The adaptive mechanisms of the marine diatom Thalassiosira weissflogii to long-term high CO2 and warming.

While it is known that increased dissolved CO2 concentrations and rising sea surface temperature (ocean warming) can act interactively on marine phytoplankton, the ultimate molecular mechanisms underlying this interaction on a long-term evolutionary scale are relatively unexplored. Here, we performed transcriptomics and quantitative metabolomics analyses, along with a physiological trait analysis, on the marine diatom Thalassiosira weissflogii adapted for approximately 3.5 years to warming and/or high CO2 conditions. We show that long-term warming has more pronounced impacts than elevated CO2 on gene expression, resulting in a greater number of differentially expressed genes (DEGs). The largest number of DEGs was observed in populations adapted to warming + high CO2, indicating a potential synergistic interaction between these factors. We further identified the metabolic pathways in which the DEGs function and the metabolites with significantly changed abundances. We found that ribosome biosynthesis-related pathways were upregulated to meet the increased material and energy demands after warming or warming in combination with high CO2. This resulted in the upregulation of energy metabolism pathways such as glycolysis, photorespiration, the tricarboxylic acid cycle, and the oxidative pentose phosphate pathway, as well as the associated metabolites. These metabolic changes help compensate for reduced photochemical efficiency and photosynthesis. Our study emphasizes that the upregulation of ribosome biosynthesis plays an essential role in facilitating the adaptation of phytoplankton to global ocean changes and elucidates the interactive effects of warming and high CO2 on the adaptation of marine phytoplankton in the context of global change.

High CO2 facilitates fatty acid biosynthesis and mitigates cellular oxidative stress caused by CAC2 dysfunction in Arabidopsis.

Increasing concentration of CO2 has significant impacts on many biological processes in plants, and its impact is closely associated with changes in the ratio of photosynthesis to photorespiration. Studies have reported that high CO2 can promote carbon fixing and alleviate plant oxidative damage in response to environmental stresses. However, the effect of high CO2 on fatty acid (FA) metabolism and cellular redox balance in FA-deficient plants is rarely reported. In this study, we identified a high-CO2 -requiring mutant cac2 through forward genetic screening. CAC2 encodes biotin carboxylase, which is one of the subunits of plastid acetyl-CoA carboxylase and participates in de novo FA biosynthesis. Null mutation of CAC2 is embryonic lethal. A point mutation of CAC2 in cac2 mutants produces severe defects in chloroplast development, plant growth and photosynthetic performance. These morphological and physiological defects were largely absent under high CO2 conditions. Metabolite analyses showed that FA contents in cac2-1 leaves were decreased, while photorespiratory metabolites, such as glycine and glycolate, did not significantly change. Meanwhile, cac2 exhibited higher reactive oxygen species (ROS) levels and mRNA expression of stress-responsive genes than the wild-type, indicating that cac2 plants may suffer oxidative stress under ambient CO2 conditions. Elevated CO2 significantly increased FA contents, especially C18:3-FA, and reduced ROS accumulation in cac2-1 leaves. We propose that stress mitigation by high CO2 in cac2 could be due to increased FA levels by promoting carbon assimilation, and the prevention of over-reduction due to decreased photorespiration.

Neonatal limited bedding and nesting experience may lead to a sex-dependent increase in panic-like defensive behaviours in adult mice.

In humans, adverse physical and/or psychological traumas in childhood may predispose to developing psychiatric disorders in adulthood, including panic disorder. To model early life adversity in mice, we subjected male and female C57BL/6 J mice to a limited bedding and nesting (LBN) protocol between postnatal days 2-9 and investigated its effect on responsiveness to panicogenic challenges in adulthood. Panic-like escape behaviour was assessed during exposure to a high concentration of CO2 (20%) or in the beetle mania task (BMT), used to model respiratory and non-respiratory-related types of panic respectively. Neonatal exposure to LBN increased panic-like jumping during the CO2 challenge in male but not female mice. In an initial pharmacological validation of the BMT as a panic-inducing paradigm, undirected jumping and horizontal escape behaviours were reduced significantly by the panicolytic alprazolam (0.05 and 0.1mg.kg-1 i.p.) whilst tolerance to the close proximity of the aversive robo-beetle increased. The anxiolytic diazepam (1 mg.kg-1 i.p.) reduced only the number of horizontal escape attempts. In both sexes, previous experience of LBN significantly enhanced the number of horizontal escape episodes, indicating a pro-panic phenotype. Directed escape to access a safe ledge on the wall of the test arena, which was seen only in males, was also reduced significantly following LBN. These findings indicate that early life adversity produced by fragmented and unpredictable maternal care promotes a sex-specific increase in susceptibility to panic-like behaviour in adulthood. Whilst non-respiratory-related panic-like behaviour was enhanced in both sexes, females were resilient to respiratory-related challenges.

