An external delta-carbonic anhydrase in a free-living marine dinoflagellate may circumvent diffusion-limited carbon acquisition

The oceans globally constitute an important sink for carbon dioxide (CO(2)) due to phytoplankton photosynthesis. However, the marine environment imposes serious restraints to carbon fixation. First, the equilibrium between CO(2) and bicarbonate (HCO(3)(-)) is pH dependent, and, in normal, slightly alkaline seawater, [CO(2)] is typically low (approximately 10 mum). Second, the rate of CO(2) diffusion in seawater is slow, so, for any cells unable to take up bicarbonate efficiently, photosynthesis could become carbon limited due to depletion of CO(2) from their immediate vicinity. This may be especially problematic for those dinoflagellates using a form II Rubisco because this form is less oxygen tolerant than the usually found form I enzyme. We have identified a carbonic anhydrase (CA) from the free-living marine dinoflagellate Lingulodinium polyedrum that appears to play a role in carbon acquisition. This CA shares 60% sequence identity with delta-class CAs, isoforms so far found only in marine algae. Immunoelectron microscopy indicates that this enzyme is associated exclusively with the plasma membrane. Furthermore, this enzyme appears to be exposed to the external medium as determined by whole-cell CA assays and vectorial labeling of cell surface proteins with (125)I. The fixation of (14)CO(2) is strongly pH dependent, suggesting preferential uptake of CO(2) rather than HCO(3)(-), and photosynthetic rates decrease in the presence of 1 mm acetazolamide, a non-membrane-permeable CA inhibitor. This constitutes the first CA identified in the dinoflagellates, and, taken together, our results suggest that this enzyme may help to increase CO(2) availability at the cell surface.     

Mechanism of CO2 acquisition in an acid-tolerant Chlamydomonas

The acid-tolerant green alga Chlamydomonas (UTCC 121) grows in media ranging in pH from 2.5 to 7.0. Determination of the overall internal pH of the cells, using (14)C-benzoic acid (BA) or [2-(14)C]-5,5-dimethyloxazolidine-2,4-dione (DMO), showed that the cells maintain a neutral pH (6.6 to 7.2) over an external pH range of 3.0-7.0. The cells express an external carbonic anhydrase (CA) when grown in media above pH 5.5, and CA increases to a maximum at pH 7.0. Removal of external CA by trypsin digestion or by acetazolamide (AZA) inhibition indicated that CA was essential for photosynthesis at pH 7.0 and that the cells had no capacity for direct bicarbonate uptake. Monitoring of CO(2) uptake and O(2) evolution by mass spectrometry during photosynthesis did not provide any evidence of active CO(2) uptake. The CO(2) compensation concentration of the cells ranged from 9.4 microM at pH 4.5 to 16.2 microM at pH 7.0. An examination of the kinetics of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco), in homogenates of cells grown at pH 7.0, showed that the K(m) (CO(2)) was 16.3 microM. These data indicate that the pH between the cell interior and the external medium was large enough at acid pH to allow the accumulation of inorganic carbon (Ci) by the diffusive uptake of CO(2), and the expression of external CA at neutral pH values would maintain an equilibrium CO(2) concentration at the cell surface. This species does not possess a CO(2)-concentrating mechanism because the whole cell affinity for Ci appears to be determined by the low K(m) (CO(2)) Rubisco of the alga.     

干出和沉水不同条件下羊栖菜光合作用碳源获得机制比较(英文)

经济海洋褐藻羊栖菜(Hizikia fusiforme(Harv.)Okamura)低潮时常常周期性地暴露于空气中。为了认识这种海藻在潮汐循环背景下的光合特征,对其在高潮沉水和低潮干出不同条件下的光合作用碳素获得机制进行了比较。沉水时,羊栖菜主要利用海水中HCO_3~-作为外源无机碳源驱动光合作用;而在干出条件下,其光合作用的主要碳源为空气中的CO_2。在这两种不同环境条件下,光合作用与pH值的关系不同:沉水状态时,羊栖菜在高pH值(10.0)下光合活性很弱;而在干出条件下,羊栖菜在高pH值时仍有较高的光合活性。然而,光合作用无论是在沉水还是在干出条件下,对外源碳源的获得都表现出对胞外碳酸酐酶(CA)强烈的依赖性,并且其光合速率都受周围环境中无机碳源水平的限制。此外,在沉水和干出两种环境条件下,羊栖菜光合作用都表现出对氧气的敏感性。这表明,在羊栖菜中,依赖胞外CA的碳源获得机制不能使细胞内CO_2浓度提高到阻碍其光呼吸的程度。增加空气中或海水中无机碳的浓度,能促进羊栖菜的光合作用,进而增加这种海藻的水产养殖产量。     

