Dissolved Inorganic Carbon Transport Research Articles

Air-sea CO2 fluxes and cross-shelf exchange of inorganic carbon in the East China Sea from a coupled physical-biogeochemical model

The East China Sea (ECS) has been reported to be a significant sink of atmospheric CO2, but less is known about horizontal transport of dissolved inorganic carbon (DIC) across the shelf. A coupled physical-biogeochemical model has been implemented for the ECS to simulate the inorganic carbon system and estimate CO2 fluxes and cross-shelf DIC transport in the ECS. A 6-year model hindcast (2013–2018) was performed and assessed. Multiple existing datasets from in-situ observations are used to constrain and validate the model. The model reproduces the spatial and temporal patterns of nitrogen, chlorophyll and CO2 parameters in general agreement with observations. Modeling estimation reveals that the ECS takes up CO2 at an annual mean rate of about 8.20 ± 3.13 mmol m−2 d−1, and experiences substantial seasonal variability. The total annual CO2 uptake in the ECS is about 21.55 Tg C yr−1. Modeling estimation suggests that the biological processes contribute to about 15 % of the shelf CO2 uptake in the ECS, leaving ~80 % of the shelf uptake contributed by other physical-chemical processes, e.g., physical pump and/or solubility pump. The horizontal fluxes of DIC between the ECS and the adjacent ocean are more than two orders of magnitude larger than the air-sea CO2 flux on the ECS and result in a net DIC export of about ~33.8 ± 14.87 Tg C yr−1 from the shelf area. Modeling results suggest that this conveyance of DIC to the open ocean is equivalent to about 70 % of the inorganic carbon inflow from riverine and atmospheric pathways in the annual scale.

Tidal effects on carbon dioxide emission dynamics in intertidal wetland sediments

Understanding the control mechanisms of carbon dioxide (CO2) emissions in intertidal wetland sediments is beneficial for the concern of global carbon biogeochemistry and climate change. Nevertheless, multiple controls on CO2 emissions from intertidal wetland sediments to the atmosphere still need to be clarified. This study investigated the effect of tidal action on CO2 emissions from salt marsh sediments covered by Spartina alterniflora in the Jiaozhou Bay wetland using the static chamber method combined with an infrared CO2 detector. The results showed that the CO2 emission fluxes from the sediment during ebb tides were higher than those during flood tides. The whole wetland sediment acted as a weak source of atmospheric CO2 (average flux: 24.44 ± 16.80 mg C m−2 h−1) compared to terrestrial soils and was affected by the cycle of seawater inundation and exposure. The tidal influence on vertical dissolved inorganic carbon (DIC) transport in the sediment was also quantitated using a two-end member mixing model. The surface sediment layer (5–15 cm) with maximum DIC concentration during ebb tides became the one with minimum DIC concentration during flood tides, indicating the DIC transport from the surface sediment to seawater. Furthermore, aerobic respiration by microorganisms was the primary process of CO2 production in the sediment according to 16 S rDNA sequencing analysis. This study revealed the strong impact of tidal action on CO2 emissions from the wetland sediment and provided insights into the source-sink pattern of CO2 and DIC at the land-ocean interface.

An assessment of the North Atlantic (25–75°N) air-sea CO2 flux in 12 CMIP6 models

