Physiological and ecological controls on carbon sequestering in terrestrial ecosystems

Carbon cycling processes in ecosystems are generally believed to be well understood. Carbon, hydrogen, oxygen and other essential elements are chemically converted from inorganic to organic compounds primarily in the process of photosynthesis. Secondary metabolic processes cycle carbon in and among organisms and carbon is ultimately released back to the environment as CO₂ by respiratory processes. Unfortunately, our understanding of this cycle was determined under the assumption that the primary inorganic form of C (CO₂ in the atmosphere) was relatively constant. With the emerging concensus that atmospheric carbon concentration is increasing, we must now reassess our understanding of the carbon cycle. How will plants, animals and decomposers respond to a doubling of carbon supply? Will biological productivity be accelerated? If plant productivity increases will a predictable percentage of the increase be accumulated as increased standing crop? Or, is it possible that doubling the availability of CO₂ will increase metabolic activity at all trophic levels resulting in no net increase in system standing crop? The purpose of this paper is to review evidence for physiological and growth responses of plants to carbon dioxide enhancement. Essentially no research has been completed on the ecological aspects of these questions. From this review, I conclude that accurate predictions of future ecosystem responses to increasing atmospheric carbon dioxide concentration are not possible without additional understanding of physiological and ecological mechanisms.     

An independent method of deriving the carbon dioxide fertilization effect in dry conditions using historical yield data from wet and dry years

Accurate estimates of the fertilization effect that elevated carbon dioxide [CO₂] has on crop yields are valuable for estimation of future crop production, yet there is still some controversy over these estimates due to possible CO₂‐by‐water‐status interactions in chamber studies and the difficulty of conducting field experiments with elevated [CO₂]. This study presents a new method to estimate the CO₂ fertilization effect (CFE) in dry conditions (CFE_dry), based on a combination of historical yield and climatic data and field experiments that do not require elevated [CO₂]. It was estimated that approximately 50 years of increasing [CO₂] (i.e., a 73 ppm increase) resulted in a 9% and 14% improvement of yield in dry conditions for maize and soybean, respectively, which are similar to estimates derived from free air CO₂ enrichment (FACE) studies. The main source of uncertainty in this approach relates to differential effects of technology trends such as new cultivars in wet vs. dry years. Estimates of this technology–water interaction can be refined by further experimentation under ambient [CO₂], offering a cost‐effective path for improving CFE estimates. The results should prove useful for modeling future yield impacts of climate change, and the approach could be used to derive estimates for other species using relatively simple yield trials.     

Estimation of the carbon dioxide (CO2) fertilization effect using growth rate anomalies of CO2 and crop yields since 1961

The effect of elevated carbon dioxide (CO₂) on crop yields is one of the most uncertain and influential parameters in models used to assess climate change impacts and adaptations. A primary reason for this uncertainty is the limited availability of experimental data on CO₂ responses for crops grown under typical field conditions. However, because of historical variations in CO₂, each year farmers throughout the world perform uncontrolled yield ‘experiments’ under different levels of CO₂. In this study, measurements of atmospheric CO₂ growth rates and crop yields for individual countries since 1961 were compared to empirically determine the average effect of a 1 ppm increase of CO₂ on yields of rice, wheat, and maize. Because the gradual increase in CO₂ is highly correlated with major changes in technology, management, and other yield controlling factors, we focused on first differences of CO₂ and yield time series. Estimates of CO₂ responses obtained from this approach were highly uncertain, reflecting the relatively small importance of year‐to‐year CO₂ changes for yield variability. Combining estimates from the top 20 countries for each crop resulted in estimates with substantially less uncertainty than from any individual country. The results indicate that while current datasets cannot reliably constrain estimates beyond previous experimental studies, an empirical approach supported by large amounts of data may provide a potentially valuable and independent assessment of this critical model parameter. For example, analysis of reliable yield records from hundreds of individual, independent locations (as opposed to national scale yield records with poorly defined errors) may result in empirical estimates with useful levels of uncertainty to complement estimates from experimental studies.     