Inorganic carbon acquisition in two green marine Stichococcus species

The mechanism of inorganic carbon (C(i)) uptake was examined in the marine green microalgae Stichococcus cylindricus and Stichococcus minor. External carbonic anhydrase (CA) activity was not detected in either species, by potentiometric assay or by mass spectrometry. Photosynthetic characteristics of C(i) uptake indicate that both species have high apparent affinity for CO(2) with a low K(1/2) (CO(2)) of about 10 μm. The O(2) evolution rates in light exceeded the spontaneous CO(2) formation rate by 2.5-fold in both species, which thus have active bicarbonate uptake. Mass spectrometric monitoring of CO(2) and O(2) fluxes showed that rates of O(2) evolution exceeded those of CO(2) depletion by about three- and twofold in S. minor and S. cylindricus, respectively, and also showed, in cells photosynthesizing at pH 8.2, a rapid depletion of CO(2) upon illumination to a CO(2) compensation concentration of 15.42 and 12.03 μm in S. minor and S. cylindricus, respectively. Both species also exhibit active CO(2) uptake: addition of bovine CA at CO(2) compensation concentration caused a rapid rise in CO(2) as the CO(2) -HCO(3) (-) equilibrium was restored. Accumulation of unfixed C(i) by cells at pH 8.2 was calculated to be 84.33 mm in S. cylindricus, and 30.37 mm in S. minor to give internal accumulations of 23- and 8-fold, respectively, compared to the external C(i) concentration.     

Physiological characterization and light response of the CO2-concentrating mechanism in the filamentous cyanobacterium Leptolyngbya sp. CPCC 696

We studied the interactions of the CO(2)-concentrating mechanism and variable light in the filamentous cyanobacterium Leptolyngbya sp. CPCC 696 acclimated to low light (15 μmol m(-2) s(-1) PPFD) and low inorganic carbon (50 μM Ci). Mass spectrometric and polarographic analysis revealed that mediated CO(2) uptake along with both active Na(+)-independent and Na(+)-dependent HCO(3)(-) transport, likely through Na(+)/HCO(3)(-) symport, were employed to concentrate Ci internally. Combined transport of CO(2) and HCO(3)(-) required about 30 kJ mol(-1) of energy from photosynthetic electron transport to support an intracellular Ci accumulation 550-fold greater than the external Ci. Initially, Leptolyngbya rapidly induced oxygen evolution and Ci transport to reach 40-50% of maximum values by 50 μmol m(-2) s(-1) PPFD. Thereafter, photosynthesis and Ci transport increased gradually to saturation around 1,800 μmol m(-2) s(-1) PPFD. Leptolyngbya showed a low intrinsic susceptibility to photoinhibition of oxygen evolution up to PPFD of 3,000 μmol m(-2) s(-1). Intracellular Ci accumulation showed a lag under low light but then peaked at about 500 μmol photons m(-2) s(-1) and remained high thereafter. Ci influx was accompanied by a simultaneous, light-dependent, outward flux of CO(2) and by internal CO(2)/HCO(3)(-) cycling. The high-affinity and high-capacity CCM of Leptolyngbya responded dynamically to fluctuating PPFD and used excitation energy in excess of the needs of CO(2) fixation by increasing Ci transport, accumulation and Ci cycling. This capacity may allow Leptolyngbya to tolerate periodic exposure to excess high light by consuming electron equivalents and keeping PSII open.     

