Controls on the origin and cycling of riverine dissolved inorganic carbon in the Brazos River, Texas

Rivers draining watersheds that include carbonate bedrock or organic matter (OM)-rich sedimentary rocks frequently have ¹⁴C-depleted dissolved inorganic carbon (DIC) relative to rivers draining carbonate- and OM-free watersheds, due to dissolution of carbonate and/or decomposition of ancient OM. However, our results from a subtropical river, the Brazos River in Texas, USA, show that in this watershed human activities appear to dominate basin lithology in controlling the origin and metabolism of DIC. The middle Brazos flows through limestone and coal-bearing bedrock, but DIC isotope data suggest no limestone dissolution or respiration of ancient OM, and instead reflect efficient air?Cwater CO₂ exchange, degradation of relatively young OM and photosynthesis in the river as a result of river damming and urban treated wastewater input. The lower Brazos drains only small areas of carbonate and coal-bearing bedrock, but DIC isotope data suggest the strong influence of carbonate dissolution, with a potentially minor contribution from decomposition of old soil organic matter (SOM). Oyster shells and crushed carbonate minerals used in road construction are likely sources of carbonate in the lower Brazos, in addition to natural marl and pedogenic carbonate. Additionally, the generally low pCO₂ and high DIC concentration in the Brazos River lead to a low CO₂ outgassing:DIC export ratio, distinguishing the Brazos River from other rivers.     

Why are East Asian ecosystems important for carbon cycle research?

The global carbon cycle is one of the most important bio-geochemical cycles. Through photosynthesis, green plants absorb CO₂ from the atmosphere to produce organic matters, such as sugars, and covert solar energy into chemical energy. The organic matters are then used by all other life forms including humans. When ecosystems and atmosphere are in dynamic equilibrium, the flow of CO₂ from the atmosphere into the biosphere because of photosynthesis should be equivalent to the flow of CO₂ released back into the atmosphere by respiration. However, during the past century atmospheric CO₂ concentration has increased substantially because of the burning of fossil fuels. It is highly likely that the atmospheric increase has resulted in global warming and sea level rise, as suggested by the Intergovernmental Panel on Climate Change (IPCC) .     

Significance of ocean carbonate budgets for the global carbon cycle

Changes in the trace gas composition of the atmosphere over glacial–interglacial cycles are linked to changes in the oceanic carbon cycle. This paper examines the role of biologically driven fluxes of organic and inorganic carbon in modifying the carbon dioxide chemistry of the oceans, and the corresponding implications for the partitioning of CO₂ between the atmosphere and ocean. Relevant details of the marine carbon system are presented together with an assessment of the significance of remineralization and dissolution processes. Recent estimates of the marine carbonate fluxes show significant uncertainties and inconsistencies which must be resolved in order to assess fully the role of the oceans' biota in the marine carbon system. Various types of ocean carbon cycle models have been developed in order to interpret the changes in past atmospheric carbon dioxide. Some take account of the role of the oceans' biota, focussing in the main on the cycling of organic matter. Relatively few have considered the role of the carbonate pump and the subtle interactions between organic and inorganic carbon cycling. The significance of carbonate formation and dissolution, and of the effects of global change on the marine carbonate system, for air–sea fluxes of CO₂ are discussed. Finally some recommendations for future research are made in order to improve our understanding of how spatial and temporal variation in marine carbonate fluxes, in conjunction with processes determining the oxidation and burial of organic matter in the oceans, affect levels of CO₂ in the atmosphere.     

Short-term measurement of carbon isotope fractionation in plants

Combustion-based studies of the carbon-13 content of plants give only an integrated, long-term value for the isotope fractionation associated with photosynthesis. A method is described here which permits determination of this isotope fractionation in 2 to 3 hours. To accomplish this, the plant is enclosed in a glass chamber, and the quantity and isotopic content of the CO₂ remaining in the atmosphere are monitored during photosynthesis. Isotope fractionation studies by this method give results consistent with what is expected from combustion studies of C₃, C₄, and Crassulacean acid metabolism plants. This method will make possible a variety of new studies of environmental and species effects in carbon isotope fractionation.     