The global ocean stores about 1/4 of anthropogenic CO2, and the North Atlantic (NA) is the second largest region of the oceanic carbon sink. The Coupled Model Intercomparison Project Phase 6 (CMIP6) concentrates the latest Earth system models for studying the interaction between the carbon cycle and climate change. We find that the simulation of air-sea CO2 fluxes (FCO2) by CMIP6 models is consistent with the observation-based results in the NA. The spatial correlation coefficients of 12-model ensemble mean results and observations are 0.87 for the long-term annual mean results and above 0.62 for the seasonally averaged results. The main simulation biases of FCO2 are that most models overestimate the long-term annual mean carbon sink and seasonal variation of FCO2 in the subpolar NA because there the sink in winter is overestimated. In a large region of the subpolar NA, the bias of winter FCO2 is mainly caused by the bias of winter surface dissolved inorganic carbon (DIC) and total alkalinity. Differences in the simulated winter DIC among CMIP6 models in the subpolar North Atlantic are primarily caused by model differences in DIC's vertical structure and transport; this is mainly associated with differences in the meridional overturning at depth and surface CO2 solubility. We find the biological processes play a minor role in the surface and interior DIC differences among CMIP6 models in the subpolar NA. Compared with the CMIP5 models, the simulated FCO2 is closer to observation-based products in some models and this is mainly due to improved simulated wind fields.

Pore water exchange‐driven inorganic carbon export from intertidal salt marshes

AbstractRespiration in intertidal salt marshes generates dissolved inorganic carbon (DIC) that is exported to the coastal ocean by tidal exchange with the marsh platform. Understanding the link between physical drivers of water exchange and chemical flux is a key to constraining coastal wetland contributions to regional carbon budgets. The spatial and temporal (seasonal, annual) variability of marsh pore water exchange and DIC export was assessed from a microtidal salt marsh (Sage Lot Pond, Massachusetts). Spatial variability was constrained from 224Ra : 228Th disequilibria across two hydrologic units within the marsh sediments. Disequilibrium between the more soluble 224Ra and its sediment‐bound parent 228Th reveals significant pore water exchange in the upper 5 cm of the marsh surface (0–36 L m−2 d−1) that is most intense in low marsh elevation zones, driven by tidal overtopping. Surficial sediment DIC transport ranges from 0.0 to 0.7 g C m−2 d−1. The sub‐surface sediment horizon intersected by mean low tide was disproportionately impacted by tidal pumping (20–80 L m−2 d−1) and supplied a seasonal DIC flux of 1.7–5.4 g C m−2 d−1. Export exceeded 10 g C m−2 d−1 for another marsh unit, demonstrating that fluxes can vary substantially across salt marshes under similar conditions within the same estuary. Seasonal and annual variability in marsh pore water exchange, constrained from tidal time‐series of radium isotopes, was driven in part by variability in mean sea level. Rising sea levels will further inundate high marsh elevation zones, which may lead to greater DIC export.

Dissolved Inorganic Carbon Transport in the Surface‐Mixed Layer of the Louisiana Shelf in Northern Gulf of Mexico

AbstractRivers and wetlands are a major source of terrestrial derived carbon for coastal ocean margins. This results in a net loss of terrestrial carbon into the shelf water and their subsequent transport to interior ocean basin. This study investigates the transport of dissolved inorganic carbon (DIC) in the surface‐mixed layer of Louisiana Shelf in northern Gulf of Mexico (nGOM) adjacent to the Wax Lake Delta (WLD) and Barataria Bay (BB), which represent contrasting net land gain and net land loss areas in this region. DIC samples were collected, in conjunction with short‐lived radium isotopes 224Ra (t1/2 = 3.66 days) and 223Ra (t1/2 = 11.43 days) samples during June and September 2019, to quantify shelf transport of DIC in the surface‐mixed layer during period of high and low river flow, respectively. Radium distribution implied shelf mixing rates of 140–6,759 and 63–2,724 m2 s−1 for WLD and BB regions, respectively, with more than tenfold decrease in rates between the two seasons. Net shelf transport of DIC was found to be highest for the WLD region in June, highlighting the importance of freshwater discharge in exporting DIC. An upscaling of our study for the entire Louisiana Shelf indicates that 1.54–20.19 × 109 mol C d−1 transported in June 2019 and 0.34–8.12 × 109 mol C d−1 in the form of DIC was exported across the shallow region of the shelf during high and low river flow seasons, representing an important source of DIC to the NGOM.