SOURCE OF INORGANIC CARBON FOR PHOTOSYNTHESIS IN TWO MARINE DINOFLAGELLATES1

Inorganic carbon uptake was investigated in two marine dinoflagellates, Amphidinium carterae Hulburt and Heterocapsa oceanica Stein. Mass spectrometric and potentiometric assays indicated that both species lacked external carbonic anhydrase (CA). The presence of internal CA was demonstrated by potentiometric assay and by the inhibition of photosynthesis upon the addition of 500 μM ethoxyzolamide a membrane‐permeable inhibitor of CA. The capacity for bicarbonate transport was investigated by comparing the calculated rate of spontaneous CO₂ formation at pH 8.2 and 25°C with the rate of photosynthesis after the addition of 100 μM NaHCO₃. Both species appeared to have a very limited capacity for direct bicarbonate uptake. Monitoring of CO₂ and O₂ fluxes in both species by mass spectrometry demonstrated a rapid uptake of CO₂ on illumination, to concentrations below the CO₂ equilibrium concentration, indicating an effective selective uptake of CO₂. This dependence of photosynthesis on free CO₂ alone suggests that these species are CO₂ limited in their natural environment because the CO₂ concentration of seawater is very low.     

Inorganic carbon acquisition in potentially toxic and non-toxic diatoms: the effect of pH-induced changes in seawater carbonate chemistry

The effects of pH-induced changes in seawater carbonate chemistry on inorganic carbon (C(i)) acquisition and domoic acid (DA) production were studied in two potentially toxic diatom species, Pseudo-nitzschia multiseries and Nitzschia navis-varingica, and the non-toxic Stellarima stellaris. In vivo activities of carbonic anhydrase (CA), photosynthetic O(2) evolution and CO(2) and HCO(3)(-) uptake rates were measured by membrane inlet MS in cells acclimated to low (7.9) and high pH (8.4 or 8.9). Species-specific differences in the mode of carbon acquisition were found. While extracellular carbonic anhydrase (eCA) activities increased with pH in P. multiseries and S. stellaris, N. navis-varingica exhibited low eCA activities independent of pH. Half-saturation concentrations (K(1/2)) for photosynthetic O(2) evolution, which were highest in S. stellaris and lowest in P. multiseries, generally decreased with increasing pH. In terms of carbon source, all species took up both CO(2) and HCO(3)(-). K(1/2) values for inorganic carbon uptake decreased with increasing pH in two species, while in N. navis-varingica apparent affinities did not change. While the contribution of HCO(3)(-) to net fixation was more than 85% in S. stellaris, it was about 55% in P. multiseries and only approximately 30% in N. navis-varingica. The intracellular content of DA increased in P. multiseries and N. navis-varingica with increasing pH. Based on our data, we propose a novel role for eCA acting as C(i)-recycling mechanism. With regard to pH-dependence of growth, the 'HCO(3)(-) user' S. stellaris was as sensitive as the 'CO(2) user' N. navis-varingica. The suggested relationship between DA and carbon acquisition/C(i) limitation could not be confirmed.     

Meeting the photosynthetic demand for inorganic carbon in an alga-invertebrate association: preferential use of CO2 by symbionts in the giant clam Tridacna gigas

Unlike most marine invertebrates which excrete respiratory CO2, giant clams (Tridacna gigas) must acquire inorganic carbon (Ci) in order to support their symbiotic population of photosynthetic dinoflagellates. Their capacity to meet this demand will be reflected in the Ci concentration of their haemolymph during periods of high photosynthesis. The Ci concentration in haemolymph was found to be inversely proportional to irradiance with a minimum Ci concentration of 0.75 mM at peak light levels increasing to 1.2 mM in the dark. The photosynthetic rate of isolated zooxanthellae under conditions that prevail in the haemolymph at peak light levels was significantly less than the potential Pmax (maximum photosynthetic rate) indicating that zooxanthellae are carbon limited in hospite. This is consistent with previous studies on the hermatypic coral symbiosis. The Pmax was not affected by pH but there was a dramatic increase in the half-saturation constant for Ci (K0.5 (Ci)) with increasing pH (6.5-9.0) and only a small decrease in K0.5 (CO2) over the same range. These results indicate that zooxanthellae in giant clams use CO2 as the primary source of their Ci in contrast to symbionts in corals, which use bicarbonate. The physiological implications are discussed and comparison is made with the coral symbiosis.     