Physiological and Proteomic Responses of the Tetraploid Robinia pseudoacacia L. to High CO2 Levels.

The increase in atmospheric CO2 concentration is a significant factor in triggering global warming. CO2 is essential for plant photosynthesis, but excessive CO2 can negatively impact photosynthesis and its associated physiological and biochemical processes. The tetraploid Robinia pseudoacacia L., a superior and improved variety, exhibits high tolerance to abiotic stress. In this study, we investigated the physiological and proteomic response mechanisms of the tetraploid R. pseudoacacia under high CO2 treatment. The results of our physiological and biochemical analyses revealed that a 5% high concentration of CO2 hindered the growth and development of the tetraploid R. pseudoacacia and caused severe damage to the leaves. Additionally, it significantly reduced photosynthetic parameters such as Pn, Gs, Tr, and Ci, as well as respiration. The levels of chlorophyll (Chl a and b) and the fluorescent parameters of chlorophyll (Fm, Fv/Fm, qP, and ETR) also significantly decreased. Conversely, the levels of ROS (H2O2 and O2·-) were significantly increased, while the activities of antioxidant enzymes (SOD, CAT, GR, and APX) were significantly decreased. Furthermore, high CO2 induced stomatal closure by promoting the accumulation of ROS and NO in guard cells. Through a proteomic analysis, we identified a total of 1652 DAPs after high CO2 treatment. GO functional annotation revealed that these DAPs were mainly associated with redox activity, catalytic activity, and ion binding. KEGG analysis showed an enrichment of DAPs in metabolic pathways, secondary metabolite biosynthesis, amino acid biosynthesis, and photosynthetic pathways. Overall, our study provides valuable insights into the adaptation mechanisms of the tetraploid R. pseudoacacia to high CO2.

Mitochondrial and peroxisomal NAD+ uptake are important for improved photosynthesis and seed yield under elevated CO2 concentrations.

As sessile organisms, plants must adapt their physiology and developmental processes to cope with challenging environmental circumstances, such as the ongoing elevation in atmospheric carbon dioxide (CO2 ) levels. Nicotinamide adenine dinucleotide (NAD+ ) is a cornerstone of plant metabolism and plays an essential role in redox homeostasis. Given that plants impaired in NAD metabolism and transport often display growth defects, low seed production and disturbed stomatal development/movement, we hypothesized that subcellular NAD distribution could be a candidate for plants to exploit the effects of CO2 fertilization. We report that an efficient subcellular NAD+ distribution is required for the fecundity-promoting effects of elevated CO2 levels. Plants with reduced expression of either mitochondrial (NDT1 or NDT2) or peroxisomal (PXN) NAD+ transporter genes grown under elevated CO2 exhibited reduced total leaf area compared with the wild-type while PXN mutants also displayed reduced leaf number. NDT2 and PXN lines grown under elevated CO2 conditions displayed reduced rosette dry weight and lower photosynthetic rates coupled with reduced stomatal conductance. Interestingly, high CO2 doubled seed production and seed weight in the wild-type, whereas the mutants were less responsive to increases in CO2 levels during reproduction, producing far fewer seeds than the wild-type under both CO2 conditions. These data highlight the importance of mitochondrial and peroxisomal NAD+ uptake mediated by distinct NAD transporter proteins to modulate photosynthesis and seed production under high CO2 levels.