Origin and cycling of riverine inorganic carbon in the Sava River watershed (Slovenia) inferred from major solutes and stable carbon isotopes

The Sava River and its tributaries in Slovenia represent waters strongly influenced by chemical weathering of limestone and dolomite. The carbon isotopic compositions of dissolved inorganic carbon (DIC) and suspended organic carbon (POC) fractions as well as major solute concentrations yielded insights into the origin and fluxes of carbon in the upper Sava River system. The major solute composition was dominated by carbonic acid dissolution of calcite and dolomite. Waters were generally supersaturated with respect to calcite, and dissolved CO₂ was about fivefold supersaturated relative to the atmosphere. The δ¹³C of DIC ranged from −13.5 to −3.3‰. Mass balances for riverine inorganic carbon suggest that carbonate dissolution contributes up to 26%, degradation of organic matter ∼17% and exchange with atmospheric CO₂ up to 5%. The concentration and stable isotope diffusion models indicated that atmospheric exchange of CO₂ predominates in streams draining impermeable shales and clays while in the carbonate-dominated watersheds dissolution of the Mesozoic carbonates predominates.     

Oceanic sinks for atmospheric CO2

There is approximately 50 times more inorganic carbon in the global ocean than in the atmosphere. On time scales of decades to millions of years, the interaction between these two geophysical fluids determines atmospheric CO₂ levels. During glacial periods, for example, the ocean serves as the major sink for atmospheric CO₂, while during glacial–interglacial transitions, it is a source of CO₂ to the atmosphere. The mechanisms responsible for determining the sign of the net exchange of CO₂ between the ocean and the atmosphere remain unresolved. There is evidence that during glacial periods, phytoplankton primary productivity increased, leading to an enhanced sedimentation of particulate organic carbon into the ocean interior. The stimulation of primary production in glacial episodes can be correlated with increased inputs of nutrients limiting productivity, especially aeolian iron. Iron directly enhances primary production in high nutrient (nitrate and phosphate) regions of the ocean, of which the Southern Ocean is the most important. This trace element can also enhance nitrogen fixation, and thereby indirectly stimulate primary production throughout the low nutrient regions of the central ocean basins. While the export flux of organic carbon to the ocean interior was enhanced during glacial periods, this process does not fully account for the sequestration of atmospheric CO₂. Heterotrophic oxidation of the newly formed organic carbon, forming weak acids, would have hydrolyzed CaCO₃ in the sediments, increasing thereby oceanic alkalinity which, in turn, would have promoted the drawdown of atmospheric CO₂. This latter mechanism is consistent with the stable carbon isotope pattern derived from air trapped in ice cores. The oceans have also played a major role as a sink for up to 30% of the anthropogenic CO₂ produced during the industrial revolution. In large part this is due to CO₂ solution in the surface ocean; however, some, poorly quantified fraction is a result of increased new production due to anthropogenic inputs of combined N, P and Fe. Based on ‘circulation as usual’, models predict that future anthropogenic CO₂ inputs to the atmosphere will, in part, continue to be sequestered in the ocean. Human intervention (large-scale Fe fertilization; direct CO₂ burial in the deep ocean) could increase carbon sequestration in the oceans, but could also result in unpredicted environmental perturbations. Changes in the oceanic thermohaline circulation as a result of global climate change would greatly alter the predictions of C sequestration that are possible on a ‘circulation as usual’ basis.     

The mechanism of carbon isotope fractionation in photosynthesis and carbon dioxide component of the greenhouse effect

The relationship between the global climate warming, which is largely induced by increased CO₂ atmospheric concentration, and the changes in carbon isotope fractionation in plants was explained in terms of the previously proposed oscillatory mechanism of photosynthesis, according to which CO₂ assimilation and photorespiration are two reciprocally coupled oscillating mechanisms controlled by ribulose bisphosphate carboxylase/oxygenase switching. This explanation is confirmed by the changes in carbon isotope fractionation in the annual rings of trees and demonstrates that the light carbon isotope ¹²C enrichment before 1990s resulted from increased photosynthetic assimilation of CO₂. The subsequent sharp ¹³C enrichment of the tree ring carbon until the present time suggests that the compensatory role of photosynthesis in boreal forests has been lost for the global climate.     

Air-sea carbon dioxide equilibrium: Will it be possible to use seaweeds for carbon removal offsets?

