Major Source Of Organic Carbon Research Articles

Algal contribution to organic carbon sequestration and its signatures in a tropical seagrass meadow

Algae form prolific coastal habitats and contribute the largest carbon dioxide (CO2) flux globally. Most of the algal biomass is transported to the coastal ocean as particulate organic matter (OM), of which the contribution of carbon occurs mainly in depositional environments. Identifying the algal contribution alongside other major sources of organic carbon (OC) is crucial for determining allochthonous and autochthonous contributions in blue carbon habitats. In this study, we identified the algal contribution to OC storage in the seagrass meadow of Tanjung Adang Shoal (Johor, Malaysia) using dual-stable isotope measurements (δ13C and δ15N) and environmental DNA (eDNA) analysis. Stable isotope analysis showed that mangrove plants are the largest contributor to organic carbon pool in the sediments, followed by seston, macroalgae, epiphytes and seagrasses. The combined proportions of sestonic, macroalgal and epiphytic inputs as algal-derived organic matter had contributed a cumulative of 41.4–55.4% of organic matter in the bulk sediment. The eDNA method was used to examine bulk sediments and determine the types of algae present in the seagrass meadow at the genus/family level. Results showed that the main macroalgae present were from member of Coralinalles, Cladophoraceae and Ulvaceae, while diatoms were the predominant microalgae found in the sediment. These findings shed light on the importance of algae in carbon sequestration in the seagrass ecosystem. Understanding the role of algae in carbon storage and their interactions with sediment is crucial in the face of global and local environmental changes.

Sources, Migration, Transformation, and Environmental Effects of Organic Carbon in Eutrophic Lakes: A Critical Review.

Organic carbon (OC) plays a leading role in the carbon cycle of lakes and is crucial to carbon balances at regional and even global scales. In eutrophic lakes, in addition to external river inputs, the decomposition of endogenous grass and algae is a major source of organic carbon. Outbreaks of algal blooms (algal eutrophication) and the rapid growth of aquatic grasses (grass eutrophication) can lead to the accumulation and decay of large amounts of algae and aquatic grass debris, which increases the intensity of the carbon cycle of lakes and greatly impacts aquatic environments and ecosystems. The structures, decomposition processes, and distribution characteristics of algae and higher aquatic plant debris in eutrophic lakes are different from mesotrophic and oligotrophic lakes. Studying their accumulation dynamics and driving mechanisms is key to further understanding lake carbon cycles and their many interdependent pathways. This paper focuses on the carbon sources, tracing technologies, migration and transformation processes, and environmental effects of OC in eutrophic lakes. Based on the existing knowledge, we further combed the literature to identify the most important knowledge gaps preventing an in-depth understanding of the processes and driving mechanisms of the organic carbon cycle in eutrophic lakes.

Organic carbon and silt determining subcritical water repellency and field capacity of soils in arid and semi-arid region

Soil water repellency (SWR) is frequently observed in different types of land use and climates. Since SWR potentially enhances the difficulty of water infiltration in soil, the phenomenon can severely impact the water use of plants in arid regions. Therefore, understanding the origin of SWR is crucial in arid and semi-arid regions. This study investigated the fundamental and hydrological properties of soils in three arid ecosystems (desert, farmland, and forest). Analysis was done to determine any potential links between these properties, vegetation cover, and the severity of SWR. It was found that SWR was positively correlated with soil organic carbon (SOC), silt content, and field capacity of soil, where the SWR was in subcritical SWR range. The linear correlation and hierarchical clustering analysis confirmed that the SOC and silt content was the critical factor affecting the occurrence and persistence of SWR. The major source of organic carbon and nutrients to the soil was vegetation, which also had an impact on the distribution of soil carbon. The most striking observation was that the silt content was strongly correlated with both field capacity (r = 0.817, p = 0.001) and SWR (r = 0.710, p = 0.010), which can be attributed to the SOC on silt. In arid and semi-arid regions, the specific surface area of silt was relatively larger than that of sand. Meanwhile, compared to the clay in soil, the proportion of silt was much higher. The results imply that silt could significantly affect the soil hydrological properties and that silt content could serve as a new proxy for predicting water repellency in arid and semi-arid regions.

