Low Affinity For CO2 Research Articles

Isoreticularity in a gallate-based metal–organic framework: Impact of the extension of the ligand on the porosity, stability and adsorption of CO2

Isoreticularity is one of the key aspects in the design of new Metal-Organic Frameworks (MOFs), but has been up to now scarcely applied to gallate (1,2-3-trioxobenzene) ligands. With the aim of building new materials isoreticular to the M(Hngal) (H4gal = gallic acid) small pore MOF series, we here designed the new elongated mixed gallate-carboxylate ligand 4-(3,4,5-trihydroxyphenyl)benzoic acid (H4bpgal). A thorough investigation of its reactivity with MgCl2 has led to the successful isolation of the targeted material Mg(H2bpgal)·n(solv) (solv = water, methanol, ethanol). A combination of experimental tools, including single-crystal X-ray diffraction (XRD), powder XRD, and 1H and 13C solid-state NMR studies confirmed the close structural relationship with the smaller analogue, including the presence of acidic OH sites arising from phenol groups bound to Mg cations. Contrary to the small pore analogue, Mg(H2bpgal) is of limited interest for gas capture, because of its moderate stability upon exposure to dioxygen and low affinity for CO2, both resulting from the intrinsic characteristics of the ligand (redox activity and flexibility, respectively). Nevertheless, it presents among the highest surface area and pore volume reported to date for a gallate MOF (1475 m2 g−1 and 0.59 cm3 g−1, respectively); these characteristics might be of interest in other fields of applications, including electrochemical energy storage or combined controlled drug release.

Equisetum praealtum and E. hyemale have abundant Rubisco with a high catalytic turnover rate and low CO2 affinity.

The kinetic properties of Rubisco, a key enzyme for photosynthesis, have been examined in numerous plant species. However, this information on some plant groups, such as ferns, is scarce. This study examined Rubisco carboxylase activity and leaf Rubisco levels in seven ferns, including four Equisetum plants (E. arvense, E. hyemale, E. praealtum, and E. variegatum), considered living fossils. The turnover rates of Rubisco carboxylation (kcatc) in E. praealtum and E. hyemale were comparable to those in the C4 plants maize (Zea mays) and sorghum (Sorghum bicolor), whose kcatc values are high. Rubisco CO2 affinity, estimated from the percentage of Rubisco carboxylase activity under CO2 unsaturated conditions in kcatc in these Equisetum plants, was low and also comparable to that in maize and sorghum. In contrast, kcatc and CO2 affinities of Rubisco in other ferns, including E. arvense and E. variegatum were comparable with those in C3 plants. The N allocation to Rubisco in the ferns examined was comparable to that in the C3 plants. These results indicate that E. praealtum and E. hyemale have abundant Rubisco with high kcatc and low CO2 affinity, whereas the carboxylase activity and abundance of Rubisco in other ferns were similar to those in C3 plants. Herein, the Rubisco properties of E. praealtum and E. hyemale were discussed regarding their evolution and physiological implications.

Construction of bismuth oxide iodide (BiOI)/zinc titanium oxide (Zn2TiO4) p-n heterojunction nanofibers with abundant oxygen vacancies for improving photocatalytic carbon dioxide reduction

The conversion of carbon dioxide (CO2) into value-added syngas represents an effective approach for addressing both environmental issues and carbon neutrality issue. However, the slow charge dynamics and low CO2 affinity severely limit the photocatalytic CO2 reduction reaction. In this study, bismuth oxide iodide (BiOI)/zinc titanium oxide (Zn2TiO4) composite nanofibers were successfully prepared by immobilizing BiOI nanosheets on Zn2TiO4 electrospun nanofibers through a solvothermal reaction method. The results of photocatalytic research indicate that the BiOI/Zn2TiO4 composite nanofibers exhibit improved photocatalytic activity in CO2 reduction compared to pristine BiOI nanosheets and Zn2TiO4 nanofibers. The highest carbon monoxide (CO) release rate of BiOI/Zn2TiO4 nanofibers could reach 9.10 µmol‧g−1‧h−1, which is 18.6 times and 6.6 times higher than that of pristine BiOI nanosheets and Zn2TiO4 nanofibers, respectively. The enhanced photocatalytic activity can be credited to the formed BiOI/Zn2TiO4 p-n heterojunction, which can boost electron separation, reduce charge recombination at the interface, and promote the reaction process. The presence of oxygen vacancies in BiOI/Zn2TiO4 nanofibers can not only provide active site to facilitate the adsorption and activation of CO2 molecules, but also adjust the energy band structure of the catalyst to accelerate carriers transfer. After four cycles of testing, the CO release rate of BiOI/Zn2TiO4 nanofibers remains nearly constant, demonstrating its excellent stability. This work develops a feasible strategy to improve the efficiency of photoreduction of CO2 through energy band engineering and surface defect technology.