Photosynthetic inorganic carbon uptake and accumulation in two marine diatoms

Some physiological characteristics of photosynthetic inorganic carbon uptake have been examined in the marine diatoms Phaeodactylum tricornutum and Cyclotella sp. Both species demonstrated a high affinity for inorganic carbon in photosynthesis at pH7.5, having K_1/2(CO₂) in the range 1.0 to 4.0mmol m^?3 and O_2? and temperature-insensitive CO₂ compensation concentrations in the range 10.8 to 17.6 cm³ m^?3. Intracellular accumulation of inorganic carbon was found to occur in the light; at an external pH of 7.5 the concentration in P. tricornutum was twice, and that in Cyclotella 3.5 times, the concentration in the suspending medium. Carbonic anhydrase (CA) was detected in intact Cyclotella cells but not in P. tricornutum, although internal CA was detected in both species. The rates of photosynthesis at pH 8.0 of P. tricornutum cells and Cyclotella cells treated with 0.1 mol m^?3 acetazolamide, a CA inhibitor, were 1.5- to 5-fold the rate of CO₂ supply, indicating that both species have the capacity to take up HCO₃^? as a source of substrate for photosynthesis. No Na⁺ dependence for HCO₃^? could be detected in either species. These results indicate that these two marine diatoms have the capacity to accumulate inorganic carbon in the light as a consequence, in part, of the active uptake of bicarbonate.     

Impacts of increased atmospheric CO_2 concentration on photosynthesis and growth of micro-and macro-algae

Marine photosynthesis drives the oceanic biological CO2 pump to absorb CO2 from the atmosphere, which sinks more than one third of the industry-originated CO2 into the ocean. The increasing atmos-pheric CO2 and subsequent rise of pCO2 in seawater, which alters the carbonate system and related chemical reactions and results in lower pH and higher HCO3- concentration, affect photosynthetic CO2 fixation processes of phytoplanktonic and macroalgal species in direct and/or indirect ways. Although many unicellular and multicellular species can operate CO2-concentrating mechanisms (CCMs) to util-ize the large HCO3- pool in seawater, enriched CO2 up to several times the present atmospheric level has been shown to enhance photosynthesis and growth of both phytoplanktonic and macro-species that have less capacity of CCMs. Even for species that operate active CCMs and those whose photo-synthesis is not limited by CO2 in seawater, increased CO2 levels can down-regulate their CCMs and therefore enhance their growth under light-limiting conditions (at higher CO2 levels, less light energy is required to drive CCM). Altered physiological performances under high-CO2 conditions may cause genetic alteration in view of adaptation over long time scale. Marine algae may adapt to a high CO2 oceanic environment so that the evolved communities in future are likely to be genetically different from the contemporary communities. However, most of the previous studies have been carried out under indoor conditions without considering the acidifying effects on seawater by increased CO2 and other interacting environmental factors, and little has been documented so far to explain how physi-ology of marine primary producers performs in a high-CO2 and low-pH ocean.     

Evidence for K+-dependent HCO3- utilization in the marine diatom Phaeodactylum tricornutum

Photosynthetic utilization of inorganic carbon in the marine diatom Phaeodactylum tricornutum was investigated by the pH drift experiment, measurement of K(1/2) values of dissolved inorganic carbon (DIC) with pH change, and comparison of the rate of photosynthesis with the rate of the theoretical CO(2) formation from uncatalyzed HCO(3)(-) conversion in the medium. The higher pH compensation point (10.3) and insensitivity of the photosynthetic rate to acetazolamide indicate that the alga has good capacity for direct HCO(3)(-) utilization. The photosynthetic rate reached 150 times the theoretical CO(2) supply rate at 100 micromol L(-1) DIC (pH 9.0) in the presence of 10 mmol L(-1) K(+) and 46 times that in the absence of K(+), indicating that for pH 9.4-grown P. tricornutum, HCO(3)(-) in the medium is taken up through K(+)-dependent and -independent HCO(3)(-) transporters. The K(1/2) (CO(2)) values at pH 8.2 were about 4 times higher than those at pH 9.0, whereas the K(1/2) (HCO(3)(-)) values at pH 8.2 were slightly lower than those at pH 9.0 whether without or with K(+), providing further evidence for the presence of the two HCO(3)(-) transport patterns in this alga. Photosynthetic rate and affinity for HCO(3)(-) in the presence of K(+), respectively, were about 2- and 7-fold higher than those in the absence of K(+), indicating that K(+)-dependent HCO(3)(-) transport is a predominant pattern of HCO(3)(-) cellular uptake in low DIC concentration. However, as P. tricornutum was cultured at pH 7.2 or 8.0, photosynthetic affinities to HCO(3)(-) were not affected by K(+), implying that K(+)-dependent HCO(3)(-) transport is induced when P. tricornutum is cultured at high alkaline pH.     