Impacts of elevated CO2 concentration on the productivity and surface energy budget of the soybean and maize agroecosystem in the Midwest USA

The physiological response of vegetation to increasing atmospheric carbon dioxide concentration ([CO₂]) modifies productivity and surface energy and water fluxes. Quantifying this response is required for assessments of future climate change. Many global climate models account for this response; however, significant uncertainty remains in model simulations of this vegetation response and its impacts. Data from in situ field experiments provide evidence that previous modeling studies may have overestimated the increase in productivity at elevated [CO₂], and the impact on large‐scale water cycling is largely unknown. We parameterized the Agro‐IBIS dynamic global vegetation model with observations from the SoyFACE experiment to simulate the response of soybean and maize to an increase in [CO₂] from 375 ppm to 550 ppm. The two key model parameters that were found to vary with [CO₂] were the maximum carboxylation rate of photosynthesis and specific leaf area. Tests of the model that used SoyFACE parameter values showed a good fit to site‐level data for all variables except latent heat flux over soybean and sensible heat flux over both crops. Simulations driven with historic climate data over the central USA showed that increased [CO₂] resulted in decreased latent heat flux and increased sensible heat flux from both crops when averaged over 30 years. Thirty‐year average soybean yield increased everywhere (ca. 10%); however, there was no increase in maize yield except during dry years. Without accounting for CO₂ effects on the maximum carboxylation rate of photosynthesis and specific leaf area, soybean simulations at 550 ppm overestimated leaf area and yield. Our results highlight important model parameter values that, if not modified in other models, could result in biases when projecting future crop–climate–water relationships.     

Olfactory carbon dioxide detection by insects and other animals

Carbon dioxide is a small, relatively inert, but highly volatile gas that not only gives beer its bubbles, but that also acts as one of the primary driving forces of anthropogenic climate change. While beer brewers experiment with the effects of CO₂ on flavor and climate scientists are concerned with global changes to ambient CO₂ levels that take place over the course of decades, many animal species are keenly aware of changes in CO₂ concentration that occur much more rapidly and on a much more local scale. Although imperceptible to us, these small changes in CO₂ concentration can indicate imminent danger, signal overcrowding, and point the way to food. Here I review several of these CO₂-evoked behaviors and compare the systems insects, nematodes, and vertebrates use to detect environmental CO₂.     

Response of isoprene emission to ambient CO2 changes and implications for global budgets

We explore the potential role of atmospheric carbon dioxide (CO₂) on isoprene emissions using a global coupled land–atmosphere model [Community Atmospheric Model–Community Land Model (CAM–CLM)] for recent (year 2000, 365 ppm CO₂) and future (year 2100, 717 ppm CO₂) conditions. We incorporate an empirical model of observed isoprene emissions response to both ambient CO₂ concentrations in the long‐term growth environment and short‐term changes in intercellular CO₂ concentrations into the MEGAN biogenic emission model embedded within the CLM. Accounting for CO₂ inhibition has little impact on predictions of present‐day global isoprene emission (increase from 508 to 523 Tg C yr^?1). However, the large increases in future isoprene emissions typically predicted in models, which are due to a projected warmer climate, are entirely offset by including the CO₂ effects. Projected global isoprene emissions in 2100 drop from 696 to 479 Tg C yr^?1 when this effect is included, maintaining future isoprene sources at levels similar to present day. The isoprene emission response to CO₂ is dominated by the long‐term growth environment effect, with modulations of 10% or less due to the variability in intercellular CO₂ concentration. As a result, perturbations to isoprene emissions associated with changes in ambient CO₂ are largely aseasonal, with little diurnal variability. Future isoprene emissions increase by more than a factor of two in 2100 (to 1242 Tg C yr^?1) when projected changes in vegetation distribution and leaf area density are included. Changing land cover and the role of nutrient limitation on CO₂ fertilization therefore remain the largest source of uncertainty in isoprene emission prediction. Although future projections suggest a compensatory balance between the effects of temperature and CO₂ on isoprene emission, the enhancement of isoprene emission due to lower ambient CO₂ concentrations did not compensate for the effect of cooler temperatures over the last 400 thousand years of the geologic record (including the Last Glacial Maximum).     

Quantitative and qualitative evaluation of selected wheat varieties released since 1903 to increasing atmospheric carbon dioxide: can yield sensitivity to carbon dioxide be a factor in wheat performance?