High rise in carbonaceous aerosols under very low anthropogenic emissions over eastern Himalaya, India: Impact of lockdown for COVID-19 outbreak

The present study has been conducted to investigate the relative changes of carbonaceous aerosols (CA) over a high altitude Himalayan atmosphere with and without (very low) anthropogenic emissions. Measurements of atmospheric organic (OC) and elemental carbon (EC) were conducted during the lockdown period (April 2020) due to global COVID 19 outbreak and compared with the normal period (April 2019). The interesting, unexpected and surprising observation is that OC, EC and the total CA (TCA) during the lockdown (OC: 12.1 ± 5.5 μg m−3; EC: 2.2 ± 1.1 μg m−3; TCA: 21.5 ± 10 μg m−3) were higher than the normal period (OC: 7.04 ± 2.2 μg m−3; EC: 1.9 ± 0.7 μg m−3; TCA: 13.2 ± 4.1 μg m−3). The higher values for OC/EC ratio too was observed during the lockdown (5.7 ± 0.9) compared to the normal period (4.2 ± 1.1). Much higher surface O3 during the lockdown (due to very low NO) could better promote the formation of secondary OC (SOC) through the photochemical oxidation of biogenic volatile organic compounds (BVOCs) emitted from Himalayan coniferous forest cover. SOC during the lockdown (7.6 ± 3.5 μg m−3) was double of that in normal period (3.8 ± 1.4 μg m−3). Regression analysis between SOC and O3 showed that with the same amount of increase in O3, the SOC formation increased to a larger extent when anthropogenic emissions were very low and biogenic emissions dominate (lockdown) compared to when anthropogenic emissions were high (normal). Concentration weighted trajectory (CWT) analysis showed that the anthropogenic activities over Nepal and forest fire over north-east India were the major long-distant sources of the CA over Darjeeling during the normal period. On the other hand, during lockdown, the major source regions of CA over Darjeeling were regional/local. The findings of the study indicate the immense importance of Himalayan biosphere as a major source of organic carbon.

Long-range Transboundary Atmospheric Transport of Polycyclic Aromatic Hydrocarbons, Carbonaceous Compositions, and Water-soluble Ionic Species in Southern Thailand

ABSTRACT This study investigated atmospheric particulate matter (PM) with an aerodynamic diameter of < 2.5 µm (PM2.5) observed at the Prince of Songkla University (Phuket Campus) in southern Thailand. All samples (n = 75) were collected using MiniVol™ portable air samplers from March 2017 to February 2018. Carbonaceous aerosol compositions, i.e., organic carbon (OC) and elemental carbon (EC), water-soluble ionic species (WSIS), and polycyclic aromatic hydrocarbons (PAHs) in the PM2.5 samples were identified and quantified. We found that the average PM2.5 concentration was 42.26 ± 13.45 µg m–3, while the average concentrations of OC and EC were 3.05 ± 1.70 and 0.63 ± 0.58 µg m–3, respectively. The OC/EC ratio was in the range of 2.69–16.9 (mean: 6.05 ± 2.70), and the average concentration of 10 selected ions was 6.91 ± 3.54 µg m–3. The average concentration of SO42– was the highest throughout the entire study period (2.33 ± 1.73 µg m–3); the average contribution of SO42– to the major ionic components was 34%. Surprisingly, the average concentrations of NO3– and NH4+ were relatively low. The mean ratio of [NO3–]/[SO42–] was 0.33 ± 0.24. Strong positive correlation was found between K+ and both OC and EC (r = 0.90 and r = 0.93, respectively). It is also precious to highlight that biomass burning (BB) is the major source of OC, EC and K+, which multiple studies have confirmed that the role of K+ as a biomass marker. Results showed that BB episodes might play a major role in producing the observed high levels of OC. The relatively high abundance of both B[g,h,i]P and Ind suggests that motor vehicles, petroleum/oil combustion, and industrial waste burning are the primary emission sources of PAHs in the ambient air of Phuket. Interestingly, principal component analysis (PCA) indicated that vehicular exhausts are the main source of carbonaceous aerosol compositions found in the ambient air of Phuket, whereas the contributions of biomass burning, diesel emissions, sea salt aerosols and industrial emissions were also important.

Diatom-Bacteria Interactions Modulate the Composition and Productivity of Benthic Diatom Biofilms.