An in-depth understanding of fast interfacial electron transfer in amide-bonded photocatalyst for boosting photoreduction of CO2

The photocatalytic reduction of CO2 is typically slow due to inert charge kinetics and low CO2 affinity. This research aimed to enhance the photocarrier transfer/separation and CO2 adsorption/activation process over amine-functionalized TiO2 nanoparticles/GO (f-TGO). Density functional theory (DFT) calculations established models of TGO and f-TGO (with CO2 adsorbed on the top, middle, and bottom), proving that the presence of the amino group fortifies the adsorption/activation of CO2, while the amido bond acts as a charge delivery channel to accelerate electron transfer in f-TGO. The optimized f-TGO demonstrated good performance in photocatalytic CO2RR under mild conditions, achieving a turnover frequency (TOF) of 40.9 and 19.9 μmol g−1h−1 for CO and CH4, respectively, outperforming most reported TiO2-based catalysts. In addition, in situ DRIFTS and theoretical calculation revealed the reaction intermediates, demonstrated the kinetic behaviors of the reaction pathway, and thus laid the groundwork for investigating the CO2RR mechanism. This study proposes a viable approach to modify the electronic structure of photocatalysts and provides new insight for designing high-performance photocatalysts, focusing particularly on how interfacial carrier transfer affects photoreduction CO2.

Modifying Copper Local Environment with Electrolyte Additives to Alter CO2 Electroreduction vs Hydrogen Evolution

CO2 electroreduction (CO2ER) by using renewable energy resources is a promising method to mitigate the CO2 level in the atmosphere as well as produce valuable chemicals. Local environment at the electrode–electrolyte interface plays a key role in CO2ER activity and selectivity along with its competing hydrogen evolution reaction (HER). In addition to the catalyst and reactor design, electrolyte also has a significant impact on the interface. Herein, electrolyte additives were used to modify the local environment around the Cu catalyst during CO2ER. For this purpose, 10 mM ionic additives with bis(trifluoromethylsulfonyl)imide ([NTF2]−) and dicyanamide ([DCA]−) as anions and 1-butyl-3-methylimidazolium ([BMIM]+), potassium (K+), or sodium (Na+) as cations have been added to an aqueous potassium bicarbonate solution (0.1 M KHCO3). COMSOL Multiphysics was also used to calculate the local pH and CO2 concentration at the electrode–electrolyte interface in different electrolytes. Results showed that the local environment modifications by the electrolyte additives altered the activity and selectivity of Cu in CO2ER. It was found that the CO2ER activity at −0.92 V was enhanced when using anions with high CO2 affinity and high hydrophobicity, such as [NTF2]−. Among [NTF2]−-based additives, [BMIM][NTF2] had a higher faradaic efficiency (FE) for formate (38.7%) compared to K[NTF2] (23.2%) and Na[NTF2] (18.5%) at −0.92 V likely due to the presence of imidazolium cations that can further stabilize the intermediates on the surface and enhance CO2ER. Electrolytes containing [DCA]−-based additives with high hydrophilicity and low CO2 affinity had a very high HER selectivity (>90% FEH2) and low CO2ER selectivity regardless of the cation nature. This observation is attributed to the presence of hydrophilic [BMIM][DCA] in the vicinity of the catalyst, which impacts the microenvironment around the catalyst. We observed that [DCA]− anions have a high affinity to adsorb on Cu catalysts as soon as the catalyst is submerged in the electrolyte. Although FTIR showed that [DCA]− anions desorb from the surface at negative potentials, it is likely that [DCA]− anions still remain in the proximity of the electrode, next to the adsorbed cations, impacting the transport of H2O and CO2, and altering the product selectivity. COMSOL calculations showed that the local pH is directly proportional to the H2 evolution activity. Also, hydrophilic salts such as those with the [DCA]− anion had a more alkaline local pH, which led to a lower CO2 concentration in the vicinity of the catalyst.

