Carbonic Anhydrase and Its Role in Photosynthesis

Evaluation of ketoprofen toxicity in two freshwater species: Effects on biochemical, physiological and population endpoints

Among the most used non-steroidal anti-inflammatory drugs (NSAIDs), ketoprofen (KTF) assumes an important position. Nevertheless, its ecotoxicological effects in non-target organisms are poorly characterized, despite its use and frequency of occurrence in aquatic matrices. Thus, the aim of this study was to evaluate the possible toxicological effects of KTF contamination, in two freshwater species, Lemna minor and Daphnia magna, by measuring biochemical, physiological and population parameters. To attain this objective, both species were exposed to KTF at the same concentrations (0, 0.24, 1.2, 6 and 30 μg/L). L. minor plants were exposed during 4 d to these levels of KTF, and the enzymatic activity (catalase (CAT), glutathione S-transferases (GSTs) and carbonic anhydrase (CA)), and pigments content (chlorophylls a, b and total and carotenoids) were analyzed to evaluate the toxicity of this drug. D. magna was acutely and chronically exposed to KTF, and enzymatic activities (CAT, GSTs and cyclooxygenase (COX)), the feeding rates, and reproduction traits were assessed. In L.minor, KTF provoked alterations in all enzyme activities, however, it was not capable of causing any alteration in any pigment levels. On the other hand, KTF also provoked alterations in all enzymatic activities in D. magna, but did not affect feeding rates and life-history parameters. In conclusion, exposure to KTF, provoked biochemical alterations in both species. However, these alterations were not reflected into deleterious effects on physiological and populational traits of L. minor and D. magna.

Emerging roles for carbonic anhydrase in mesophyll conductance and photosynthesis

Carbonic anhydrase (CA) is an abundant protein in most photosynthesizing organisms and higher plants. This review paper considers the physiological importance of the more abundant CA isoforms in photosynthesis, through their effects on CO₂ diffusion and other processes in photosynthetic organisms. In plants, CA has multiple isoforms in three different families (α, β and γ) and is mainly known to catalyze the CO₂↔HCO3- equilibrium. This reversible conversion has a clear role in photosynthesis, primarily through sustaining the CO₂ concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Despite showing the same major reaction mechanism, the three main CA families are evolutionarily distinct. For different CA isoforms, cellular localization and total gene expression as a function of developmental stage are predicted to determine the role of each family in relation to the net assimilation rate. Reaction-diffusion modeling and observational evidence support a role for CA activity in reducing resistance to CO₂ diffusion inside mesophyll cells by facilitating CO₂ transfer in both gas and liquid phases. In addition, physical and/or biochemical interactions between CAs and other membrane-bound compartments, for example aquaporins, are suggested to trigger a CO₂ -sensing response by stomatal movement. In response to environmental stresses, changes in the expression level of CAs and/or stimulated deactivation of CAs may correspond with lower photosynthetic capacity. We suggest that further studies should focus on the dynamics of the relationship between the activity of CAs (with different subcellular localization, abundance and gene expression) and limitations due to CO₂ diffusivity through the mesophyll and supply of CO₂ to photosynthetic reactions.

Role of Extracellular Carbonic Anhydrase in Dissolved Inorganic Carbon Uptake in Alkaliphilic Phototrophic Biofilm

Alkaline Soda Lakes are extremely productive ecosystems, due to their high dissolved inorganic carbon (DIC) concentrations. Here, we studied the dynamics of the carbonate system, in particular, the role of extracellular carbonic anhydrase (eCA) of an alkaliphilic phototrophic biofilm composed of bacteria enriched from soda lake benthic mats. By using measurements with microsensors and membrane inlet mass spectrometry, combined with mathematical modeling, we show how eCA controls DIC uptake. In our experiments, the activity of eCA varied four-fold, and was controlled by the bicarbonate concentration during growth: a higher bicarbonate concentration led to lower eCA activity. Inhibition of eCA decreased both the net and the gross photosynthetic productivities of the investigated biofilms. After eCA inhibition, the efflux of carbon dioxide (CO2) from the biofilms increased two- to four-fold. This could be explained by the conversion of CO2, leaking from cyanobacterial cells, by eCA, to bicarbonate. Bicarbonate is then taken up again by the cyanobacteria. In suspensions, eCA reduced the CO2 leakage to the bulk medium from 90 to 50%. In biofilms cultivated at low bicarbonate concentration (~0.13 mM), the oxygen production was reduced by a similar ratio upon eCA inhibition. The role of eCA in intact biofilms was much less significant compared to biomass suspensions, as CO2 loss to the medium is reduced due to mass transfer resistance.