The sensitivity of yield and quality parameters to carbon dioxide concentration [CO₂] was determined for individual lines of hard‐red spring wheat released in 1903, 1921, 1965 and 1996. All cultivars were evaluated with respect to growth and vegetative characteristics, grain yield and nutritional quality in response to [CO₂] increases that corresponded roughly to the CO₂ concentrations at the beginning of the 20th century, the current [CO₂], and the future projected [CO₂] for the end of the 21st century, respectively. Leaf area ratio (cm² g^?1) declined and net assimilation rate (g m² day^?1) increased in response to increasing [CO₂] for all cultivars during early vegetative growth. By maturity, vegetative growth of all cultivars significantly increased with the increase in [CO₂]. Seed yield increased significantly as [CO₂] increased, with yield sensitivity to rising [CO₂] inversely proportional to the year of cultivar release. Greater [CO₂] yield sensitivity in older cultivars was associated with whole‐plant characteristics such as increased tillering and panicle formation. Grain and flour protein, however, declined significantly with increasing [CO₂] and with year of release for all cultivars, although absolute values were higher for the older cultivars. Overall, these data indicate that yield response at the whole‐plant level to recent and projected increases in [CO₂] has declined with the release of newer cultivars, as has protein content of grain and flour. However, if agronomic practice can be adapted to maximize individual plant performance, [CO₂] responsive characteristics of older cultivars could, potentially, be incorporated as factors in future wheat selection.     

Super-optimal temperatures are detrimental to peanut (Arachis hypogaea L.) reproductive processes and yield at both ambient and elevated carbon dioxide

Continuing increases in atmospheric carbon dioxide concentration (CO₂) will likely be accompanied by global warming. Our research objectives were (a) to determine the effects of season‐long exposure to daytime maximum/nighttime minimum temperatures of 32/22, 36/26, 40/30 and 44/34°C at ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ on reproductive processes and yield of peanut, and (b) to evaluate whether the higher photosynthetic rates and vegetative growth at elevated CO₂ will negate the detrimental effects of high temperature on reproductive processes and yield. Doubling of CO₂ increased leaf photosynthesis and seed yield by 27% and 30%, respectively, averaged across all temperatures. There were no effects of elevated CO₂ on pollen viability, seed‐set, seed number per pod, seed size, harvest index or shelling percentage. At ambient CO₂, seed yield decreased progressively by 14%, 59% and 90% as temperature increased from 32/22 to 36/26, 40/30 and 44/34°C, respectively. Similar percentage decreases in seed yield occurred at temperatures above 32/22°C at elevated CO₂ despite greater photosynthesis and vegetative growth. Decreased seed yields at high temperature were a result of lower seed‐set due to poor pollen viability, and smaller seed size due to decreased seed growth rates and decreased shelling percentages. Seed harvest index decreased from 0.41 to 0.05 as temperature increased from 32/22 to 44/34°C under both ambient and elevated CO₂. We conclude that there are no beneficial interactions between elevated CO₂ and temperature, and that seed yield of peanut will decrease under future warmer climates, particularly in regions where present temperatures are near or above optimum.     

Contrasting crop species responses to CO2 and temperature: rice,soybean and citrus

The continuing increase in atmospheric carbon dioxide concentration ([CO₂]) and projections of possible future increases in global air temperatures have stimulated interest in the effects of these climate variables on plants and, in particular, on agriculturally important food crops. Mounting evidence from many different experiments suggests that the magnitude and even direction of crop responses to [CO₂] and temperature is almost certain to be species dependent and very likely, within a species, to be cultivar dependent. Over the last decade, [CO₂] and temperature experiments have been conducted on several crop species in the outdoor, naturally-sunlit, environmentally controlled, plant growth chambers by USDA-ARS and the University of Florida, at Gainesville, Florida, USA. The objectives for this paper are to summarize some of the major findings of these experiments and further to compare and contrast species responses to [CO₂] and temperature for three diverse crop species: rice (Oryza sativa, L.), soybean (Glycine max, L.) and citrus (various species). Citrus had the lowest growth and photosynthetic rates but under [CO₂] enrichment displayed the greatest percentage increases over ambient [CO₂] control treatments. In all three species the direct effect of [CO₂] enrichment was always an increase in photosynthetic rate. In soybean, photosynthetic rate depended on current [CO₂] regardless of the long-term [CO₂] history of the crop. In rice, photosynthetic rate measured at a common [CO₂], decreased with increasing long-term [CO₂] growth treatment due to a corresponding decline in RuBP carboxylase content and activity. Rice specific respiration decreased from subambient to ambient and superambient [CO₂] due to a decrease in plant tissue nitrogen content and a decline in specific maintenance respiration rate. In all three species, crop water use decreased with [CO₂] enrichment but increased with increases in temperature. For both rice and soybean, [CO₂] enrichment increased growth and grain yield. Rice grain yields declined by roughly 10 % per each 1 °C rise in day/night temperature above 28/21 °C.     