Efficient CO2 photoreduction triggered by oxygen vacancies in ultrafine Bi5O7Br nanowires

Sluggish charge kinetics, poor photoabsorption and low CO2 affinity have been regarded as the main obstacles inhibiting the efficiency of CO2 photoreduction. Herein, freestanding ultrafine Bi5O7Br nanowires with abundant oxygen vacancies were initially fabricated to synchronously optimize these critical processes. The 1D ultrafine configuration and abundant oxygen vacancies endow the Bi5O7Br nanowires with extended photoadsorption, boosted charge separation and enhanced interfacial CO2 adsorption and activation. Density functional calculations reveal that the presence of oxygen vacancies on the Bi5O7Br surface can not only afford abundant localized electrons and lower the CO2 reaction energy barriers, but also have a stronger covalent interaction and more efficient electron exchange and transfer between CO2 and oxygen vacancies. Without any co-catalyst or sacrifice reagent, OV-rich Bi5O7Br nanowires show a 27.76-fold enhancement of CO2 photoreduction activity relative to bulk Bi5O7Br in the gas-solid system. This work may inspire the future design of ultrafine catalysts for artificial photosynthesis.

Production of CH4 and CO on CuxO and NixOy coatings through CO2 photoreduction

CO2 capture and photocatalytic reduction to hydrocarbons is an interesting yet challenging area that requires photocatalysts with the capability to capture and photoconvert CO2 simultaneously. Furthermore, earth-abundant photocatalysts with high efficiency and product selectivity are essential for commercialization. Thus, two earth-abundant photocatalysts based on copper and nickel oxides were selected to produce solar fuels from CO2 photoreduction. The photocatalysts were immobilized on commercial glass fibers substrates by a facile one-step microwave-hydrothermal method. CuxO (x = 1, 2) and NixOy (x = 1, 2 and y = 1, 3) coatings on glass fiber were evaluated as photocatalysts in two different reactors to investigate the selectivity in a continuous reactor and a batch system. Two different light sources were employed: a heterochromatic lamp to simulate part of the solar light in the continuous reactor and a LED visible light in the batch reactor. CuO/Cu2O photocatalysts exhibited a selective production of CH4 (95 µmol g−1 h−1) and CH3OH (177 µmol g−1 h−1) from CO2 photoreduction in the continuous and batch continuous systems, respectively. The superior performance was attributed to the unique rod-shape morphology, the presence of oxygen vacancies, and efficient charge transfer in the CuO/Cu2O heterostructure with high affinity towards CO2, resulting the formation of mono- and bidentate carbonate species during the CO2 photoreduction reaction. NixOy coating with 2D cubic shape produced CO (103 µmol g−1 h−1) and HCOOH (4245 µmol g−1 h−1), associating with the low CO2 affinity and less efficient charge separation compared to CuO/Cu2O heterostructure.

Preference of carbon absorption determines the competitive ability of algae along atmospheric CO2 concentration.

Although many studies have focused on the effects of elevated atmospheric CO2 on algal growth, few of them have demonstrated how CO2 interacts with carbon absorption capacity to determine the algal competition at the population level. We conducted a pairwise competition experiment of Phormidium sp., Scenedesmus quadricauda, Chlorella vulgaris and Synedra ulna. The results showed that when the CO2 concentration increased from 400 to 760 ppm, the competitiveness of S. quadricauda increased, the competitiveness of Phormidium sp. and C. vulgaris decreased, and the competitiveness of S. ulna was always the lowest. We constructed a model to explore whether interspecific differences in affinity and flux rate for CO2 and HCO3 − could explain changes in competitiveness between algae species along the gradient of atmospheric CO2 concentration. Affinity and flux rates are the capture capacity and transport capacity of substrate respectively, and are inversely proportional to each other. The simulation results showed that, when the atmospheric CO2 concentration was low, species with high affinity for both CO2 and HCO3 − (HCHH) had the highest competitiveness, followed by the species with high affinity for CO2 and low affinity for HCO3 − (HCLH), the species with low affinity for CO2 and high affinity for HCO3 − (LCHH) and the species with low affinity for both CO2 and HCO3 − (LCLH); when the CO2 concentration was high, the species were ranked according to the competitive ability: LCHH > LCLH > HCHH > HCLH. Thus, low resource concentration is beneficial to the growth and reproduction of algae with high affinity. With the increase in atmospheric CO2 concentration, the competitive advantage changed from HCHH species to LCHH species. These results indicate the important species types contributing to water bloom under the background of increasing global atmospheric CO2, highlighting the importance of carbon absorption characteristics in understanding, predicting and regulating population dynamics and community composition of algae.

