preferentially metabolized by glial cells. After 1-h substrate infusion, the incorporation of 13C in brain metabolites was determined by NMR spectroscopy. Under hypoxia, an important hyperglycemia was noted. As a consequence, when using labeled glucose, the specific enrichment of brain glucose C1 was lower (48.2±5.1%) than under normoxia (66.9±2.5%). However, relative to this specific enrichment, the 13C incorporation in amino acids was increased under hypoxia. This suggested primarily a decreased exchange between blood and brain lactate. The glutamate C2/C4 enrichment ratio was higher under hypoxia (0.62±0.01) than normoxia (0.51±0.06), indicating a lower glutamate turnover relative to the neuronal TCA cycle activity. The glutamine C2/C4 enrichment ratio was also higher under hypoxia (0.87±0.07 instead of 0.65±0.11), indicating a new balance in the contributions of different carbon sources at the acetyl-CoA level. When using [2- 13C]acetate as substrate, no difference in glutamine enrichment appeared under hypoxia, whereas a significant decrease in glutamate, aspartate, alanine and lactate enrichments was noted. This indicated a lower trafficking between astrocytes and neurons and a reduced tricarboxylic acid cycle intermediate recycling of pyruvate.

Origin and fate of dissolved inorganic carbon ininterstitial waters of two freshwater intertidalareas: A case study of the Scheldt Estuary, Belgium

Processes affecting the concentration and isotopiccomposition of dissolved inorganic carbon (DIC) wereinvestigated in pore waters of two freshwaterintertidal areas of the Scheldt Estuary, Belgium. Porewater δ13CDIC values from marshes andmudflats varied from −27 to +13.4‰, these very largevariations reflect the contribution of differentcarbon sources to the DIC pool.