Response of maize biomass and soil water fluxes on elevated CO2 and drought—From field experiments to process‐based simulations

The rising concentration of atmospheric carbon dioxide (CO₂) is known to increase the total aboveground biomass of several C3 crops, whereas C4 crops are reported to be hardly affected when water supply is sufficient. However, a free‐air carbon enrichment (FACE) experiment in Braunschweig, Germany, in 2007 and 2008 resulted in a 25% increased biomass of the C4 crop maize under restricted water conditions and elevated CO₂ (550 ppm). To project future yields of maize under climate change, an accurate representation of the effects of eCO₂ and drought on biomass and soil water conditions is essential. Current crop growth models reveal limitations in simulations of maize biomass under eCO₂ and limited water supply. We use the coupled process‐based hydrological‐plant growth model Catchment Modeling Framework‐Plant growth Modeling Framework to overcome this limitation. We apply the coupled model to the maize‐based FACE experiment in Braunschweig that provides robust data for the investigation of combined CO₂ and drought effects. We approve hypothesis I that CO₂ enrichment has a small direct‐fertilizing effect with regard to the total aboveground biomass of maize and hypothesis II that CO₂ enrichment decreases water stress and leads to higher yields of maize under restricted water conditions. Hypothesis III could partly be approved showing that CO₂ enrichment decreases the transpiration of maize, but does not raise soil moisture, while increasing evaporation. We emphasize the importance of plant‐specific CO₂ response factors derived by use of comprehensive FACE data. By now, only one FACE experiment on maize is accomplished applying different water levels. For the rigorous testing of plant growth models and their applicability in climate change studies, we call for datasets that go beyond single criteria (only yield response) and single effects (only elevated CO₂).     

CO2 enrichment using CRAM fermentation improves growth,physiological traits and yield of cherry tomato (Solanum lycopersicum L.)

Carbon dioxide (CO₂) concentration in greenhouses is sub-optimal for vegetable production. Many techniques have been used to increase CO₂ concentration in greenhouses but most of them are expensive with certain limitations and drawbacks. We adopted a new strategy to elevate CO₂ concentration in the greenhouse throughout the day via crop residues and animal manure composting (CRAM). During the whole cultivation period, CRAM-treated greenhouse had doubled CO₂ concentration which significantly increased the yield of cherry tomatoes (Solanum lycopersicum L.), i.e., up to 38%. The influence of CRAM procedure on cherry tomato quality was also investigated and the concentrations of total soluble solids (TSS) and soluble sugar were found to be significantly higher in cherry tomatoes grown under composting greenhouse than that of non-composting greenhouse. Additionally, CRAM-CO₂ enrichment also resulted in increased concentrations of ascorbic acid (Vitamin C) and titrate acid as compared to control. In contrast, the concentration of nitrate was considerably decreased in cherry tomato grown under CO₂ enriched condition than that of control. The increase in active oxygen metabolisms such as POD, CAT and SOD while a decrease in MDA, as well as APX was observed for cherry tomatoes grown under CO₂ enriched condition. Hence, CO₂ fertilization by using CRAM in greenhouse significantly improved quality and increased the yield of cherry tomatoes.     