Synergistic Polarization Engineering on Bulk and Surface for Boosting CO2 Photoreduction.

Sluggish charge kinetics and low CO2 affinity seriously inhibit CO2 photoreduction. Herein, the synchronous promotion of charge separation and CO2 affinity of Bi4 Ti3 O12 is realized by coupling corona poling and surface I-grafting. Corona poling enhances ferroelectric polarization of Bi4 Ti3 O12 by aligning the domains direction, which profoundly promotes charge transfer along opposite directions across bulk. Surface I-grafting forms a surface local electric field for further separating charge carriers and provides abundant active sites to enhance CO2 adsorption. The two modifications cooperatively further increase the ferroelectric polarization of Bi4 Ti3 O12 , which maximize the separation efficiency of photogenerated charges, resulting in an enhanced CO production rate of 15.1 μmol g-1 h-1 (nearly 9 times) with no sacrificial agents or cocatalysts. This work discloses that ferroelectric polarization and surface ion grafting can promote CO2 photoreduction in a synergistic way.

Synergistic Polarization Engineering on Bulk and Surface for Boosting CO2 Photoreduction

AbstractSluggish charge kinetics and low CO2 affinity seriously inhibit CO2 photoreduction. Herein, the synchronous promotion of charge separation and CO2 affinity of Bi4Ti3O12 is realized by coupling corona poling and surface I‐grafting. Corona poling enhances ferroelectric polarization of Bi4Ti3O12 by aligning the domains direction, which profoundly promotes charge transfer along opposite directions across bulk. Surface I‐grafting forms a surface local electric field for further separating charge carriers and provides abundant active sites to enhance CO2 adsorption. The two modifications cooperatively further increase the ferroelectric polarization of Bi4Ti3O12, which maximize the separation efficiency of photogenerated charges, resulting in an enhanced CO production rate of 15.1 μmol g−1 h−1 (nearly 9 times) with no sacrificial agents or cocatalysts. This work discloses that ferroelectric polarization and surface ion grafting can promote CO2 photoreduction in a synergistic way.

In Synechococcus sp. competition for energy between assimilation and acquisition of C and those of N only occurs when growth is light limited.

The carbon-concentrating mechanisms (CCMs) of cyanobacteria counteract the low CO2 affinity and CO2:O2 selectivities of the Rubisco of these photolithotrophs and the relatively low oceanic CO2 availability. CCMs have a significant energy cost; if light is limiting, the use of N sources whose assimilation demands less energy could permit a greater investment of energy into CCMs and inorganic C (Ci) assimilation. To test this, we cultured Synechococcus sp. UTEX LB 2380 under either N or energy limitation, in the presence of NO3- or NH4+. When growth was energy-limited, NH4+-grown cells had a 1.2-fold higher growth rate, 1.3-fold higher dissolved inorganic carbon (DIC)-saturated photosynthetic rate, 19% higher linear electron transfer, 80% higher photosynthetic 1/K1/2(DIC), 2.0-fold greater slope of the linear part of the photosynthesis versus DIC curve, 3.5-fold larger intracellular Ci pool, and 2.3-fold higher Zn quota than NO3--grown cells. When energy was not limiting growth, there were not differences between NH4+- and NO3--grown cells, except for higher linear electron transfer and larger intracellular Ci pool.We conclude that, when energy limits growth, cells that use the cheaper N source divert energy from N assimilation to C acquisition and assimilation; this does not happen when energy is not limiting.