Persulfidation protects from oxidative stress under nonphotorespiratory conditions in Arabidopsis.

Hydrogen sulfide is a signaling molecule in plants that regulates essential biological processes through protein persulfidation. However, little is known about sulfide-mediated regulation in relation to photorespiration. Here, we performed label-free quantitative proteomic analysis and observed a high impact on protein persulfidation levels when plants grown under nonphotorespiratory conditions were transferred to air, with 98.7% of the identified proteins being more persulfidated under suppressed photorespiration. Interestingly, a higher level of reactive oxygen species (ROS) was detected under nonphotorespiratory conditions. Analysis of the effect of sulfide on aspects associated with non- or photorespiratory growth conditions has demonstrated that it protects plants grown under suppressed photorespiration. Thus, sulfide amends the imbalance of carbon/nitrogen and restores ATP levels to concentrations like those of air-grown plants; balances the high level of ROS in plants under nonphotorespiratory conditions to reach a cellular redox state similar to that in air-grown plants; and regulates stomatal closure, to decrease the high guard cell ROS levels and induce stomatal aperture. In this way, sulfide signals the CO2 -dependent stomata movement, in the opposite direction of the established abscisic acid-dependent movement. Our findings suggest that the high persulfidation level under suppressed photorespiration reveals an essential role of sulfide signaling under these conditions.

Plant NADPH-dependent thioredoxin reductases are crucial for the metabolism of sink leaves and plant acclimation to elevated CO2.

Plants contain three NADPH-thioredoxin reductases (NTR) located in the cytosol/mitochondria (NTRA/B) and the plastid (NTRC) with important metabolic functions. However, mutants deficient in all NTRs remained to be investigated. Here, we generated and characterised the triple Arabidopsis ntrabc mutant alongside with ntrc single and ntrab double mutants under different environmental conditions. Both ntrc and ntrabc mutants showed reduced growth and substantial metabolic alterations, especially in sink leaves and under high CO2 (HC), as compared to the wild type. However, ntrabc showed higher effective quantum yield of PSII under both constant and fluctuating light conditions, altered redox states of NADH/NAD+ and glutathione (GSH/GSSG) and lower potential quantum yield of PSII in sink leaves in ambient but not high CO2 concentrations, as compared to ntrc, suggesting a functional interaction between chloroplastic and extra-chloroplastic NTRs in photosynthesis regulation depending on leaf development and environmental conditions. Our results unveil a previously unknown role of the NTR system in regulating sink leaf metabolism and plant acclimation to HC, while it is not affecting full plant development, indicating that the lack of the NTR system can be compensated, at least to some extent, by other redox mechanisms.

Crosstalk between ABA and ethylene in regulating stomatal behavior in tomato under high CO2 and progressive soil drying.

Increasing atmospheric CO2 concentrations accompanied by intensifying drought markedly impact plant growth and physiology. This study aimed to explore the role of abscisic acid (ABA) in mediating the response of stomata to elevated CO2 (e[CO2]) and drought. Tomato plants with different endogenous ABA concentrations [Ailsa Craig (AC), the ABA-deficient mutant flacca, and ABA-overproducing transgenic tomato SP5] were grown in ambient (a[CO2], 400 µmol mol-1) and elevated (e[CO2],800 µmol mol-1) CO2 environments and subjected to progressive soil drying. Compared with a[CO2] plants, e[CO2] plants had significantly lower stomatal conductance in AC and SP5 but not in flacca. Under drought, e[CO2] plants had better water status and higher water use efficiency. e[CO2] promoted the accumulation of ABA in leaves of plants subjected to drought, which coincided with the up-regulation of ABA biosynthetic genes and down-regulation of ABA metabolic genes. Although the increase of ABA induced by drought in flacca was much less than in AC and SP5, flacca accumulated large amounts of ethylene, suggesting that in plants with ABA deficiency, ethylene might play a compensatory role in inducing stomatal closure during soil drying. Collectively, these findings improve our understanding of plant performance in a future drier and higher-CO2 environment.

DNA methylation and gene transcription act cooperatively in driving the adaptation of a marine diatom to global change.