Understanding crop physiology to select breeding targets and improve crop management under increasing atmospheric CO2 concentrations

The present overview paper reviews knowledge on plant metabolism under elevated atmospheric CO₂ concentrations (e[CO₂]) with regard to underpinning options for the management of crop production systems and the selection of crop traits beneficial for future conditions.Better understanding of intra-specific variability in responses to e[CO₂] is of great importance to breed or select best possible genotypes for future conditions. Yield increases per 100 μL L⁻¹ increase in [CO₂] varied between none and over 30% among varieties of important crops. Carbon source–sink relationships are believed to play a major role in determining the ability of a plant to utilise e[CO₂] and avoid downward acclimation of photosynthesis upon prolonged e[CO₂] exposure. Corresponding traits (e.g. tillering capacity, stem carbohydrate storage capacity, or seed size and numbers) are currently under investigation in Free Air Carbon dioxide Enrichment (FACE) facilities, such as AGFACE (Australian Grains FACE).The stimulatory effect of e[CO₂] on plant growth is dependent on adequate nutrient supply. For example, N concentrations in plant tissues generally decrease under e[CO₂], which in leaves is commonly related to a decrease in Rubisco concentration and activity, and therefore linked to photosynthetic downward acclimation. This effect is also of direct concern for food production where decreased N and protein content can have negative effects on product quality (e.g. grain protein). Plant nutrient metabolism appears to adjust to a new physiological equilibrium under e[CO₂] which limits the extent to which nutrient application can ameliorate the situation. What the control points are for an adjustment of plant N metabolism is unclear. Rubisco metabolism in leaves, N assimilation, N translocation or N uptake are all potential key steps that may be inhibited or downregulated under e[CO₂]. To achieve the best possible growth response whilst maintaining product quality, it is important to understand plant nutrient metabolism under e[CO₂].Comparatively little is known about mechanisms of potential changes in plant stress tolerance under e[CO₂]. Defence metabolites such as antioxidants are, in part, directly linked to primary carbohydrate mechanism and so potentially impacted by e[CO₂]. It is unknown whether photoprotective and antioxidative defence systems, key to plant stress tolerance, will be affected, and if so, whether the response will be strengthened or weakened by e[CO₂]. Better understanding of underlying principles is particularly important because it is virtually impossible to test all possible stress factor combinations with e[CO₂] in realistic field settings.     

The impact of elevated CO2 on yield loss from a C3 and C4 weed in field-grown soybean

Soybean (Glycine max) was grown at ambient and enhanced carbon dioxide (CO₂, + 250 μL L^?1 above ambient) with and without the presence of a C3 weed (lambsquarters, Chenopodium album L.) and a C4 weed (redroot pigweed, Amaranthus retroflexus L.), in order to evaluate the impact of rising atmospheric carbon dioxide concentration [CO₂] on crop production losses due to weeds. Weeds of a given species were sown at a density of two per metre of row. A significant reduction in soybean seed yield was observed with either weed species relative to the weed‐free control at either [CO₂]. However, for lambsquarters the reduction in soybean seed yield relative to the weed‐free condition increased from 28 to 39% as CO₂ increased, with a 65% increase in the average dry weight of lambsquarters at enhanced [CO₂]. Conversely, for pigweed, soybean seed yield losses diminished with increasing [CO₂] from 45 to 30%, with no change in the average dry weight of pigweed. In a weed‐free environment, elevated [CO₂] resulted in a significant increase in vegetative dry weight and seed yield at maturity for soybean (33 and 24%, respectively) compared to the ambient CO₂ condition. Interestingly, the presence of either weed negated the ability of soybean to respond either vegetatively or reproductively to enhanced [CO₂]. Results from this experiment suggest: (i) that rising [CO₂] could alter current yield losses associated with competition from weeds; and (ii) that weed control will be crucial in realizing any potential increase in economic yield of agronomic crops such as soybean as atmospheric [CO₂] increases.     

Use of Chlorella vulgaris for CO(2) mitigation in a photobioreactor

Carbon dioxide (CO₂) is a colorless gas that exists at a concentration of approximately 330 ppm in the atmosphere and is released in great quantities when fossil fuels are burned. The current flux of carbon out of fossil fuels is about 600 times greater than that into fossil fuels. With increased concerns about global warming and greenhouse gas emissions, there have been several approaches proposed for managing the levels of CO₂ emitted into the atmosphere. One of the most understudied methods for CO₂ mitigation is the use of biological processes in engineered systems such as photobioreactors. This research project describes the effectiveness of Chlorella vulgaris, used in a photobioreactor with a very short gas residence time, in sequestering CO₂ from an elevated CO₂ airstream. We evaluated a flow-through photobioreactor's operational parameters, as well as the growth characteristics of the C. vulgaris inoculum when exposed to an airstream with over 1850 ppm CO₂. When using dry weight, chlorophyll, and direct microscopic measurements, it was apparent that the photobioreactor's algal inoculum responded well to the elevated CO₂ levels and there was no build-up of CO₂ or carbonic acid in the photobioreactor. The photobioreactor, with a gas residence time of approximately 2 s, was able to remove up to 74% of the CO₂ in the airstream to ambient levels. This corresponded to a 63.9-g/m³/h bulk removal for the experimental photobioreactor. Consequently, this photobioreactor shows that biological processes may have some promise for treating point source emissions of CO₂ and deserve further study. Received 25 April 2002/ Accepted in revised form 27 July 2002     