To limit global warming below 2°C by 2100, we must drastically reduce greenhouse gas emissions and additionally remove ~100–900 Gt CO₂ from the atmosphere (carbon dioxide removal, CDR) to compensate for unavoidable emissions. Seaweeds (marine macroalgae) naturally grow in coastal regions worldwide where they are crucial for primary production and carbon cycling. They are being considered as a biological method for CDR and for use in carbon trading schemes as offsets. To use seaweeds in carbon trading schemes requires verification that seaweed photosynthesis that fixes CO₂ into organic carbon results in CDR, along with the safe and secure storage of the carbon removed from the atmosphere for more than 100 years (sequestration). There is much ongoing research into the magnitude of seaweed carbon storage pools (e.g., as living biomass and as particulate and dissolved organic carbon in sediments and the deep ocean), but these pools do not equate to CDR unless the amount of CO₂ removed from the atmosphere as a result of seaweed primary production can be quantified and verified. The draw-down of atmospheric CO₂ into seawater is via air-sea CO₂ equilibrium, which operates on time scales of weeks to years depending upon the ecosystem considered. Here, we explain why quantifying air-sea CO₂ equilibrium and linking this process to seaweed carbon storage pools is the critical step needed to verify CDR by discrete seaweed beds and nearshore and open ocean aquaculture systems prior to their use in carbon trading.     

Niches of the pre-photosynthetic biosphere and geologic preservation of Earth's earliest ecology

The tree of terrestrial life probably roots in non‐photosynthetic microbes. Chemoautotrophs were the first primary producers, and the globally dominant niches in terms of primary productivity were determined by availability of carbon dioxide and hydrogen for methanogenesis and sulfite reduction. Methanogen niches were most abundant where CO₂‐rich ocean water flowed through serpentinite. Black smoker vents from basalt supplied comparable amount of H₂. Hydrogen from arc volcanoes supported a significant methanogenic niche at the Earth's surface. SO₂ from arc volcanoes reacted with organic matter and hydrogen, providing a significant surface niche. Methane ascended to the upper atmosphere where photolysis produced C‐rich haze and CO, and H escaped into space. The CO and C‐rich haze supported secondary surface niches. None of these ecologies were bountiful; less than 1% of the CO₂ vented by ridge axes, arcs, and metamorphism became organic matter before it was buried in carbonate. In contrast, a photosynthetic biosphere leaves copious amounts of organic carbon, locally concentrated in sediments. Black shales are a classic geologic biosignature for photosynthesis that can survive subduction and high‐grade metamorphism.     

The role of coccolithophore calcification in bioengineering their environment

Coccolithophorids are enigmatic plankton that produce calcium carbonate coccoliths, which over geological time have buried atmospheric CO₂ into limestone, changing both the atmosphere and geology of the Earth. However, the role of coccoliths for the proliferation of these organisms remains unclear; suggestions include roles in anti-predation, enhanced photosynthesis and sun-screening. Here we test the hypothesis that calcification stabilizes the pH of the seawater proximate to the organisms, providing a level of acidification countering the detrimental basification that occurs during net photosynthesis. Such bioengineering provides a more stable pH environment for growth and fits the empirical evidence for changes in rates of calcification under different environmental conditions. Under this scenario, simulations suggest that the optimal production ratio of inorganic to organic particulate C (PIC : POC_prod) will be lower (by approx. 20%) with ocean acidification and that overproduction of coccoliths in a future acidified ocean, where pH buffering is weaker, presents a risk to calcifying cells.     

On the nature of the light-induced component of dark respiration of plants

The data on the isotope composition of carbon of CO₂ of the light-induced dark respiration component have been analyzed using the oscillation model of photosynthesis. It was concluded that this component originates during the transformation of sucrose accumulated by the plant in the oxygenase phase of photosynthetic oscillations in the light period into organic acids. The transformation occurs in the dark period. It this process, C-3 and C-4 atoms, which determine the “heavy“ isotope composition of carbon of CO₂ of the light-induced dark respiration component, are split off from the hexose link of sucrose.     