Meridional Ocean Carbon Transport

The ocean's ability to take up and store CO2 is a key factor for understanding past and future climate variability. However, qualitative and quantitative understanding of surface‐to‐interior pathways, and how the ocean circulation affects the CO2 uptake, is limited. Consequently, how changes in ocean circulation may influence carbon uptake and storage and therefore the future climate remains ambiguous. Here we quantify the roles played by ocean circulation and various water masses in the meridional redistribution of carbon. We do so by calculating streamfunctions defined in dissolved inorganic carbon (DIC) and latitude coordinates, using output from a coupled biogeochemical‐physical model. By further separating DIC into components originating from the solubility pump and a residual including the biological pump, air‐sea disequilibrium, and anthropogenic CO2, we are able to distinguish the dominant pathways of how carbon enters particular water masses. With this new tool, we show that the largest meridional carbon transport occurs in a pole‐to‐equator transport in the subtropical gyres in the upper ocean. We are able to show that this pole‐to‐equator DIC transport and the Atlantic meridional overturning circulation (AMOC)‐related DIC transport are mainly driven by the solubility pump. By contrast, the DIC transport associated with deep circulation, including that in Antarctic bottom water and Pacific deep water, is mostly driven by the biological pump. As these two pumps, as well as ocean circulation, are widely expected to be impacted by anthropogenic changes, these findings have implications for the future role of the ocean as a climate‐buffering carbon reservoir.

Dynamics of dissolved inorganic carbon in the South China Sea: A modeling study

Using a validated, three-dimensional, coupled physical-biogeochemical model, this study examines the dynamics of dissolved inorganic carbon (DIC) in the South China Sea (SCS). The model identifies characteristic differences in DIC concentrations between the SCS and adjacent West Philippine Sea (WPS), showing an increase in DIC of up to 140 μmol kg−1 in the upper 2000 m layer from the WPS to the vicinity of the northern SCS. This increasing pattern continues from the northern to the southern SCS, but to a much lower degree (<20 μmol kg−1 increase). Overall, spatial and seasonal variation of DIC in the SCS are evidently modulated by both intrinsic dynamics (e.g., river plumes, vertical mixing, basin-scale upwelling and eddies), and extrinsic exchanges via various straits. The inputs of DIC to the SCS are mainly through the Luzon Strait in the upper layer (depth, h < 600 m) and from the deep WPS at > 1600 m. Exports of DIC occur through other straits and the SCS intermediate water outflow (600 m to 1600 m) via the Luzon Strait. The model shows upward transport of DIC in the upper 400 m and below 1100 m, in contrast to downward DIC transport between 400 m and 1100 m. Seasonally, both DIC concentrations and fluxes in the upper layer have larger dynamic ranges in winter than in summer, attributable to the combined effects of enhanced Kuroshio intrusion, stronger vertical upwelling and greater biogeochemical alterations.A balance of various constitutive DIC terms revealed that horizontal and vertical advection is an order of magnitude greater than the other terms of the DIC budget, which has an opposite sign and thus tends to neutralize each other. The model indicated a decrease in the DIC inventory in the euphotic zone in spring and summer caused by net DIC consumption, and a contrasting increase in fall and winter primarily influenced by DIC vertical transport. In intermediate and deep layers, physical transport processes play a dominant role in the seasonal variation of DIC inventories. In the upper 150 m, DIC fluxes and their stoichiometry with nutrient fluxes, which potentially impact the air-sea CO2 fluxes, are largely driven by excess vertical DIC fluxes compared to horizontal fluxes via the Luzon Strait.

Carbonic anhydrases are influenced by the size and symbiont identity of the aggregating sea anemone Anthopleura elegantissima.