Genetic changes together with epigenetic modifications such as DNA methylation have been demonstrated to regulate many biological processes and thereby govern the response of organisms to environmental changes. However, how DNA methylation might act cooperatively with gene transcription and thereby mediate the long-term adaptive responses of marine microalgae to global change is virtually unknown. Here we performed a transcriptomic analysis, and a whole-genome bisulfite sequencing, along with phenotypic analysis of a model marine diatom Phaeodactylum tricornutum adapted for 2 years to high CO2 and/or warming conditions. Our results show that the methylated islands (peaks of methylation) mCHH were positively correlated with expression of genes in the subregion of the gene body when the populations were grown under high CO2 or its combination with warming for ~2 years. We further identified the differentially expressed genes (DEGs), and hence the metabolic pathways in which they function, at the transcriptomics level in differentially methylated regions (DMRs). Although DEGs in DMRs contributed only 18-24% of the total DEGs, we found that those DEGs acted cooperatively with DNA methylation and then regulated key processes such as central carbon metabolism, amino acid metabolism, ribosome biogenesis, terpenoid backbone biosynthesis, and degradation of misfolded proteins. Taken together, by integrating transcriptomic, epigenetic, and phenotypic analysis, our study provides evidence for DNA methylation acting cooperatively with gene transcription to contribute to the adaptation of microalgae to global changes.

Ocean acidification alters foraging behaviour in Dungeness crab through impairment of the olfactory pathway.

Crustacean olfaction is fundamental to most aspects of living and communicating in aquatic environments and more broadly, for individual- and population-level success. Accelerated ocean acidification from elevated CO2 threatens the ability of crabs to detect and respond to important olfactory-related cues. Here, we demonstrate that the ecologically and economically important Dungeness crab (Metacarcinus magister) exhibits reduced olfactory-related antennular flicking responses to a food cue when exposed to near-future CO2 levels, adding to the growing body of evidence of impaired crab behaviour. Underlying this altered behaviour, we find that crabs have lower olfactory nerve sensitivities (twofold reduction in antennular nerve activity) in response to a food cue when exposed to elevated CO2 . This suggests that near-future CO2 levels will impact the threshold of detection of food by crabs. We also show that lower olfactory nerve sensitivity in elevated CO2 is accompanied by a decrease in the olfactory sensory neuron (OSN) expression of a principal chemosensory receptor protein, ionotropic receptor 25a (IR25a) which is fundamental for odorant coding and olfactory signalling cascades. The OSNs also exhibit morphological changes in the form of decreased surface areas of their somata. This study provides the first evidence of the effects of high CO2 levels at multiple levels of biological organization in marine crabs, linking physiological and cellular changes with whole animal behavioural responses.

A reduction in metabolism explains the tradeoffs associated with the long-term adaptation of phytoplankton to high CO2 concentrations.

Phytoplankton are responsible for nearly half of global primary productivity and play crucial roles in the Earth's biogeochemical cycles. However, the long-term adaptive responses of phytoplankton to rising CO2 remains unknown. Here we examine the physiological and proteomics responses of a marine diatom, Phaeodactylum tricornutum, following long-term (c. 900 generations) selection to high CO2 conditions. Our results show that this diatom responds to long-term high CO2 selection by downregulating proteins involved in energy production (Calvin cycle, tricarboxylic acid cycle, glycolysis, oxidative pentose phosphate pathway), with a subsequent decrease in photosynthesis and respiration. Nearly similar extents of downregulation of photosynthesis and respiration allow the high CO2 -adapted populations to allocate the same fraction of carbon to growth, thereby maintaining their fitness during the long-term high CO2 selection. These results indicate an important role of metabolism reduction under high CO2 and shed new light on the adaptive mechanisms of phytoplankton in response to climate change.

Insights on Cryogenic Distillation Technology for Simultaneous CO2 and H2S Removal for Sour Gas Fields.