Contribution of stem CO2 fixation to whole-plant carbon balance in nonsucculent species

In many plant species that remain leafless part of the year, CO₂ fixation occurring in green stems represents an important carbon gain. Traditionally, a distinction has been made between stem photosynthesis and corticular photosynthesis. All stem photosynthesis is, sensu stricto, cortical, since it is carried out largely by the stem cortex. We proposed the following nomenclature: stem net photosynthesis (SNP), which includes net CO₂ fixation by stems with stomata in the epidermis and net corticular CO₂ fixation in suberized stems, and stem recycling photosynthesis (SRP), which defines CO₂ ling in suberized stems. The proposed terms should reflect differences in anatomical and physiological traits. SNP takes place in the chlorenchyma below the epidermis with stomata, where the net CO₂ uptake occurs, and it resembles leaf photosynthesis in many characteristics. SRP is found in species where the chlorenchyma is beneath a well-developed stomata-free periderm and where reassimilation of internally respired CO₂ occurs. SNP is common in plants from desert ecosystems, rates reaching up to 60% of the leaf photosynthetic rate. SRP has been demonstrated in trees from temperate forests and it offsets partially a carbon loss by respiration of stem nonphotosynthetic tissues. Reassimilation can vary between 7 and 123% of respired CO₂, the latter figure implying net CO₂ uptake from the atmosphere. Both types of stem photosynthesis contribute positively to the carbon economy of the species, in which they occur; they are advantageous to the plant because they allow the maintenance of physiological activity during stress, an increase of integrated water use efficiency, and they provide the carbon source used in the production of new organs.     

Methanogenesis in Eocene Arctic soils inferred from δC of tree fossil carbonates

We report on the carbon and oxygen stable isotope composition of fossil tree material collected at the White Mountain locality of the Buchanan Lake Formation on Axel Heiberg Island in the High Arctic of Canada. The fossils are Middle Eocene in age and have been permineralized with carbonate. Microscopic examination of fossils revealed them to be the remains of Metasequoia stems, composed of secondary carbonate (calcite) and original wood intermingled at the cellular level. Because the specimens show little compression, crushing, or tissue degradation, we believe that carbonate permineralization occurred soon after burial, and therefore provides insight into Eocene carbon cycling at the locality. The carbon isotope signature of the carbonate suggests that methanogenesis resulted in a ¹³C-enriched CO₂ pool that equilibrated with soil water and gave rise to unusually ¹³C-enriched CaCO₃. Tree fossil carbonate exhibited strikingly high δ¹³C values (+4.0 to +7.4‰) compared to published Phanerozoic pedogenic carbonate δ¹³C values. These δ¹³C values, in conjunction with fractionation factors (α) previously determined for carbonate precipitation and methanogenic pathways, indicate an Eocene soil CO₂ pool containing 80-95% CO₂ produced as a by-product of acetate-fermentation methanogenesis. Because methane in the atmosphere is a powerful greenhouse gas, we suggest that methane emissions from Axel Heiberg soils contributed to the relatively warm Arctic climate during the Middle Eocene.     

湖泊生态系统碳汇特征及其潜在碳中和价值研究

为了应对气候挑战,达成碳达峰远景目标,需要正确评估自然资源碳中和价值。湖泊作为具有独特生态、人文价值的地理单元,因碳循环强度高、碳排放总量大,是传统意义上的碳源。通过梳理近期相关研究成果,对比不同类型湖泊碳汇/源状况,湖泊生态系统以一系列碳汇特征表现出潜在的碳中和价值。强烈的光合作用可以使水体CO₂欠饱和,但由呼吸-光合作用、碳酸盐岩溶蚀作用带来的水体碱度、CO₂分压pCO₂提高也有利于湖泊碳汇增益。CO₂在水体中大量溶解,积极参与到湖泊碳循环,将pCO₂高于40 Pa作为判断湖泊为碳源的依据可能忽视了水体碱度上升带来的碳汇。在湖泊沉积物中有机碳的累积受到生态系统光合-呼吸作用的影响,当异养微生物群落能及时分解沉入湖底的衰亡组织、有机质时,沉积物中有机碳不会大量累积,当呼吸对光合的相对滞后,有机碳才会大量累积。湖泊生态系统的生产力决定了固碳能力,是湖泊发挥碳汇效益的重要“碳库”。由水生植物固定下的CO₂总量不如浮游植物,但在过程中发挥了“压舱石”般的稳定作...     