Benthic diatoms are dominant primary producers in intertidal mudflats and constitute a major source of organic carbon to consumers and decomposers residing within these ecosystems. They typically form biofilms whose species richness, community composition and productivity can vary in response to environmental drivers and their interactions with other organisms (e.g., grazers). Here, we investigated whether bacteria can affect diatom community composition and vice versa, and how this could influence the biodiversity-productivity relation. Using axenic experimental communities with three common benthic diatoms (Cylindrotheca closterium, Navicula phyllepta, and Seminavis robusta), we observed an increase in algal biomass production in diatom co-cultures in comparison to monocultures. The presence of bacteria decreased the productivity of diatom monocultures while bacteria did not seem to affect the overall productivity of diatoms grown in co-cultures. The effect of bacteria on diatom growth, however, appeared to be species-specific, resulting in compositional shifts when different diatom species were grown together. The effect of the diatoms on the bacteria also proved to be species-specific as each diatom species developed a bacterial community that differed in its composition. Together, our results suggest that interactions between bacteria and diatoms residing in mudflats are a key factor in the structuring of the benthic microbial community composition and the overall functioning of that community.

Chemical Characterization of Fine Atmospheric Particles of Water-Soluble Ions and Carbonaceous Species in a Tropical Urban Atmosphere over the Eastern Indo-Gangetic Plain

ABSTRACT Ambient fine aerosols and their sources were evaluated in an eastern Indian megacity, Kolkata (KOL), from September 2010 to August 2011. A submicron aerosol sampler (SAS) with two stage stacked filter units (SFU) was devised for simultaneous but discrete collection of water-soluble inorganic ions (WSII) and carbonaceous aerosols (CA; elemental carbon (EC) and organic carbon (OC)). Characteristics of the WSII and CA were identified using ion chromatography and an OC-EC analyzer, respectively, adopting the Interagency Monitoring of PROtected Visual Environments (IMPROVE-A) protocol. The mean annual concentrations of the WSII showed a predominance of cations (anions), consisting of Ca2+, Mg2+, and Na+ (Cl–, NO3–, and SO42–), with secondary aerosols (NH4+, NO3–, and SO42–) and Ca2+ each constituting 25% and 30%, respectively, of the total WSII (TWSII). The highest mean monthly concentration of SO42– and NO3– was observed during the winter month of February and the summer months of March and May, respectively. A pronounced peak in the monthly mean for the non-sea salt-K+ (nss-K+) concentration was noticed during October and April, implying the strong influence of biomass burning emissions during these months. Among the sea salt (SS), anthropogenic (AN), and dust (DT) sources of the TWSII, a predominant contribution from DT in August and from AN in November, April, and May was inferred. The mean annual concentration of OC was three times higher than that of EC, with 43% of it being secondary OC. Whereas the major sources of OC were inferred to be paved dust, coal combustion, and biomass burning, those of EC were industrial and motor-vehicle non-exhaust emissions, coal combustion, and motor vehicle exhaust.

Spatial variation in concentration and sources of organic carbon in the Lena River, Siberia

AbstractGlobal warming in permafrost areas is expected to change fluxes of riverine organic carbon (OC) to the Arctic Ocean. Here OC concentrations, stable carbon isotope signatures (δ13C), and carbon‐nitrogen ratios (C/N) are presented from 22 sampling stations in the Lena River and 40 of its tributaries. Sampling was conducted during two expeditions: the first in July 2012 in the south and southeastern region and the second in June 2013 in the northern region of the Lena basin. The data showed significant spatial differences in concentrations and major sources of OC. Mean subcatchment slopes were correlated with OC concentrations, implying that mountainous areas in general had lower concentrations than lowland areas. δ13C and C/N data from tributaries originating in mountainous areas indicated that both dissolved and particulate OC (DOC and POC) were mainly derived from soil organic matter (SOM). In contrast, tributaries originating in lowland areas had larger contributions from fresh vegetation to DOC, while aquatically produced OC was the major source of POC. We suggest that these differences in dominant sources indicated differences in dominant flow pathways. Tributaries with larger influence of fresh vegetation probably had surficial flow pathways, while tributaries with more SOM influence had deeper water flow pathways. Thus, the future export of OC to the Arctic Ocean will likely be controlled by changes in spatial patterns in hydroclimatology and the depth of the active layers influencing the dominant water flow pathways in Arctic river basins.