Natural gas demand has dramatically increased due to the emerging growth of the world economy and industry. Presently, CO2 and H2S content in gas fields accounts for up to 90% and 15%, respectively. Apart from fulfilling the market demand, CO2 and H2S removal from natural gas is critical due to their corrosive natures, the low heating value of natural gas and the greenhouse gas effect. To date, several gas fields have remained unexplored due to limited technologies to monetize the highly sour natural gas. A variety of conventional technologies have been implemented to purify natural gas such as absorption, adsorption and membrane and cryogenic separation. The application of these technologies in natural gas upgrading are also presented. Among these commercial technologies, cryogenic technology has advanced rapidly in gas separation and proven ideally suitable for bulk CO2 removal due to its independence from absorbents or adsorbents, which require a larger footprint, weight and energy. Present work comprehensively reviews the mechanisms and potential of the advanced nonconventional cryogenic separation technologies for processing of natural gas streams with high CO2 and H2S content. Moreover, the prospects of emerging cryogenic technologies for future commercialization exploitation are highlighted.

Enhancing the productivity of ryegrass at elevated CO2 is dependent on tillering and leaf area development rather than leaf-level photosynthesis.

Whilst a range of strategies have been proposed for enhancing crop productivity, many recent studies have focused primarily on enhancing leaf photosynthesis under current atmospheric CO2 concentrations. Given that the atmospheric CO2 concentration is likely to increase significantly in the foreseeable future, an alternative/complementary strategy might be to exploit any variability in the enhancement of growth/yield and photosynthesis at higher CO2 concentrations. To explore this, we investigated the responses of a diverse range of wild and cultivated ryegrass genotypes, with contrasting geographical origins, to ambient and elevated CO2 concentrations and examined what genetically tractable plant trait(s) might be targeted by plant breeders for future yield enhancements. We found substantial ~7-fold intraspecific variations in biomass productivity among the different genotypes at both CO2 levels, which were related primarily to differences in tillering/leaf area, with only small differences due to leaf photosynthesis. Interestingly, the ranking of genotypes in terms of their response to both CO2 concentrations was similar. However, as expected, estimates of whole-plant photosynthesis were strongly correlated with plant productivity. Our results suggest that greater yield gains under elevated CO2 are likely through the exploitation of genetic differences in tillering and leaf area rather than focusing solely on improving leaf photosynthesis.

Uncoupling growth and succinic acid production in an industrial Saccharomyces cerevisiae strain.

This study explores the relation between biomass-specific succinic acid (SA) production rate and specific growth rate of an engineered industrial strain of Saccharomyces cerevisiae, with the aim to investigate the extent to which growth and product formation can be uncoupled. Ammonium-limited aerobic chemostat and retentostat cultures were grown at different specific growth rates under industrially relevant conditions, that is, at a culture pH of 3 and with sparging of a 1:1 CO2 -air mixture. Biomass-specific SA production rates decreased asymptotically with decreasing growth rate. At near-zero growth rates, the engineered strain maintained a stable biomass-specific SA production rate for over 500 h, with a SA yield on glucose of 0.61 mol mol-1 . These results demonstrate that uncoupling of growth and SA production could indeed be achieved. A linear relation between the biomass-specific SA production rate and glucose consumption rate indicated the coupling of SA production rate and the flux through primary metabolism. The low culture pH resulted in an increased death rate, which was lowest at near-zero growth rates. Nevertheless, a significant amount of non-viable biomass accumulated in the retentostat cultures, thus underlining the importance of improving low-pH tolerance in further strain development for industrial SA production with S. cerevisiae.

Differential responses of the sunn4 and rdn1-1 super-nodulation mutants of Medicago truncatula to elevated atmospheric CO2.