Physiological and structural tradeoffs underlying the leaf economics spectrum

The leaf economics spectrum (LES) represents a suite of intercorrelated leaf traits concerning construction costs per unit leaf area, nutrient concentrations, and rates of carbon fixation and tissue turnover. Although broad trade-offs among leaf structural and physiological traits have been demonstrated, we still do not have a comprehensive view of the fundamental constraints underlying the LES trade-offs. Here, we investigated physiological and structural mechanisms underpinning the LES by analysing a novel data compilation incorporating rarely considered traits such as the dry mass fraction in cell walls, nitrogen allocation, mesophyll CO₂ diffusion and associated anatomical traits for hundreds of species covering major growth forms. The analysis demonstrates that cell wall constituents are major components of leaf dry mass (18-70%), especially in leaves with high leaf mass per unit area (LMA) and long lifespan. A greater fraction of leaf mass in cell walls is typically associated with a lower fraction of leaf nitrogen (N) invested in photosynthetic proteins; and lower within-leaf CO₂ diffusion rates, as a result of thicker mesophyll cell walls. The costs associated with greater investments in cell walls underpin the LES: long leaf lifespans are achieved via higher LMA and in turn by higher cell wall mass fraction, but this inevitably reduces the efficiency of photosynthesis.

Exclusive localization of carbonic anhydrase in bacteriocytes of the deep-sea clam Calyptogena okutanii with thioautotrophic symbiotic bacteria

Deep-sea Calyptogena clams harbor thioautotrophic intracellular symbiotic bacteria in their gill epithelial cells. The symbiont fixes CO2 to synthesize organic compounds. Carbonic anhydrase (CA) from the host catalyzes the reaction CO2 + H2O ↔ HCO3(-) + H(+), and is assumed to facilitate inorganic carbon (Ci) uptake and transport to the symbiont. However, the localization of CA in gill tissue remains unknown. We therefore analyzed mRNA sequences, proteins and CA activity in Calyptogena okutanii using expression sequence tag, SDS-PAGE and LC-MS/MS. We found that acetazolamide-sensitive soluble CA was abundantly expressed in the gill tissue of C. okutanii, and the enzyme was purified by affinity chromatography. Mouse monoclonal antibodies against the CA of C. okutanii were used in western blot analysis and immunofluorescence staining of the gill tissues of C. okutanii, which showed that CA was exclusively localized in the symbiont-harboring cells (bacteriocytes) in gill epithelial cells. Western blot analysis and measurement of activity showed that CA was abundantly (26-72% of total soluble protein) detected in the gill tissues of not only Calyptogena clams but also deep-sea Bathymodiolus mussels that harbor thioautotrophic or methanotrophic symbiotic bacteria, but was not detected in a non-symbiotic mussel, Mytilus sp. The present study showed that CA is abundant in the gill tissues of deep-sea symbiotic bivalves and specifically localizes in the cytoplasm of bacteriocytes of C. okutanii. This indicates that the Ci supply process to symbionts in the vacuole (symbiosome) in bacteriocytes is essential for symbiosis.

Morphological and photosynthetic variations in the process of spermatia formation from vegetative cells in Porphyra yezoensis Ueda (Bangiales, Rhodophyta) and their responses to desiccation

Porphyra yezoensis has a macroscopic foliage gametophyte phase with only a single cell layer, and is ideally suited for the study of the sexual differentiation process, from the vegetative cell to the spermatia. Firstly, we compared variations in the responses of the vegetative and male sectors to desiccation. Later, cell tracking experiments were carried out during the formation of spermatia from vegetative cells. The two sectors showed similar tolerance to desiccation, and the formation of spermatia from vegetative cells was independent of the degree of desiccation. Both light and scanning electron microscopy (SEM) observations of the differentiation process showed that the formation of spermatia could be divided into six phases: the one-cell, two-cell, four-cell, eight-cell, pre-release and spermatia phases. Photomicrographs of Fluorescent Brightener staining showed that the released spermatia had no cell walls. Photosynthetic data showed that there was a significant rise in Y(II) in the four-cell phase, indicating an increase in photosynthetic efficiency of PSII during this phase. We propose that this photosynthetic rise may be substantial and provide the increased energy needed for the formation and release of spermatia in P. yezoensis.