Carbon Bioavailability in a High Arctic Fjord Influenced by Glacial Meltwater, NE Greenland

The land-to-ocean flux of organic carbon is increasing in glacierized regions in response to increasing temperatures in the Arctic (Hood et al., 2015). In order to understand the response in ecosystem metabolism to the organic carbon input it is essential to determine the bioavailability of the different carbon sources in the system. We quantified the bacterial turnover of organic carbon in a high Arctic fjord system (Young Sound, NE Greenland) during the ice-free period (July-October 2014) and assessed the quality and quantity of the 3 major organic carbon sources; 1) local phytoplankton production 2) runoff from land-terminating glaciers and a lowland river and 3) inflow from the ocean shelf. We found that despite relatively low concentrations of DOC in the rivers, the bioavailability of the river–DOC was significantly higher than in the fjord, and characterized by high cell-specific bacterial production and low C:N ratios. In contrast, the allochthonous DOC source entering via inflow of coastal shelf waters had elevated DOC concentrations with low bioavailability, high C:N and low specific bacterial production. The phytoplankton production in the fjord could not sustain the bacterial carbon demand, but was still the major source of organic carbon for bacterial growth. We assessed the bacterial community composition and found that communities were specific for the different water types i.e. the bacterial community of the coastal coastal inflow water could be traced mainly in the subsurface water, while the glacial river community strongly dominated the surface water in the fjord.

Total organic carbon transport by the Alaknanda River, Garhwal Himalayas, India

Transport and fate of organic carbon by the fluvial system play a significant role in the global biogeochemical cycle of carbon. Previous studies show that the transportation of modern organic carbon from the Himalayan River system accounts for 10–20% of the total global flux to the oceans. Till date, no study has been published which dealt with the transport of organic carbon in the headwaters of the Ganga River. The Alaknanda River is a headwater stream of the Ganga, which flows in the Western Himalayas of India. Water and freshly deposited channel sediment samples were collected during the months of March 2014 and August 2014 and analysed for dissolved organic carbon (DOC), particulate organic carbon (POC) and channel organic carbon (COC). The observed variability of organic carbon concentration was correlated with factors such as discharge, physiography and suspended sediment concentration (SSC). The results show that seasonal erosivity in the basin influences its DOC concentration and physiography, thus acting as a key parameter which controls transportation, oxidation and residence time of the organic matter. The allochthonous input of sediments from the erosional activities is the major source of organic carbon. At Devprayag, Alaknanda contributes 66% of the total DOC flux carried by the Ganga River. The comparison with the previously published values indicate that due to differences in physiography and chemical weathering rate, the Ganga River transports organic carbon mainly as a dissolved load in its upstream and predominantly as POC down the Himalayan foothills.

Effects of fine root characteristics of beech on carbon turnover in the topsoil and subsoil of a sandy Cambisol

Fine roots that enter mineral soil at different depths are a major source of organic carbon stored in forest soil. Little is known about the key factors that govern the mineralization kinetics of fine roots in topsoil compared with subsoil. Therefore, we analysed the effects of concentration, spatial distribution and size of fine beech roots on their rates of decomposition in the topsoil and subsoil of a sandy Cambisol. Undisturbed (intact soil columns) and disturbed (soil sieved &lt; 2 mm) samples from the topsoil (2–10 cm) and subsoil (145–153 cm) were incubated for 365 days to determine the carbon dioxide emissions. The treatments included applications of fine roots that varied in size (length: &lt;2 mm and 1–2 cm), different rates of application (2 and 8 g kg‐1) and spatial distribution (homogeneous and locally concentrated, i.e. localized). The mineralization was affected significantly at both depths by the rate of application (large rate &gt; small rate) and in the topsoil by distribution (localized &gt; homogeneous). The spatial distribution of large roots, but not smaller ones, affected rates of emission in subsoil but not in topsoil. Correlation analyses suggest an effect of the calcium and potassium supply on the microbial biomass and on the turnover of roots if these are locally concentrated. The data of this study suggest that in sandy soil the availability of macronutrients has to be considered complementary to root characteristics such as concentration, size and distribution to elucidate their decomposition kinetics throughout the soil profile.HighlightsAnalysis of effects of characteristics of fine beech roots on organic matter decomposition in top‐ and subsoil. The rate of decomposition in the topsoil was larger if the fine roots were locally concentrated. The rate of mineralization of fine beech roots in the subsoil increased with increasing root size. Soil microbial biomass carbon is affected by the supply of calcium and potassium if the fine beech roots are locally concentrated.