Effect of controlled carbon dioxide on in vitro shoot multiplication in Feronia limonia (L.) Swingle

The effect of controlled carbon dioxide environment on in vitro shoot growth and multiplication in Feronia limonia (a tropical fruit plant, Family- Rutaceae) was studied. Carbon dioxide available in the ambient air of the growth room was insufficient for in vitro growth of the shoots alone. Also, the presence of sucrose only as the C-source in the medium (without CO₂), was found to be inadequate for sustainable growth and multiplication of shoots. The carbon dioxide enrichment promoted shoot multiplication and overall growth. The promotory effect of CO₂ was independent of the presence of sucrose in the medium. In the presence of both CO₂ and sucrose, an additive effect was observed producing maximum shoot growth. In the absence of sucrose a higher concentration of CO₂ (10.0)g m⁻³ was required to achieve photoautotrophic shoot multiplication comparable to ambient air controls. Highest leaf area per shoot cluster promoting shoot growth and multiplication was recorded under this treatment. Shoots growing on sucrose containing medium under controlled CO₂ environment of 0.6 g m⁻³ concentration evoked better response than ambient air controls (shoots growing on sucrose containing medium) in growth room. This treatment produced the overall best response. The present study highlighted the possibility of photoautotrophic multiplication which might prove useful for successful hardening and acclimatization in tissue culture plants.     

Long-term CO2 production from deeply weathered soils of a tropical rain forest: evidence for a potential positive feedback to climate warming

Currently, it is unknown what role tropical forest soils will play in the future global carbon cycle under higher temperatures. Many tropical forests grow on deeply weathered soils and although it is generally accepted that soil carbon decomposition increases with higher temperatures, it is not known whether subsurface carbon pools are particularly responsive to increasing soil temperatures. Carbon dioxide (CO₂) diffusing out of soils is an important flux in the global carbon. Although soil CO₂ efflux has been the subject of many studies in recent years, it remains difficult to deduct controls of this flux because of the different sources that produce CO₂ and because potential environmental controls like soil temperature and soil moisture often covary. Here, we report results of a 5‐year study in which we measured soil CO₂ production on two deeply weathered soil types at different depths in an old‐growth tropical wet forest in Costa Rica. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). Annual soil CO₂ efflux varied between 2.8–3.6 μmol CO₂‐C m^?2 s^?1 (old alluvium) and 3.4–3.9 μmol CO₂‐C m^?2 s^?1 (residual). More than 75% of the CO₂ was produced in the upper 0.5 m (including litter layer) and less than 7% originated from the soil below 1 m depth. This low contribution was explained by the lack of water stress in this tropical wet forest which has resulted in very low root biomass below 2 m depth. In the top 0.5 m CO₂ production was positively correlated with both temperature and soil moisture; between 0.6 and 2 m depth CO₂ production correlated negatively with soil moisture in one soil and positively with photosynthetically active radiation in the other soil type. Below 2 m soil CO₂ production strongly increased with increasing temperature. In combination with reduced tree growth that has been shown for this ecosystem, this would be a strong positive feedback to ecosystem warming.     

Will intra‐specific differences in transpiration efficiency in wheat be maintained in a high CO2 world? A FACE study