Breaking the rules of Rubisco catalysis.

The canon of knowledge on the catalytic properties of the photosynthetic enzyme Rubisco has shackled efforts to understand its diversity. Now the chains are off. While investigating the variability in Rubisco function among diatoms, Young et al. (see pages 3445–3456 in this issue) have demonstrated that it is our thinking, not Rubisco catalysis, that has been constrained. The photosynthetic CO2-fixing enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), is renowned for its slow catalytic rate and difficulty in distinguishing between the substrate of photosynthesis, CO2, and one of the products, O2. The oxygenase activity was discovered 45 years ago (Bowes et al., 1971), nearly 20 years after the discovery of the carboxylase activity (Quayle et al., 1954; Weissbach et al., 1954). The photorespiratory losses associated with Rubisco oxygenation are substantial (Sharkey, 1988), and the persistence of oxygenation throughout 3.5 billion years of evolution – particularly the last 400 million years when the O2 and CO2 pressures in the atmosphere have favored substantial oxygenation and photorespiration – is still a mystery (Hagemann et al. 2016). It is conceivable that evolutionary constraints related to the origins of Rubisco from enzymes with roles in sulfur metabolism (Ashida et al., 2003) may have imposed limitations on the catalytic mechanism for CO2 fixation, though none have come to light. Oxygenation has also been proposed as an energy dissipative mechanism that would be beneficial under some stressful conditions (Osmond, 1981), but this does not explain why it exists in all Rubiscos, including those from anaerobic organisms (Andrews and Lorimer, 1987). The inefficiency of Rubisco is due not only to the strong competitive interaction of O2 and CO2 at the active site, but also to a slow catalytic turnover rate per active site (k cat) and a low affinity for CO2 (high K C). Substrate-saturated k cat values (maximum CO2 fixation per catalytic site) occur in the range 1–12s–1 and the k cat/K C ratios (which reflect the ability of Rubisco to function when CO2 is limiting) occur in the range 2–40×104 M–1 s–1 (Badger et al., 1998). This feeble catalytic potency at limiting CO2 is several orders of magnitude slower than the diffusion limit to catalysis that many enzymes approach (108–109 M–1 s–1) and it means that plants need copious amounts of Rubisco (as much as 50% of soluble leaf protein, consuming up to 25% of leaf nitrogen) to support adequate rates of photosynthesis in our current atmosphere (Andrews and Lorimer, 1987). The requirement for such large quantities of Rubisco gives it the dubious honor of being the most abundant protein on Earth (Ellis, 1979). Even with so much Rubisco present, it is still often the rate-limiting enzyme in photosynthesis.

Calcification and ocean acidification: new insights from the coccolithophore Emiliania huxleyi

Calcification and ocean acidification: new insights from the coccolithophore <i>Emiliania huxleyi</i>

Carbon Concentrating Mechanisms in Eukaryotic Marine Phytoplankton

The accumulation of inorganic carbon from seawater by eukaryotic marine phytoplankton is limited by the diffusion of carbon dioxide (CO2) in water and the dehydration kinetics of bicarbonate to CO2 and by ribulose-1,5-bisphosphate carboxylase/oxygenase's (RubisCO) low affinity for its inorganic carbon substrate, CO2. Nearly all marine phytoplankton have adapted to these limitations and evolved inorganic carbon (or CO2) concentrating mechanisms (CCMs) to support photosynthetic carbon fixation at the concentrations of CO2 present in ocean surface waters (< 10-30 microM). The biophysics and biochemistry of CCMs vary within and among the three dominant groups of eukaryotic marine phytoplankton and may involve the activity of external or intracellular carbonic anhydrase, HCO3- transport, and perhaps a C4 carbon pump. In general, coccolithophores have low-efficiency CCMs, and diatoms and the haptophyte genus Phaeocystis have high-efficiency CCMs. Dinoflagellates appear to possess moderately efficient CCMs, which may be necessitated by the very low CO2 affinity of their form II RubisCO. The energetic and nutrient costs of CCMs may modulate how variable CO2 affects primary production, element composition, and species composition of phytoplankton in the ocean.