The curious consistency of carbon biosignatures over billions of years of Earth-life coevolution

The oldest and most wide-ranging signal of biological activity (biosignature) on our planet is the carbon isotope composition of organic materials preserved in rocks. These biosignatures preserve the long-term evolution of the microorganism-hosted metabolic machinery responsible for producing deviations in the isotopic compositions of inorganic and organic carbon. Despite billions of years of ecosystem turnover, evolutionary innovation, organismic complexification, and geological events, the organic carbon that is a residuum of the global marine biosphere in the rock record tells an essentially static story. The ~25‰ mean deviation between inorganic and organic ¹³C/¹²C values has remained remarkably unchanged over >3.5 billion years. The bulk of this record is conventionally attributed to early-evolved, RuBisCO-mediated CO₂ fixation that, in extant oxygenic phototrophs, produces comparable isotopic effects and dominates modern primary production. However, billions of years of environmental transition, for example, in the progressive oxygenation of the Earth’s atmosphere, would be expected to have accompanied shifts in the predominant RuBisCO forms as well as enzyme-level adaptive responses in RuBisCO CO₂-specificity. These factors would also be expected to result in preserved isotopic signatures deviating from those produced by extant RuBisCO in oxygenic phototrophs. Why does the bulk carbon isotope record not reflect these expected environmental transitions and evolutionary innovations? Here, we discuss this apparent discrepancy and highlight the need for greater quantitative understanding of carbon isotope fractionation behavior in extant metabolic pathways. We propose novel, laboratory-based approaches to reconstructing ancestral states of carbon metabolisms and associated enzymes that can constrain isotopic biosignature production in ancient biological systems. Together, these strategies are crucial for integrating the complementary toolsets of biological and geological sciences and for interpretation of the oldest record of life on Earth.Subject terms: Bacterial evolution, Applied microbiology, Biogeochemistry     

Carbon respired by terrestrial ecosystems – recent progress and challenges

Net ecosystem production is the residual of two much larger fluxes: photosynthesis and respiration. While photosynthesis is a single process with a well‐established theoretical underpinning, respiration integrates the variety of plant and microbial processes by which CO₂ returns from ecosystems to the atmosphere. Limits to current capacity for predicting ecosystem respiration fluxes across biomes or years result from the mismatch between what is usually measured – bulk CO₂ fluxes – and what process‐based models can predict – fluxes of CO₂ from plant (autotrophic) or microbial (heterotrophic) respiration. Papers in this Thematic Issue and in the recent literature, document advances in methods for separating respiration into autotrophic and heterotrophic components using three approaches: (1) continuous measurements of CO₂ fluxes and assimilation of these data into process‐based models; (2) application of isotope measurements, particularly radiocarbon; and (3) manipulation experiments. They highlight the role of allocation of C fixed by plants to respiration, storage, growth or transfer to other organisms as a control of seasonal and interannual variability in soil respiration and the oxidation state of C in the terrestrial biosphere. A second theme is the potential for comparing C isotope signatures in organic matter, CO₂ evolved in incubations and microbial biomarkers to elucidate the pathways (respiration, recycling, or transformation) of C during decomposition. Together, these factors determine the continuum of timescales over which C is returned to the atmosphere by respiration and enable testing of theories of plant and microbial respiration that go beyond empirical models and allow predictions of future respiration responses to future change in climate, pollution and land use.     

CO2‐concentrating mechanisms in three southern hemisphere strains of Emiliania huxleyi

Rising global CO₂ is changing the carbonate chemistry of seawater, which is expected to influence the way phytoplankton acquire inorganic carbon. All phytoplankton rely on ribulose‐bisphosphate carboxylase oxygenase (RUBISCO) for assimilation of inorganic carbon in photosynthesis, but this enzyme is inefficient at present day CO₂ levels. Many algae have developed a range of energy demanding mechanisms, referred to as carbon concentrating mechanisms (CCMs), which increase the efficiency of carbon acquisition. We investigated CCM activity in three southern hemisphere strains of the coccolithophorid Emiliania huxleyi W. W. Hay & H. P. Mohler. Both calcifying and non‐calcifying strains showed strong CCM activity, with HCO₃^? as a preferred source of photosynthetic carbon in the non‐calcifying strain, but a higher preference for CO₂ in the calcifying strains. All three strains were characterized by the presence of pyrenoids, external carbonic anhydrase (CA) and high affinity for CO₂ in photosynthesis, indicative of active CCMs. We postulate that under higher CO₂ levels cocco‐lithophorids will be able to down‐regulate their CCMs, and re‐direct some of the metabolic energy to processes such as calcification. Due to the expected rise in CO₂ levels, photosynthesis in calcifying strains is expected to benefit most, due to their use of CO₂ for carbon uptake. The non‐calcifying strain, on the other hand, will experience only a 10% increase in HCO₃^?, thus making it less responsive to changes in carbonate chemistry of water.     