BACKGROUND AND AIMS: Nitrogen fixation in legumes requires tight control of carbon and nitrogen balance. Thus, legumes control nodule numbers via an autoregulation mechanism. 'Autoregulation of nodulation' mutants super-nodulate are thought to be carbon-limited due to the high carbon-sink strength of excessive nodules. This study aimed to examine the effect of increasing carbon supply on the performance of super-nodulation mutants. METHODS: We compared the responses of Medicago truncatula super-nodulation mutants (sunn-4 and rdn1-1) and wild type to five CO2 levels (300-850 µmol mol-1). Nodule formation and nitrogen fixation were assessed in soil-grown plants at 18 and 42 d after sowing. KEY RESULTS: Shoot and root biomass, nodule number and biomass, nitrogenase activity and fixed nitrogen per plant of all genotypes increased with increasing CO2 concentration and reached a maximum at 700 µmol mol-1. While the sunn-4 mutant showed strong growth retardation compared with wild-type plants, elevated CO2 increased shoot biomass and total nitrogen content of the rdn1-1 mutant up to 2-fold. This was accompanied by a 4-fold increase in nitrogen fixation capacity in the rdn1-1 mutant. CONCLUSIONS: These results suggest that the super-nodulation phenotype per se did not limit growth. The additional nitrogen fixation capacity of the rdn1-1 mutant may enhance the benefit of elevated CO2 for plant growth and N2 fixation.

Effect of different light intensity on physiology, antioxidant capacity and photosynthetic characteristics on wheat seedlings under high CO2 concentration in a closed artificial ecosystem.

The growth of plants under high carbon dioxide (CO2) concentrations (≥ 1000 ppm) is explored for the climate change and the bioregenerative life support system (BLSS) environment of long-duration space missions. Wheat (Triticum aestivum L.) is a grass cultivated for cereal grain-a global staple food including astronauts. Light and CO2 are both indispensable conditions for wheat seedlings. This study provides insights on the physiology, antioxidant capacity and photosynthetic characteristics of wheat seedlings under a range of photosynthetic photon flux densities in a closed system simulating BLSS with high CO2 concentration. We found that the Fv/Fm, Fv/F0, chlorophyll content, intrinsic water use efficiencies (WUEi), membrane stability index (MSI), and malondialdehyde (MDA) of wheat seedlings grown under an intermediate light intensity of 600 µmol m-2 s-1 environment were all largest. Interestingly, the high light intensity of 1200 mol m-2 s-1 treatment group exhibits the highest net photosynthetic rate but the lowest MDA content. The stomatal conductance and F0 of high light intensity of 1000 µmol m-2 s-1 treatment group were both significantly higher than that of other groups. Our study provides basic knowledge on the wheat growth in different environments, especially in a closed ecosystem with artificial lights.

CO2 biofixation and fatty acid composition of two indigenous Dunaliella sp. isolates (ABRIINW-CH2 and ABRIINW-SH33) in response to extremely high CO2 levels.

Global warming, as a result of atmospheric CO2 increase, is regarded as an important universal concern. Microalgae are considered as appropriate microorganisms for CO2 assimilation. Here we aimed to investigate carbon biofixation ability of two indigenous isolates of Dunaliella spp. (ABRIINW-CH2 and ABRIINW-SH33) under elevated CO2 concentrations of 10, 20, and 30% (v/v) as well as their lipid content, productivity, and fatty acid profile under adjusted pH conditions. The maximum biomass production and CO2 biofixation rates were obtained under 10% CO2. High CO2 concentrations were favorable for the accumulation of lipids, lipid productivity, and polyunsaturated fatty acids formation. The highest lipid content and lipid productivity was obtained at 10% CO2. The highest fraction of the fatty acids (FA) profile was allocated to omega-3 FAs at 20% CO2. Accordingly, these isolates were able to tolerate extremely high CO2 concentrations and present even enhanced growth as well as formation of valuable products.

Provision of carbon skeleton for lipid synthesis from the breakdown of intracellular protein and soluble sugar in Phaeodactylum tricornutum under high CO2.

BACKGROUND: Increasing CO2 emissions have resulted in ocean acidification, affecting marine plant photosynthesis and changing the nutrient composition of marine ecosystems. The physiological and biochemical processes of marine phytoplankton in response to ocean acidification have been reported, but have been mainly focused on growth and photosynthetic physiology. To acquire a thorough knowledge of the molecular regulation mechanisms, model species with clear genetic background should be selected for systematic study. Phaeodactylum tricornutum is a pennate diatom with the characteristics of small genome size, short generation cycle, and easy to transform. Furthermore, the genome of P. tricornutum has been completely sequenced. RESULTS AND DISCUSSION: In this study, P. tricornutum was cultured at high and normal CO2 concentrations. Cell composition changes during culture time were investigated. The 13C isotope tracing technique was used to determine fractional labeling enrichments for the main cellular components. The results suggested that when lipid content increased significantly under high CO2 conditions, total protein and soluble sugar contents decreased. The 13C labeling experiment indicated that the C skeleton needed for fatty acid C chain elongation in lipid synthesis under high CO2 conditions is not mainly derived from NaHCO3 (carbon fixed by photosynthesis). CONCLUSION: This study indicated that breakdown of intracellular protein and soluble sugar provide C skeleton for lipid synthesis under high CO2 concentration.