Novel gene products associated with NdhD3/D4-containing NDH-1 complexes are involved in photosynthetic CO2 hydration in the cyanobacterium, Synechococcus sp. PCC7942.

Cyanobacteria possess light-dependent CO2 uptake activity that results in the net hydration of CO2 to HCO3- and may involve a protein-mediated carbonic anhydrase (CA)-like activity. This process is vital for the survival of cyanobacteria and may be a contributing factor in the ecological success of this group of organisms. Here, via isolation of mutants of Synechococcus sp. PCC7942 that cannot grow under low-CO2 conditions, we have identified two novel genes, chpX and chpY, that are involved in light-dependent CO2 hydration and CO2 uptake reactions; co-inactivation of both these genes abolished both activities. The function and mechanism of the CO2 uptake systems supported by each chp gene product differs, with each associated with functionally distinct NAD(P)H dehydrogenase (NDH-1) complexes. The ChpX system has a low affinity for CO2 and is dependent on photosystem I cyclic electron transport, whereas the inducible ChpY system has a high affinity for CO2 and is dependent on linear electron transport. We believe that ChpX and ChpY are involved in a unique, net hydration of CO2 to HCO3-, that is coupled electron flow within the NDH-1 complex on the thylakoid membrane.

The "green" form I ribulose 1,5-bisphosphate carboxylase/oxygenase from the nonsulfur purple bacterium Rhodobacter capsulatus.

Form I ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) of the Calvin-Benson-Bassham cycle may be divided into two broad phylogenetic groups, referred to as red-like and green-like, based on deduced large subunit amino acid sequences. Unlike the form I enzyme from the closely related organism Rhodobacter sphaeroides, the form I RubisCO from R. capsulatus is a member of the green-like group and closely resembles the enzyme from certain chemoautotrophic proteobacteria and cyanobacteria. As the enzymatic properties of this type of RubisCO have not been well studied in a system that offers facile genetic manipulation, we purified the R. capsulatus form I enzyme and determined its basic kinetic properties. The enzyme exhibited an extremely low substrate specificity factor, which is congruent with its previously determined sequence similarity to form I enzymes from chemoautotrophs and cyanobacteria. The enzymological results reported here are thus strongly supportive of the previously suggested horizontal gene transfer that most likely occurred between a green-like RubisCO-containing bacterium and a predecessor to R. capsulatus. Expression results from hybrid and chimeric enzyme plasmid constructs, made with large and small subunit genes from R. capsulatus and R. sphaeroides, also supported the unrelatedness of these two enzymes and were consistent with the recently proposed phylogenetic placement of R. capsulatus form I RubisCO. The R. capsulatus form I enzyme was found to be subject to a time-dependent fallover in activity and possessed a high affinity for CO2, unlike the closely similar cyanobacterial RubisCO, which does not exhibit fallover and possesses an extremely low affinity for CO2. These latter results suggest definite approaches to elucidate the molecular basis for fallover and CO2 affinity.

The occurrence of the chloroplast pyrenoid is correlated with the activity of a CO2-concentrating mechanism and carbon isotope discrimination in lichens and bryophytes