Prebiotic Synthesis of Methionine and Other Sulfur-Containing Organic Compounds on the Primitive Earth: A Contemporary Reassessment Based on an Unpublished 1958 Stanley Miller Experiment

Original extracts from an unpublished 1958 experiment conducted by the late Stanley L. Miller were recently found and analyzed using modern state-of-the-art analytical methods. The extracts were produced by the action of an electric discharge on a mixture of methane (CH₄), hydrogen sulfide (H₂S), ammonia (NH₃), and carbon dioxide (CO₂). Racemic methionine was formed in significant yields, together with other sulfur-bearing organic compounds. The formation of methionine and other compounds from a model prebiotic atmosphere that contained H₂S suggests that this type of synthesis is robust under reducing conditions, which may have existed either in the global primitive atmosphere or in localized volcanic environments on the early Earth. The presence of a wide array of sulfur-containing organic compounds produced by the decomposition of methionine and cysteine indicates that in addition to abiotic synthetic processes, degradation of organic compounds on the primordial Earth could have been important in diversifying the inventory of molecules of biochemical significance not readily formed from other abiotic reactions, or derived from extraterrestrial delivery.     

Hydrogen peroxide and the evolution of oxygenic photosynthesis

The early atmosphere of the Earth is considered to have been reducing (H₂ rich) or neutral (CO₂-N₂). The present atmosphere by contrast is highly oxidizing (20% O₂). The source of this oxygen is generally agreed to have been oxygenic photosynthesis, whereby organisms use water as the electron donor in the production of organic matter, liberating oxygen into the atmosphere. A major question in the evolution of life is how oxygenic photosynthesis could have evolved under anoxic conditions — and also when this capability evolved. It seems unlikely that water would be employed as the electron donor in anoxic environments that were rich in reducing agents such as ferrous or sulfide ions which could play that role. The abiotic production of atmospheric oxidants could have provided a mechanism by which locally oxidizing conditions were sustained within spatially confined habitats thus removing the available reductants and forcing photosynthetic organisms to utilize water as the electron donor. We suggest that atmospheric H₂O₂ played the key role in inducing oxygenic photosynthesis because as peroxide increased in a local environment, organisms would not only be faced with a loss of reductant, but they would also be pressed to develop the biochemical apparatus (e.g., catalase) that would ultimately be needed to protect against the products of oxygenic photosynthesis. This scenario allows for the early evolution of oxygenic photosynthesis while global conditions were still anaerobic.     

树干皮层光合作用--生理生态功能和测定方法

大部分植物的树干(枝条)等部位含有能进行光合作用的绿色组织,树皮叶绿素含量最高可达750 mg/m2。这些绿色组织能够再固定树干内部的CO2(来源于自身组织呼吸或者木质部液流运输),使树干向大气排放的CO2量减少60%—90%皮层光合作用是树干生理活动的重要组成部分,其与树干呼吸和液流速率之间均有密切关系,对植物的碳平衡有重要作用。概述了皮层光合作用的生理生态功能;介绍了皮层光合作用测定和计算方法;讨论了皮层光合作用研究存在的问题;通过加入皮层光合作用的测量修正质量平衡法,以减少树干呼吸测定的不确定性。建议综合运用稳定碳同位素示踪、CO2和O2微传感器、树干液流技术等,准确地区分树干内部CO2的来源及比例,分析各个组分与影响因素的关系。同时,在微观上揭示皮层光合作用的基因组调控功能,在宏观上探讨尺度扩展、模型模拟,并与涡度协方差技术和遥感技术相融合以提高区域尺度估算的精度。