Carbonic anhydrases (CA; EC 4.2.1.1) play a vital role in dissolved inorganic carbon (DIC) transport to photosynthetic microalgae residing in symbiotic cnidarians. The temperate sea anemone Anthopleura elegantissima can occur in three symbiotic states: hosting Breviolum muscatinei (brown), hosting Elliptochloris marina (green) or without algal symbionts (aposymbiotic). This provides a basis for A. elegantissima to be a model for detailed studies of the role of CA in DIC transport. This study investigated the effects of symbiosis, body size and light on CA activity and expression, and suggests that A. elegantissima has a heterotrophy-dominated trophic strategy. We identified putative A. elegantissima CA genes and performed phylogenetic analyses to infer subcellular localization in anemones. We performed experiments on field-collected anemones to compare: (1) CA activity and expression from anemones in different symbiotic states, (2) CA activity in brown anemones as a function of size, and (3) CA activity in anemones of different symbiotic states that were exposed to different light intensities. CA activity in brown anemones was highest, whereas activity in green and aposymbiotic anemones was low. Several CAs had expression patterns that mirrored activity, while another had expression that was inversely correlated with activity, suggesting that symbionts may induce different DIC transport pathways. Finally, CA activity was inversely correlated with anemone size. Our results suggest that the observed CA activity and expression patterns are affected not only by symbiosis, but also by other factors in the host physiology, including trophic strategy as it relates to body size and cellular pH homeostasis.

Ion transporter gene expression is linked to the thermal sensitivity of calcification in the reef coral Stylophora pistillata

Coral calcification underpins biodiverse reef ecosystems, but the physiology underlying the thermal sensitivity of corals to changing seawater temperatures remains unclear. Furthermore, light is also a key factor in modulating calcification rates, but a mechanistic understanding of how light interacts with temperature to affect coral calcification is lacking. Here, we characterized the thermal performance curve (TPC) of calcification of the wide-spread, model coral species Stylophora pistillata, and used gene expression analysis to investigate the role of ion transport mechanisms in thermally-driven declines in day and nighttime calcification. Focusing on genes linked to transport of dissolved inorganic carbon (DIC), calcium and H+, our study reveals a high degree of coherence between physiological responses (e.g. calcification and respiration) with distinct gene expression patterns to the different temperatures in day and night conditions. At low temperatures, calcification and gene expression linked to DIC transport processes were downregulated, but showed little response to light. By contrast, at elevated temperature, light had a positive effect on calcification and stimulated a more functionally diverse gene expression response of ion transporters. Overall, our findings highlight the role of mechanisms linked to DIC, calcium and H+ transport in the thermal sensitivity of coral calcification and how this sensitivity is influenced by light.

Hydrologic Transport of Dissolved Inorganic Carbon and Its Control on Chemical Weathering

Chemical weathering is one of the major processes interacting with climate and tectonics to form clays, supply nutrients to soil microorganisms and plants, and sequester atmospheric CO2. Hydrology and dissolution kinetics have been emphasized as factors controlling chemical weathering rates. However, the interaction between hydrology and transport of dissolved inorganic carbon (DIC) in controlling weathering has received less attention. In this paper, we present an analytical model that couples subsurface water and chemical molar balance equations to analyze the roles of hydrology and DIC transport on chemical weathering. The balance equations form a dynamical system that fully determines the dynamics of the weathering zone chemistry as forced by the transport of DIC. The model is formulated specifically for the silicate mineral albite, but it can be extended to other minerals, and is studied as a function of percolation rate and water transit time. Three weathering regimes are elucidated. For very small or large values of transit time, the weathering is limited by reaction kinetics or transport, respectively. For intermediate values, the system is transport controlled and is sensitive to transit time. We apply the model to a series of watersheds for which we estimate transit times and identify the type of weathering regime. The results suggest that hydrologic transport of DIC may be as important as reaction kinetics and dilution in determining chemical weathering rates.

Molecular aspects of the biophysical CO2-concentrating mechanism and its regulation in marine diatoms.