The organic-matter carbon isotope discrimination (Δ) of lichens with a wide range of photobiont and/or cyanobiont associations was used to determine the presence or absence of a carbon-concentrating mechanism (CCM). Two groups were identified within the lichens with green algal photobionts. One group was characterised by low, more C4-like Δ values (Δ 18‰). Tri-partite lichens (lichens with a green alga as the primary photobiont and cyanobacteria within internal or external cephalodia) occurred in both groups. All lichens with cyanobacterial photobionts had low Δ values (Δ < 15‰). The activity of the CCM, organic-matter Δ values, on-line Δ values and gas-exchange characteristics correlated with the presence of a pyrenoid in the algal chloroplast. Consistent with previous findings, lichens with Trebouxia as the primary photobiont possessed an active CCM while those containing Coccomyxa did not. Organic Δ values for lichens with Stichococcus as the photobiont varied between 11 and 28‰. The lichen genera Endocarpon and Dermatocarpon (Stichococcus + pyrenoid) had C4-like organic Δ values (Δ = 11 to 16.5‰) whereas the genus Chaenotheca (Stichococcus — pyrenoid) was characterised by high C3-like Δ values (Δ = 22 to 28‰), unless it associated with Trebouxia (Δ = 16‰). Gas-exchange measurements demonstrated that Dermatocarpon had an affinity for CO2 comparable to those species which possessed the CCM, with K0.5 = 200–215 μ1 · 1−1, compensation point (Γ) = 45–48 μl · l−1, compared with K0.5 = 195 μ1 · 1−1, Γ = 44μ1 · 1−1 for Trebouxioid lichens. Furthermore, lichens with Stichococcus as their photobiont released a small pool (24.2 ± 1.9 to 34.2 ± 2.5 nmol · mg−1 Chl) of inorganic carbon similar to that released by Trebouxioid lichens [CCM present, dissolved inorganic carbon (DIC) pool size = 51.0 ± 2.8 nmol · mg−1 Chl]. Lichens with Trentepohlia as photobiont did not possess an active CCM, with high C3-like organic Δ values (Δ = 18‰ to 23‰). In particular, Roccella phycopsis had very high on-line Δ values (Δ = 30 to 33‰), a low affinity for CO2 (K0.5 = 400 μ1 · 1−1,Γ = 120 μ1 · −1) and a negligible DIC pool. These responses were comparable to those from lichens with Coccomyxa as the primary photobiont with Nostoc in cephalodia (organic Δ = 17 to 25‰, on-line Δ = 16 to 21‰, k0.5 = 388 μ1 · 1−1, Γ = 85 μ1 · 1−1, DIC pool size = 8.5 ± 2.4 nmol · mg−1 Chl). The relative importance of refixation of respiratory CO2 and variations in source isotope signature were considered to account for any variation between on-line and organic Δ. Organic Δ was also measured for species of Anthocerotae and Hepaticae which contain pyrenoids and/or Nostoc enclosed within the thallus. The results of this screening showed that the pyrenoid is correlated with low, more C4-like organic Δ values (Δ = 7 to 12‰ for members of the Anthocerotae with a pyrenoid compared with Δ = 17 to 28‰ for the Hepaticae with and without Nostoc in vesicles) and confirms that the pyrenoid plays a fundamental role in the functioning of the CCM in microalgal photobionts and some bryophytes.

Kinetic properties of ribulose bisphosphate carboxylase/oxygenase from Thiobacillus thyasiris, the putative symbiont of Thyasira flexuosa (Montagu), a bivalve mussel

Some kinetic properties of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase from Thiobacillus thyasiris, a marine, facultatively heterotrophic, sulphur-oxidizing bacterium and putative symbiont of Thyasira flexuosa (Montagu), a bivalve mussel, have been determined. The kinetic parameters for the CO2/Mg2+-activated enzyme were: K m(RuBP) 24·3 μm, K m(CO2) 125·5 μm, Km(O2) 900 μm and K m(Mg2+) 1·53 mm. The low CO2 affinity suggests that T. thyasiris may possess a CO2-concentrating mechanism. RuBP oxygenase activity was inhibited by increasing CO2 concentration. Divalent metal ions were essential for RuBP carboxylase activity; activity of the Mg2+-free enzyme could be restored by the addition of Mg2+, Mn2+ or Ca2+. The pH optimum was 7·8. The temperature optimum for RuBP carboxylase activity was 55 °C, although the enzyme rapidly lost activity at this temperature. An Arrhenius plot was biphasic, with a break at 40 °C. The activation energies were 55·5 × 103 J mol−1 and 32·9 × 103 J mol−1 over the temperature ranges 10–40 °C and 40–55 °C, respectively. Q 10 was 2·12 for any 10 °C increment between 10·40 °C, and 1·47 between 40·55 °C. RuBP carboxylase activity was stable at 35 °C, the optimum growth temperature of T. thyasiris and at 7·5 °C, the temperature of the habitat of Thyasira flexuosa, but the activity was 40% and 3·5%, respectively, of the potential activity at 55 °C. RuBP carboxylase activity was stimulated by NaCl concentrations of up to 0·3 m, with a maximum (33 %), occurring between 0·1 and 0·2 m-NaCl. At higher concentrations of NaG (&gt;0·3 m) RuBP carboxylase activity was inhibited