The molecular evolution of β-carbonic anhydrase in Flaveria

Limited information exists regarding molecular events that occurred during the evolution of C(4) plants from their C(3) ancestors. The enzyme β-carbonic anhydrase (CA; EC 4.2.1.1), which catalyses the reversible hydration of CO(2), is present in multiple forms in C(3) and C(4) plants, and has given insights into the molecular evolution of the C(4) pathway in the genus Flaveria. cDNAs encoding three distinct isoforms of β-CA, CA1-CA3, have been isolated and examined from Flaveria C(3) and C(4) congeners. Sequence data, expression analyses of CA orthologues, and chloroplast import assays with radiolabelled CA precursor proteins from the C(3) species F. pringlei Gandoger and the C(4) species F. bidentis (L.) Kuntze have shown that both contain chloroplastic and cytosolic forms of the enzyme, and the potential roles of these isoforms are discussed. The data also identified CA3 as the cytosolic isoform important in C(4) photosynthesis and indicate that the C(4) CA3 gene evolved as a result of gene duplication and neofunctionalization, which involved mutations in coding and non-coding regions of the ancestral C(3) CA3 gene. Comparisons of the deduced CA3 amino acid sequences from Flaveria C(3), C(4), and photosynthetic intermediate species showed that all the C(3)-C(4) intermediates investigated and F. brownii, a C(4)-like species, have a C(3)-type CA3, while F. vaginata, another C(4)-like species, contains a C(4)-type CA3. These observations correlate with the photosynthetic physiologies of the intermediates, suggesting that the molecular evolution of C(4) photosynthesis in Flaveria may have resulted from a temporally dependent, stepwise modification of protein-encoding genes and their regulatory elements.

Plant responses to low [CO2] of the past

During the Last Glacial Maximum (LGM; 18,000-20,000 yr ago) and previous glacial periods, atmospheric [CO(2)] dropped to 180-190 ppm, which is among the lowest concentrations that occurred during the evolution of land plants. Modern atmospheric CO(2) concentrations ([CO(2)]) are more than twice those of the LGM and 45% higher than pre-industrial concentrations. Since CO(2) is the carbon source for photosynthesis, lower carbon availability during glacial periods likely had a major impact on plant productivity and evolution. From the studies highlighted here, it is clear that the influence of low [CO(2)] transcends several scales, ranging from physiological effects on individual plants to changes in ecosystem functioning, and may have even influenced the development of early human cultures (via the timing of agriculture). Through low-[CO(2)] studies, we have determined a baseline for plant response to minimal [CO(2)] that occurred during the evolution of land plants. Moreover, an increased understanding of plant responses to low [CO(2)] contributes to our knowledge of how natural global change factors in the past may continue to influence plant responses to future anthropogenic changes. Future work, however, should focus more on the evolutionary responses of plants to changing [CO(2)] in order to account for the potentially large effects of genetic change.

Irradiance and phenotype: comparative eco-development of sun and shade leaves in relation to photosynthetic CO2 diffusion

The subject of this paper, sun leaves are thicker and show higher photosynthetic rates than the shade leaves, is approached in two ways. The first seeks to answer the question: why are sun leaves thicker than shade leaves? To do this, CO2 diffusion within a leaf is examined first. Because affinity of Rubisco for CO2 is low, the carboxylation of ribulose 1,5-bisphosphate is competitively inhibited by O2, and the oxygenation of ribulose 1,5-bisphosphate leads to energy-consuming photorespiration, it is essential for C3 plants to maintain the CO2 concentration in the chloroplast as high as possible. Since the internal conductance for CO2 diffusion from the intercellular space to the chloroplast stroma is finite and relatively small, C3 leaves should have sufficient mesophyll surfaces occupied by chloroplasts to secure the area for CO2 dissolution and transport. This explains why sun leaves are thicker. The second approach is mechanistic or 'how-oriented'. Mechanisms are discussed as to how sun leaves become thicker than shade leaves, in particular, the long-distance signal transduction from mature leaves to leaf primordia inducing the periclinal division of the palisade tissue cells. To increase the mesophyll surface area, the leaf can either be thicker or have smaller cells. Issues of cell size are discussed to understand plasticity in leaf thickness.

Temperature dependence of leaf-level CO2 fixation: revising biochemical coefficients through analysis of leaf three-dimensional structure

CO2 fixation in a leaf is determined by biochemical and physical processes within the boundaries set by leaf structure. Traditionally determined temperature dependencies of biochemical processes include physical processes related to CO2 exchange that result in inaccurate estimates of parameter values. A realistic three-dimensional model of a birch (Betula pendula) leaf was used to distinguish between the physical and biochemical processes affecting the temperature dependence of CO2 exchange, to determine new chloroplastic temperature dependencies for V c(max) and Jmax based on experiments, and to analyse mesophyll diffusion in detail. The constraint created by dissolution of CO2 at cell surfaces substantially decreased the CO2 flux and its concentration inside chloroplasts, especially at high temperatures. Consequently, newly determined chloroplastic V c(max) and Jmax were more temperature dependent than originally. The role of carbonic anhydrase in mesophyll diffusion appeared to be minor under representative mid-day nonwater-limited conditions. Leaf structure and physical processes significantly affect the apparent temperature dependence of CO2 exchange, especially at optimal high temperatures when the photosynthetic sink is strong. The influence of three-dimensional leaf structure on the light environment inside a leaf is marked and affects the local choice between Jmax and V c(max)-limited assimilation rates.