Diatoms operate a CO2-concentrating mechanism (CCM) that drives upwards of 20% of annual global primary production. Recent progress in CCM research in the marine pennate diatom Phaeodactylum tricornutum revealed that this diatom directly takes up HCO3- from seawater through low-CO2-inducible plasma membrane HCO3- transporters, which belong to the solute carrier (SLC) 4 family. Apart from this, studies of carbonic anhydrases (CAs) in diatoms have revealed considerable diversity in classes and localization among species. This strongly suggests that the CA systems, which control permeability and flux of dissolved inorganic carbon (DIC) by catalysing reversible CO2 hydration, have evolved from diverse origins. Of particular interest is the occurrence of low-CO2-inducible external CAs in the centric marine diatom Thalassiosira pseudonana, offering a strategy of CA-catalysed initial CO2 entry via passive diffusion, contrasting with active DIC transport in P. tricornutum. Molecular mechanisms to transport DIC across chloroplast envelopes are likely also through specific HCO3- transporters, although details have yet to be elucidated. Furthermore, recent discovery of a luminal θ-CA in the diatom thylakoid implied a common strategy in the mechanism to supply CO2 to RubisCO in the pyrenoid, which is conserved among green algae and some heterokontophytes. These results strongly suggest an occurrence of convergent coevolution between the pyrenoid and thylakoid membrane in aquatic photosynthesis.

Carbonic anhydrase activity changes in response to increased temperature and pCO2 in Symbiodinium–zoanthid associations

Carbon dioxide (CO2) makes up less than 1% of dissolved inorganic carbon (DIC) in the ocean. To acquire carbon dioxide for photosynthesis, many marine autotrophs rely on the enzyme carbonic anhydrase (CA) to catalyze the conversion of bicarbonate ions (HCO3−) to CO2. In zoanthids and other cnidarians with Symbiodinium spp. endosymbionts, CA is essential for transporting CO2 to symbionts for photosynthesis. Temperature and ambient DIC affect CA activity, therefore, increased sea water temperatures and ocean acidification (OA) will alter CO2 transport in symbiotic cnidarians. However, these effects are likely to be species specific for both host and symbiont, as different cnidarians and Symbiodinium spp. vary in their mechanisms of DIC transport and utilization of CA. In this study, host and symbiont CA activity in the zoanthids Palythoa sp. and Zoanthus sp. varied with thermal stress and low pH. Increased temperature inhibited algal, but not host CA activity in Zoanthus sp. polyps with A4 Symbiodinium, while temperature had no effect on CA activity in Palythoa sp. with C1 Symbiodinium. High pCO2/low pH altered algal CA activity in both zoanthid species, but host CA activity changed in Zoanthus sp. polyps only. This study shows that thermal stress and OA induce species-specific changes in CA activity, and thus DIC transport in symbiotic zoanthids. These observations suggest that CA activity in symbiotic cnidarians will be altered by climate conditions predicted for the future, and for some cnidarians, changes in CA activity may inhibit photosynthesis.

The Arctic Ocean carbon sink

We present observation based estimates of the transport of dissolved inorganic carbon (DIC) across the four main Arctic Ocean gateways (Davis Strait, Fram Strait, Barents Sea Opening and Bering Strait). Combining a recently derived velocity field at these boundaries with measurements of DIC, we calculated a net summertime pan-Arctic export of 231±49TgCyr−1. On an annual basis, we estimate that at least 166±60TgCyr−1 of this is due to uptake of CO2 from the atmosphere, although time-dependent changes in carbon storage are not quantified. To further understand the region's role as a carbon sink, we calculated the volume-conserved net DIC transport from beneath a prescribed mixed layer depth of 50m, referred to as ‘interior transport’, revealing an export of 61±23TgCyr−1. Applying a carbon framework to infer the sources of interior transport implied that this export is primarily due to the sinking and remineralisation of organic matter, highlighting the importance of the biological pump. Furthermore, we qualitatively show that the present day Arctic Ocean is accumulating anthropogenic carbon beneath the mixed layer, imported in Atlantic Water.