Characteristics of size-segregated carbonaceous aerosols in the Beijing-Tianjin-Hebei region.

Mass concentrations of organic carbon (OC) and elemental carbon (EC) in size-resolved aerosols were investigated at four sites (three cities and one country) in the Beijing-Tianjin-Hebei region from September 2009 to August 2011. The size distributions of OC and EC presented large evolutions among rural and urban sites, and among four seasons, with highest peaks of OC and EC in fine mode in urban areas during winter. Geometric mean diameters (GMDs) of OC and EC in fine particles at urban sites during winter were lower than those at rural site mainly due to effects of fine particle coagulation and organic compound repartitioning. Fossil fuel emissions were a dominant source of OC and EC in urban areas, while biomass burning was a major source of OC and EC at rural site. Trajectory clustering and CWT analysis showed that regional transport was an important contributor to OC and EC in Beijing.

Source apportionment of carbonaceous fine particulate matter (PM2.5) in two contrasting cities across the Indo–Gangetic Plain

Agra and Kanpur are heavily polluted Indian cities and are the fourth and second largest cities in Uttar Pradesh State, respectively. PM2.5 was collected from December 2011 to May 2012 in Agra and from December 2011 to October 2012 in Kanpur every 6th day. The samples were chemically analyzed to determine organic carbon (OC), water soluble organic carbon (WSOC), elemental carbon (EC), secondary inorganic ions, and particle–phase organic compounds. A chemical mass balance (CMB) receptor model using organic tracers was used to estimate source contributions to PM2.5. Concentrations of carbonaceous aerosols were on average 23±16μg/m3 in Agra and 33±21μg/m3 in Kanpur during the winter and summer periods, and had a strong seasonal trend with highest levels in winter (December–February) and then decreasing to summer (March–May). Five primary sources were identified. In Agra, biomass burning was the major source of OC in the winter months with decreasing relative and absolute concentrations in summer. In Kanpur, biomass burning was also the most important primary source of OC, but was about half the concentration found in Agra. Mobile source contributions to OC were on average 25±9% and 25±22% in Agra and Kanpur, respectively, with similar absolute concentrations of 2.5±1.9μg/m3 in most months. Secondary organic aerosol (SOA) was estimated from non–biomass burning WSOC and the unapportioned OC, with each method indicating SOA as a major source of OC in the winter in both cities, apportioning 25% of OC in Agra and 65% in Kanpur. SOA in Kanpur in December was four times higher than in Agra. Overall, results suggest differences in aerosol chemical composition and sources at these two sites across the Indo–Gangetic plain with biomass burning making up a larger fraction of the particulate OC in Agra, and SOA being a more important contributor to OC mass in Kanpur.

Food web structure of the coastal area adjacent to the Tagus estuary revealed by stable isotope analysis

The identification of energy sources, pathways and trophic linkages among organisms is crucial for the understanding of food web dynamics. Stable isotopes were used to identify the trophic level of food web components and track the incorporation of organic matter of different origins in the coastal ecosystem adjacent to the Tagus estuary. It was shown that the river Tagus is a major source of organic carbon to this system. Also, the wide difference in δ 13C among the primary consumers allowed the identification of the pelagic and the benthic energy pathways. The maximum trophic level observed was 2.4 for Sepia officinalis. This value is indicative of a short food web. It was concluded that the diet of the upper trophic level species relies directly on the lower food web levels to a considerable extent, instead of relying mostly on intermediate trophic level species. Moreover, the δ 15N values of primary consumers were very close to that of particulate organic matter, probably due to poorly known processes occurring at the basis of the food web. This lowers the trophic length of the whole food web. Reliance on benthic affinity prey was high for all upper trophic level secondary consumers.