Inorganic-carbon transport in some marine eukaryotic microalgae

Inorganic-carbon transport was investigated in the eukaryotic marine microalgaeStichococcus minor, Nannochloropsis oculata and aMonallantus sp. Photosynthetic O2 evolution at constant inorganic-carbon concentration but varying pH showed thatS. minor had a greater capacity for CO2 rather than HCO 3 (-) utilization but forN. oculata andMonallantus HCO 3 (-) was the preferred source of inorganic carbon. All three microalgae had a low affinity for CO2 as shown by the measurement of inorganic-carbon-dependent photosynthetic O2 evolution at pH 5.0. At pH 8.3, where HCO 3 (-) is the predominant form of inorganic carbon, the concentration of inorganic carbon required for half-maximal rate of photosynthetic O2 evolution [K 0.5 (CO2)] was 53 μM forMonallantus sp. and 125 μM forN. oculata, values compatible with HCO 3 (-) transport. Neither extra- nor intracellular carbonic anhydrase was detected in these three microalgal species. It is concluded that these microalgae lack a specific transport system for CO2 but that HCO 3 (-) transport occurs inN. oculata andMonallantus, and in the absence of intracellular carbonic anhydrase the conversion of HCO 3 (-) to CO2 may be facilitated by the internal pH of the cell.

The carbamate equilibrium of alpha- and epsilon-amino groups of human hemoglobin at 37 degrees C.

We have investigated the carbamate equilibrium of human adult hemoglobin, human cord blood hemoglobin, methemoglobin, and carbamylated hemoglobin using a stopped flow, rapid reaction pH apparatus described previously. The carbamate formation of human adult hemoglobin at 37 degrees C and ionic strength 0.15 was measured at pH values ranging from 6.2 to 8.8 and at CO2 partial pressures between 15 and 140 Torr. From experiments with unmodified hemoglobin as well as with hemoglobin specifically carbamylated at the four NH2 termini, it was found that already at pH 8, the epsilon-amino groups contribute significantly to carbamate formation in addition to the alpha-amino groups. At pH 8.5, about 70% of the total carbamate is due to epsilon-amino groups. The carbamate formation of alpha- and epsilon-amino groups can be suppressed by complete carbamylation of the hemoglobin. The results obtained from human adult deoxy- and oxyhemoglobin were used to calculate the equilibrium constants governing carbamate formation of these hemoglobins: Kc, the carbamate equilibrium constant, Kz, the R-NH2 ionization constant, and n, the number of binding sites per hemoglobin tetramer. Accordingly, two types of alpha-amino groups, each comprising two groups per tetramer, participate in carbamate formation of deoxyhemoglobin, one of low CO2 affinity (pKc = 5.2; pKz = 7.1; n = 2) and one of high CO2 affinity (pKc = 4.4; pKz = 6.1; n = 2). The pKz values derived from carbamate measurements agree within experimental error with figures obtained by difference titration of unmodified and specifically carbamylated hemoglobin. In addition to the alpha-amino groups, 15 epsilon-amino groups with pKc = 5.0 and pKz = 9.8 form carbamate in deoxyhemoglobin. In oxyhemoglobin, the carbamate data could be fitted with only two similar alpha-amino groups per tetramer in addition to 15 epsilon-amino groups, the latter with pKc = 4.7 and pKz = 10.2. The difference titration of the alpha-amino groups of oxyhemoglobin showed abnormal titration behavior of the beta-chain alpha-NH2. The pKc and pKz values obtained for the epsilon-amino groups of unmodified hemoglobin also provide a good description of the carbamate formed by hemoglobin specifically carbamylated at the four alpha-amino groups. The oxylabile carbamate, according to these results and in agreement with earlier reports, is only formed by alpha-amino groups; it amounts to 0.18 mol/mol of hemoglobin monomer at physiological conditions of pH 7.2 and pCO2 = 40 Torr. The amount of CO2 bound by methemoglobin equals that of oxyhemoglobin. Experiments carried out in the presence of 2,3-diphosphoglycerate provided evidence for competitive binding of CO2 and diphosphoglycerate to adult and cord blood oxy- as well as deoxyhemoglobin, the sites of competition probably being alpha-amino groups.