Eddy compensation and controls of the enhanced sea‐to‐air CO2 flux during positive phases of the Southern Annular Mode

The current positive trend in the Southern Annular Mode (SAM) is thought to reduce the growth rate of the Southern Ocean CO2 sink because enhanced wind‐driven upwelling of dissolved inorganic carbon (DIC) increases outgassing of natural CO2. However, no study to date has quantified the potentially large role of mesoscale eddies in compensating intensified upwelling nor the mixed‐layer processes in terms of their effects on CO2 fluxes. Here we report on results from two new simulations in a regional 0.5° eddying model of the Southern Ocean. The first simulation is forced with interannually varying atmospheric reanalysis and coupled to a biogeochemistry model run under constant preindustrial atmospheric CO2. The second simulation is like the first except that superimposed on the forcing is a poleward shifted and intensified westerlies wind anomaly consistent with the positive phase of the SAM. In response to the SAM, the Southern Ocean's sea‐to‐air CO2 flux is enhanced by 0.1 Pg C yr−1 per standard deviation of the SAM, mostly from the Antarctic Zone (AZ), where enhanced surface DIC is only partly compensated by enhanced surface alkalinity. Increased mixed‐layer DIC in the AZ results from a combination of increased upwelling below the mixed layer and increased vertical diffusion at the base of the mixed layer. Previous studies overlooked the latter. Thus, upward supply of DIC and alkalinity depends on associated vertical gradients just below the mixed layer, which are affected by interior ocean transport. Our eddying model study suggests that about one third of the SAM enhancement of the Ekman‐induced northward DIC transport is compensated by southward transport from standing and transient eddies.

Carbon isotopic fractionation in macroalgae from Cádiz Bay (Southern Spain): Comparison with other bio-geographic regions

The 13C signature of forty-five macroalgal species from intertidal zones at Cádiz Bay was analysed in order to research the extension of diffusive vs. non-diffusive utilisation of dissolved inorganic carbon (DIC) and to perform a comparison with data published for other bio-geographic regions. The ∂ 13C values ranged from −6.8‰ to −33‰, although the span of variation was different depending on the taxa. Thus, ∂ 13C for Chlorophyta varied from −7‰ ( Codium adhaerens) to −29.6‰ ( Flabellia petiolata), while all the Phaeophyceae (excepting Padina pavonica with ∂ 13C higher than −10‰) had values between −10‰, and −20‰. The widest variation range was recorded in Rhodophyta, from values above −10‰ ( Liagora viscida) to values lower than −30‰ obtained in three species belonging to the subclass Rhodymeniophycidae. Accordingly, the mean ∂ 13C value calculated for red algae (−20.2‰) was significantly lower than that for brown (−15.9‰) and green algae (−15.6‰). Most of the analysed red algae were species inhabiting crevices and the low intertidal fringe which explains that, on average, the shaded-habitat species had a ∂ 13C value lower than those growing fully exposed to sun (i.e. in rockpools or at the upper intertidal zone). The comparison between the capacity for non-diffusive use of DIC (i.e. active or facilitated transport of HCO 3 − and/or CO 2) and the ∂ 13C values reveals that values more negative than −30‰ indicate that photosynthesis is dependent on CO 2 diffusive entry, whereas values above this threshold would not indicate necessary the operation of a non-diffusive DIC transport mechanism. Furthermore, external carbonic anhydrase activity ( extCA) and ∂ 13C values were negatively correlated indicating that the higher the dependence of the photosynthesis on the CO 2 supplied from HCO 3 − via extCA, the lower the ∂ 13C in the algal material. The comparison between the ∂ 13C values obtained for the analysed species and those published for species inhabiting other bio-geographic areas (warm-temperate, cold and polar) suggests that globally (at least for the red and brown algae) the non-diffusive entry of DIC is more widely spread among the species from Cádiz Bay than among those of polar regions. If it is assumed that non-diffusive use of DIC implies saturation of photosynthesis at the present-day CO 2 concentration in seawater, our data indicate that the potential impact of the acidification on photosynthesis in the seaweed communities will be different depending on the latitude.