The use of polar organic compounds to estimate the contribution of domestic solid fuel combustion and biogenic sources to ambient levels of organic carbon and PM 2.5 in Cork Harbour, Ireland

PM 2.5 samples collected at Cork Harbour, Ireland during summer, autumn, late autumn and winter, 2008–2009 were analyzed for polar organic compounds that are useful markers for aerosol source characterization. The determined compounds include tracers for biomass burning primary particles, fungal spores, markers for secondary organic aerosol (SOA) from isoprene, α-/ β-pinene, and d-limonene. Seasonal and temporal variations and other characteristic features of the detected tracers are discussed in terms of aerosol sources and processes. The biogenic species were detected only during the summer period where the contributions of isoprene SOA and fungal spores to the PM 2.5 organic carbon (OC) were estimated to be 1.6% and 1% respectively. The biomass burning markers, and in particular levoglucosan, were present in all samples and attributed to the combustion of cellulose-containing fuels including wood, peat, bituminous and smokeless coal. The contribution of domestic solid fuel (DSF) burning to the measured OC mass concentration was estimated at 10.8, 50, 66.4 and 74.9% for summer, autumn, late autumn and winter periods, respectively, based on factors derived from a series of burning experiments on locally available fuels. Application of an alternative approach, namely principal component analysis–multiple linear regression (PCA–MLR), to the measured concentrations of the polar organic marker compounds used in conjunction with real-time air quality data provided similar trends and estimates for DSF combustion during all seasons except summer. This study clearly demonstrates that, despite the ban on the sale of bituminous coal in Cork and other large urban areas in Ireland, DSF combustion is still the major source of OC during autumn and winter periods and also makes a significant contribution to PM 2.5 levels. The developed marker approach for estimating the contribution of DSF combustion to ambient OC concentrations can, in principle, also be applied to other locations.

Sensitivity and Bias of Molecular Marker-Based Aerosol Source Apportionment Models to Small Contributions of Coal Combustion Soot

Carbonaceous atmospheric particulate matter (PM25) collected in the midwestern United States revealed that soot emissions from incomplete coal combustion were important sources of several organic molecular markers used in source apportionment studies. Despite not constituting a major source of organic carbon in the PM25, coal soot was an important source of polyaromatic hydrocarbons, hopanes, and elemental carbon. These marker compounds are becoming widely used for source apportionment of atmospheric organic PM, meaning that significant emissions of these marker compounds from unaccounted sources such as coal soot could bias apportionment results. This concept was demonstrated using measurements of atmospheric PM collected on a 1-in-6 day schedule at three monitoring sites in Ohio: Mingo Junction (near Steubenville), Cincinnati, and Cleveland. Impacts of coal sootwere measured to be significant at Mingo Junction and small at Cleveland and Cincinnati. As a result, biases in apportionment results were substantial at Mingo Junction and insignificant at Cleveland and Cincinnati. Misapportionments of organic carbon mass at Mingo Junction were significant when coal soot was detected in the particulate samples as identified bythe presence of picene, but when coal soot was not included in the model: gasoline engines (+8% to +58% of OC), smoking engines (0% to -17% of OC), biomass combustion (+1% to +11% of OC), diesel engines (-1% to -2% of OC), natural gas combustion (0% to -2% of OC), and unapportioned OC (0% to -47% of OC). These results suggest that the role of coal soot in source apportionment studies needs to be better examined in many parts of the United States and other parts of the world.

Observation of anoxic water mass in a tropical reservoir: the Cirata Reservoir in Java, Indonesia

We conducted a transect survey of water quality and bottom sediments in a large tropical reservoir, the Cirata Reservoir, located on the Citarum River, West Java, Indonesia. In the main basin of this reservoir, the surface water contained high concentrations of chlorophyll a, up to 48 μg l−1, and most of the water body was occupied by thick anoxic water. The thickness of the surface oxygenated water was only 5–7 m, whereas that of the anoxic water mass was more than 70 m. The concentrations of phosphate and ammonia were quite high in the anoxic hypolimnion. The reasons for the formation of the huge anoxic water mass include the oligomictic status of circulation, a relatively weak mixing caused by topography, high hypolimnion temperature, and high loads of organic matter. The carbon/nitrogen (C/N) ratios and the carbon stable isotope ratios of sediments indicated that the major source of organic carbon in the sediments was algal production in the reservoir and fish culture activity. The mechanism of eutrophication in the reservoir is also discussed.