This study evaluates whether the target breeding trait of superior leaf level transpiration efficiency is still appropriate under increasing carbon dioxide levels of a future climate using a semi‐arid cropping system as a model. Specifically, we investigated whether physiological traits governing leaf level transpiration efficiency, such as net assimilation rates (A_net), stomatal conductance (g_s) or stomatal sensitivity were affected differently between two Triticum aestivum L. cultivars differing in transpiration efficiency (cv. Drysdale, superior; cv. Hartog, low). Plants were grown under Free Air Carbon dioxide Enrichment (FACE, approximately 550 µmol mol^?1 or ambient CO₂ concentrations (approximately 390 µmol mol^?1). Mean A_net (approximately 15% increase) and g_s (approximately 25% decrease) were less affected by elevated [CO₂] than previously found in FACE‐grown wheat (approximately 25% increase and approximately 32% decrease, respectively), potentially reflecting growth in a dry‐land cropping system. In contrast to previous FACE studies, analyses of the Ball et al. model revealed an elevated [CO₂] effect on the slope of the linear regression by 12% indicating a decrease in stomatal sensitivity to the combination of [CO₂], photosynthesis rate and humidity. Differences between cultivars indicated greater transpiration efficiency for Drysdale with growth under elevated [CO₂] potentially increasing the response of this trait. This knowledge adds valuable information for crop germplasm improvement for future climates.     

Production,purification, and characterization of a fusion protein of carbonic anhydrase from Neisseria gonorrhoeae and cellulose binding domain from Clostridium thermocellum

Carbon dioxide capture technologies have the potential to become an important climate change mitigation option through sequestration of gaseous CO₂. A new concept for CO₂ capture involves use of immobilized carbonic anhydrase (CA) that catalyzes the reversible hydration of CO₂ to HCO₃^? and H⁺. Cost‐efficient production of the enzyme and an inexpensive immobilization system are critical for development of economically feasible CA‐based CO₂ capture processes. An artificial, bifunctional enzyme containing CA from Neisseria gonorrhoeae and a cellulose binding domain (CBD) from Clostridium thermocellum was constructed with a His₆ tag. The chimeric enzyme exhibited both CA activity and CBD binding affinity. This fusion enzyme is of particular interest due to its binding affinity for cellulose and retained CA activity, which could serve as the basis for improved technology to capture CO₂ from flue gasses. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Responses of macroalgae to CO2 enrichment cannot be inferred solely from their inorganic carbon uptake strategy

Increased plant biomass is observed in terrestrial systems due to rising levels of atmospheric CO₂, but responses of marine macroalgae to CO₂ enrichment are unclear. The 200% increase in CO₂ by 2100 is predicted to enhance the productivity of fleshy macroalgae that acquire inorganic carbon solely as CO₂ (non‐carbon dioxide‐concentrating mechanism [CCM] species—i.e., species without a carbon dioxide‐concentrating mechanism), whereas those that additionally uptake bicarbonate (CCM species) are predicted to respond neutrally or positively depending on their affinity for bicarbonate. Previous studies, however, show that fleshy macroalgae exhibit a broad variety of responses to CO₂ enrichment and the underlying mechanisms are largely unknown. This physiological study compared the responses of a CCM species (Lomentaria australis) with a non‐CCM species (Craspedocarpus ramentaceus) to CO₂ enrichment with regards to growth, net photosynthesis, and biochemistry. Contrary to expectations, there was no enrichment effect for the non‐CCM species, whereas the CCM species had a twofold greater growth rate, likely driven by a downregulation of the energetically costly CCM(s). This saved energy was invested into new growth rather than storage lipids and fatty acids. In addition, we conducted a comprehensive literature synthesis to examine the extent to which the growth and photosynthetic responses of fleshy macroalgae to elevated CO₂ are related to their carbon acquisition strategies. Findings highlight that the responses of macroalgae to CO₂ enrichment cannot be inferred solely from their carbon uptake strategy, and targeted physiological experiments on a wider range of species are needed to better predict responses of macroalgae to future oceanic change.     

Evolutionary history and biotechnological future of carboxylases

Carbon dioxide (CO₂) is a potent greenhouse gas whose presence in the atmosphere is a critical factor for global warming. At the same time atmospheric CO₂ is also a cheap and readily available carbon source that can in principle be used to synthesize value-added products. However, as uncatalyzed chemical CO₂-fixation reactions usually require quite harsh conditions to functionalize the CO₂ molecule, not many processes have been developed that make use of CO₂. In contrast to synthetical chemistry, Nature provides a multitude of different carboxylating enzymes whose carboxylating principle(s) might be exploited in biotechnology. This review focuses on the biochemical features of carboxylases, highlights possible evolutionary scenarios for the emergence of their reactivity, and discusses current, as well as potential future applications of carboxylases in organic synthesis, biotechnology and synthetic biology.