Tidal-induced inorganic carbon dynamics in the Strait of Gibraltar

This study presents the distribution of dissolved inorganic carbon (DIC) along the Strait of Gibraltar, its tidal-induced variability, as well as the inorganic carbon exchange between the Atlantic Ocean and Mediterranean Sea. During November 2003, water column samples were collected at nine stations to measure total alkalinity (TA), pH, and dissolved oxygen (DO) for the spatial characterization of the carbonate system. At the same time, anchored samplings were carried out, above the Camarinal Sill and in the Eastern Section of the Strait, in order to assess the tidal mixing effects for oxygen and DIC distribution on the water column. Three distinct water masses can be discerned in this area: the Surface Atlantic Water (SAW), the Mediterranean Water (MW), and the less abundant North Atlantic Central Water (NACW). The observations show an increase in the DIC and a decrease in oxygen concentration with depth, related to the different physico-chemical features of each water mass. The results show the high time-dependence of the vertical distribution of DIC with the interface oscillation, affected by the intense mixing processes taking place in the Strait. Intense mixing episodes over the Camarinal Sill are responsible for an increase in the DIC concentrations in the upper layer of the Eastern Section of the Strait. Higher DIC concentrations in the Mediterranean than in the Atlantic waters are responsible for a net DIC transport of 1.47×10 12 mol C yr −1 to the Atlantic Ocean. Nevertheless, the net exchange is highly sensitive to the interface definition, as well as to the estimate of water volume transport used.

Active transport of CO2 and bicarbonate is induced in response to external CO2 concentration in the green alga Chlorella kessleri

The time-course of induction of CO(2) and HCO(3)- transport has been investigated during the acclimation of high CO(2)-grown Chlorella kessleri cells to dissolved inorganic carbon (DIC)-limited conditions. The rate of photosynthesis of the cells in excess of the uncatalysed supply rate of CO(2) from HCO(3)- was taken as an indicator of HCO(3)- transport, while a stimulation of photosynthesis on the addition of bovine carbonic anhydrase was used as an indicator of CO(2) transport. The maximum rate of photosynthesis (Pmax) was similar for high CO(2)-grown and low CO(2)-grown cells, but the apparent whole cell affinity for DIC and CO(2) of high CO(2)-grown cells was found to be about 30-fold greater than in air-grown cells, which indicates a lower affinity for DIC and CO(2). It was found that HCO(3)- and CO(2) transport were induced in 5.5 h in cells acclimating to air in the light and in the presence and absence of 21% O(2), which indicates that a change in the CO(2)/O(2) ratio in the acclimating medium does not trigger induction of DIC transport. No active DIC transport was detected in high CO(2)-grown cells maintained on high CO(2) for 5.5 h in the presence of 5 mM aminooxyacetate, an aminotransferase inhibitor. These results indicate no involvement of photorespiration in triggering induction. Active DIC transport induction was inhibited in cells treated with 5 microgram ml(-1) cycloheximide, but was unaffected by chloramphenicol treatment, indicating that the induction process requires de novo cytoplasmic protein synthesis. The total DIC concentration eliciting the induction and repression of CO(2) and HCO(3)- transport was higher at pH 7.5 than at pH 6.6. The concentrations of external CO(2) required for the induction and repression of DIC transport were 0 and 120 microM, respectively, and was independent of the pH of the acclimation medium. Prolonged exposure to a critical external CO(2) concentration elicits the induction of DIC transport in C. kessleri.