CO 2 Uptake and Fixation by Endosymbiotic Chemoautotrophs from the Bivalve Solemya velum

Chemoautotrophic symbioses, in which endosymbiotic bacteria are the major source of organic carbon for the host, are found in marine habitats where sulfide and oxygen coexist. The purpose of this study was to determine the influence of pH, alternate sulfur sources, and electron acceptors on carbon fixation and to investigate which form(s) of inorganic carbon is taken up and fixed by the gamma-proteobacterial endosymbionts of the protobranch bivalve Solemya velum. Symbiont-enriched suspensions were generated by homogenization of S. velum gills, followed by velocity centrifugation to pellet the symbiont cells. Carbon fixation was measured by incubating the cells with (14)C-labeled dissolved inorganic carbon. When oxygen was present, both sulfide and thiosulfate stimulated carbon fixation; however, elevated levels of either sulfide (>0.5 mM) or oxygen (1 mM) were inhibitory. In the absence of oxygen, nitrate did not enhance carbon fixation rates when sulfide was present. Symbionts fixed carbon most rapidly between pH 7.5 and 8.5. Under optimal pH, sulfide, and oxygen conditions, symbiont carbon fixation rates correlated with the concentrations of extracellular CO(2) and not with HCO(3)(-) concentrations. The half-saturation constant for carbon fixation with respect to extracellular dissolved CO(2) was 28 +/- 3 microM, and the average maximal velocity was 50.8 +/- 7.1 micromol min(-1) g of protein(-1). The reliance of S. velum symbionts on extracellular CO(2) is consistent with their intracellular lifestyle, since HCO(3)(-) utilization would require protein-mediated transport across the bacteriocyte membrane, perisymbiont vacuole membrane, and symbiont outer and inner membranes. The use of CO(2) may be a general trait shared with many symbioses with an intracellular chemoautotrophic partner.

The characteristics of carbonaceous species and their sources in PM2.5 in Beijing

Concentrations of organic carbon (OC) and element carbon (EC) in PM2.5 in the urban and rural areas of Beijing were determined and their sources were qualitatively discussed. PM2.5 samples were collected at five sites of Beijing during summers and winters of 2001 and 2002 and spring of 2003. Daily average 10.7±3.6 and 5.7±2.9 μg m−3 in summer and 36.7±19.4 and 15.2±11.1 μg m−3 in winter of 2002 for OC and EC, respectively, were observed in Beijing urban area. The concentrations of carbonaceous species contributed 18.4±3.5% and 37.2±12.7% of PM2.5 in summer and winter, respectively, of which OC accounted for 70% or more of the total carbon (TC). OC and EC concentrations and their mass percentages in PM2.5 were higher in winter than in summer, which could be attributed to the coal consumption of heating period. Organic aerosols would constitute up to 42% of PM2.5 in winter based on the evaluation of 1.6 for the ratio of OC to organic particulates. Secondary organic carbon (SOC) were estimated to be more than 50% and up to 95% of total OC in summer based on the minimum OC/EC ratio of 0.9. Comparisons of OC and EC with trace elements, As, Zn, K and Pb, at various sites demonstrated that coal combustion was a dominant source of carbonaceous species of PM2.5 in the urban area of Beijing during winter, and biomass burning, traffic and/or industry emission were the major sources of OC and EC in the summer.

The characteristics of carbonaceous species and their sources in PM2.5 in Beijing

Concentrations of organic carbon (OC) and element carbon (EC) in PM2.5 in the urban and rural areas of Beijing were determined and their sources were qualitatively discussed. PM2.5 samples were collected at five sites of Beijing during summers and winters of 2001 and 2002 and spring of 2003. Daily average 10.7±3.6 and 5.7±2.9 μg m −3 in summer and 36.7±19.4 and 15.2±11.1 μg m −3 in winter of 2002 for OC and EC, respectively, were observed in Beijing urban area. The concentrations of carbonaceous species contributed 18.4±3.5% and 37.2±12.7% of PM2.5 in summer and winter, respectively, of which OC accounted for 70% or more of the total carbon (TC). OC and EC concentrations and their mass percentages in PM2.5 were higher in winter than in summer, which could be attributed to the coal consumption of heating period. Organic aerosols would constitute up to 42% of PM2.5 in winter based on the evaluation of 1.6 for the ratio of OC to organic particulates. Secondary organic carbon (SOC) were estimated to be more than 50% and up to 95% of total OC in summer based on the minimum OC/EC ratio of 0.9. Comparisons of OC and EC with trace elements, As, Zn, K and Pb, at various sites demonstrated that coal combustion was a dominant source of carbonaceous species of PM2.5 in the urban area of Beijing during winter, and biomass burning, traffic and/or industry emission were the major sources of OC and EC in the summer.