Effects of instantaneous and growth CO2 levels and abscisic acid on stomatal and mesophyll conductances.

C3 photosynthesis is often limited by CO2 diffusivity or stomatal (gs ) and mesophyll (gm ) conductances. To characterize effects of stomatal closure induced by either high CO2 or abscisic acid (ABA) application on gm , we examined gs and gm in the wild type (Col-0) and ost1 and slac1-2 mutants of Arabidopsis thaliana grown at 390 or 780 µmol mol-1 CO2 . Stomata of these mutants were reported to be insensitive to both high CO2 and ABA. When the ambient CO2 increased instantaneously, gm decreased in all these plants, whereas gs in ost1 and slac1-2 was unchanged. Therefore, the decrease in gm in response to high CO2 occurred irrespective of the responses of gs . gm was mainly determined by the instantaneous CO2 concentration during the measurement and not markedly by the CO2 concentration during the growth. Exogenous application of ABA to Col-0 caused the decrease in the intercellular CO2 concentration (Ci ). With the decrease in Ci , gm did not increase but decreased, indicating that the response of gm to CO2 and that to ABA are differently regulated and that ABA content in the leaves plays an important role in the regulation of gm .

Transcriptional Activities of the Microbial Consortium Living with the Marine Nitrogen-Fixing Cyanobacterium Trichodesmium Reveal Potential Roles in Community-Level Nitrogen Cycling.

Trichodesmium is a globally distributed cyanobacterium whose nitrogen-fixing capability fuels primary production in warm oligotrophic oceans. Like many photoautotrophs, Trichodesmium serves as a host to various other microorganisms, yet little is known about how this associated community modulates fluxes of environmentally relevant chemical species into and out of the supraorganismal structure. Here, we utilized metatranscriptomics to examine gene expression activities of microbial communities associated with Trichodesmium erythraeum (strain IMS101) using laboratory-maintained enrichment cultures that have previously been shown to harbor microbial communities similar to those of natural populations. In enrichments maintained under two distinct CO2 concentrations for ∼8 years, the community transcriptional profiles were found to be specific to the treatment, demonstrating a restructuring of overall gene expression had occurred. Some of this restructuring involved significant increases in community respiration-related transcripts under elevated CO2, potentially facilitating the corresponding measured increases in host nitrogen fixation rates. Particularly of note, in both treatments, community transcripts involved in the reduction of nitrate, nitrite, and nitrous oxide were detected, suggesting the associated organisms may play a role in colony-level nitrogen cycling. Lastly, a taxon-specific analysis revealed distinct ecological niches of consistently cooccurring major taxa that may enable, or even encourage, the stable cohabitation of a diverse community within Trichodesmium consortia.IMPORTANCETrichodesmium is a genus of globally distributed, nitrogen-fixing marine cyanobacteria. As a source of new nitrogen in otherwise nitrogen-deficient systems, these organisms help fuel carbon fixation carried out by other more abundant photoautotrophs and thereby have significant roles in global nitrogen and carbon cycling. Members of the Trichodesmium genus tend to form large macroscopic colonies that appear to perpetually host an association of diverse interacting microbes distinct from the surrounding seawater, potentially making the entire assemblage a unique miniature ecosystem. Since its first successful cultivation in the early 1990s, there have been questions about the potential interdependencies between Trichodesmium and its associated microbial community and whether the host's seemingly enigmatic nitrogen fixation schema somehow involved or benefited from its epibionts. Here, we revisit these old questions with new technology and investigate gene expression activities of microbial communities living in association with Trichodesmium.