Regulation of dissolved inorganic carbon transport in green algae

The regulation of the expression of the inorganic carbon concentrating mechanism (CCM) in aquatic photoautotrophs, particularly green algae, has been thought to require light and active photosynthesis. Recent studies, however, have indicated that there may be a light-independent pathway of signal transduction in green algae that may regulate the expression of CCMs in response to changes in ambient dissolved inorganic carbon (DIC) concentration. In the green alga, Chlorella ellipsoidea, changes in the rate of transport of both CO2 and HCO3- were shown to occur in response to the CO2 concentration in the bulk medium, independent of pH, whereas other inorganic carbon species, which might induce or repress DIC transport expression, were shown to change markedly with the pH of the medium. Furthermore, neither changes in the CO2 concentration around ribulose bisphosphate carboxylase-oxygenase (Rubisco) nor light were shown to be critical factors in regulating CCM expression in this alga. CO2-insensitive mutants of Chlorella ellipsoidea were recently isolated in which DIC transport operates constitutively at maximum activity. These results strongly suggest that a direct CO2 sensing mechanism may operate at the cell surface in Chlorella ellipsoidea and that this mechanism may trigger the repression of the expression of DIC transport in response to high CO2. Some supportive evidence for this hypothesis has also been obtained in other green algae, Chlamydomonas reinhardtii and Chlorella kessleri. The possibility of the occurrence of a direct sensing mechanism for CO2, its implications, and possible coexistence of other regulatory systems for CCM expression are discussed.Key words: green algae, Chlorella, signal transduction, CO2-insensitive mutants, CO2-sensing mechanism.

A New Screening Method for Algal Photosynthetic Mutants (CO2-Insensitive Mutants of the Green Alga Chlorella ellipsoidea).

A new method has been developed for screening algal photosynthetic mutants. This method uses autoradiography to assess gross photosynthetic 14C fixation by green algal colonies on agar plates and allows the identification of clones that differ in photosynthetic characteristics from wild-type cells. Three wild-type cells, high-CO2-grown Chlorella ellipsoidea, air-grown C. ellipsoidea, and air-grown Chlorella saccharophila, had K0.5 values for dissolved inorganic carbon (DIC) of 1083, 250, and 50 [mu]M, respectively, and as plaques on agar plates at Chl densities greater than 25 [mu]g cm-2 exhibited relative amounts of 14C fixation of 15, 55, and 100%, respectively. Cells of C. ellipsoidea were mutagenized with x-rays and screened by this method. Growth of C. ellipsoidea in high CO2 represses DIC transport and thus lowers its affinity for DIC. Five of the mutants detected by this method showed high-affinity photosynthesis similar to air-grown wild-type cells even when grown in high CO2. Seven other mutants when grown in high CO2 showed affinities for DIC intermediate between air-grown and high-CO2-grown wild-type cells. The affinities of high-CO2-grown mutants were reflected precisely in their capacities to accumulate DIC intracellularly. These results indicate that the mutants are fully or partially insensitive to the repressive effect of ambient CO2 concentration on DIC transport.

Active uptake of inorganic carbon byChlorella saccharophilais not repressed by growth in high CO2

Regulation of transport of dissolved inorganic carbon (DIC) in response to CO 2 concentration in the external medium has been compared in two closely-related green algae, Chlorella ellipsoidea and Chlorella saccharophila. C. ellipsoidea, when grown in high CO 2 , had reduced activities of both CO 2 and HCO 3 - transport and DIC transport activities were increased after the cells had acclimated to air. However, high CO 2 -grown C. saccharophila had a comparable level of photosynthetic affinity for DIC to that of air-grown C. ellipsoidea and this was accompanied by a capacity to accumulate high internal concentrations of DIC. The high photosynthetic affinity and the high intracellular DIC accumulation did not change in cells grown in air except that the occurrence of external carbonic anhydrase (CA) in air-grown C. saccharophila stimulated the intracellular DIC accumulation in the absence of added CA. These data indicate that active DIC transport is constitutively expressed in C. saccharophila, presumably because this alga is insensitive to the repressive effect of high CO 2 on DIC transport. This strongly supports the existence of a direct sensing mechanism for external CO 2 in Chlorella species, but also indicates that external CA is regulated independently of DIC transport in Chlorella species.