ACQUISITION OF INORGANIC CARBON BY THE MARINE HAPTOPHYTE ISOCHRYSIS GALBANA (PRYMNESIOPHYCEAE)1

Inorganic carbon acquisition has been investigated in the marine haptophyte Isochrysis galbana. External carbonic anhydrase (CA) was present in air‐grown (0.034% CO₂) cells but completely repressed in high (3%) CO₂‐grown cells. External CA was not inhibited by 1.0 mM acetazolamide. The capacity of cells to take up bicarbonate was examined by comparing the rate of photosynthetic O₂ evolution with the calculated rate of spontaneous CO₂ supply; at pH 8.2 the rates of O₂ evolution exceeded the CO₂ supply rate 14‐fold, indicating that this alga was able to take up HCO₃ ^? . Monitoring CO₂ concentrations by mass spectrometry showed that suspensions of high CO₂‐grown cells caused a rapid drop in the extracellular CO₂ in the light and addition of bovine CA raised the CO₂ concentration by restoring the HCO₃ ^? ‐CO₂ equilibrium, indicating that cells were maintaining the CO₂ in the medium below its equilibrium value during photosynthesis. A rapid increase in extracellular CO₂ concentration occurred on darkening the cells, indicating that the cells had accumulated an internal pool of unfixed inorganic carbon. Active CO₂ uptake was blocked by the photosynthetic electron transport inhibitor 3‐(3′,4′‐dichlorphenyl)‐1,1‐dimethylurea, indicating that CO₂ transport was supported by photosynthetic reactions. These results demonstrate that this species has the capacity to take up HCO₃ ^? and CO₂ actively as sources of substrate for photosynthesis and that inorganic carbon transport is not repressed by growth on high CO₂, although external CA expression is regulated by CO₂ concentration.     

水体无机碳条件对常见沉水植物生长和生理的影响

为了解水华引起的水体无机碳变化对沉水植物生长的影响,对8种沉水植物:金鱼藻、穗花狐尾藻、篦齿眼子菜、光叶眼子菜、微齿眼子菜、伊乐藻、菹草和黑藻在不同无机碳浓度下的生物量、株高、叶绿素以及光合和呼吸速率进行了比较研究.结果表明8种沉水植物均能利用HCO3-作为光合无机碳源,在1.5 mmoL/L外源HCO3-浓度下能促进金鱼藻、菹草和伊乐藻的生长,提高其光合速率;在2.5 mmol/L外源HCO3-浓度下能促进狐尾藻、光叶眼子菜、黑藻、微齿眼子菜和蓖齿眼子菜的生长,提高其光合速率.在CO32-为优势碳源时,8种沉水植物表现出不同的适应性,发现微齿眼子菜、篦齿眼子菜和黑藻在整个实验范围内生长未受抑制,且在不同浓度下表现生长和光合速率的促进,说明这三种沉水植物对[HCO3-]/[CO32-]比值和pH具有较广适应范围.而当CO32-成为优势碳源时,金鱼藻和伊乐藻的生长受到抑制,狐尾藻、菹草和光叶眼子菜均死亡,表明[HCO3-]/[CO32-]比值和pH是这5种沉水植物生长的重要限制因子.     

海洋浮游藻类无机碳利用机理的研究

为了认识海洋浮游藻类在碳充足和碳受限条件下对水体中溶解无机碳(DIC)的利用方式与可能机理,对13种海洋浮游藻类在不同pH和CO2浓度及不同DIC条件下细胞外碳酸酐酶(CA)的活性进行了分析测定.结果显示:13种藻中,只有Amphidinium carterae和Prorocentrum minimum在碳充足条件下具细胞外CA活性.Melosira sp.、Phaeodactylum tricornutum、Skeletonema costatum、Thalassiosira rotula、Emiliania huxleyi和Pleurochrysis carterae则在碳受限条件下才具细胞外CA活性.Chaetoceros compressus、Glenodinium foliaceum、Coccolithus pelagicus、 Gephrocapsa oceanica和Heterosigma akashiwo即使在碳受限条件下也未检测到细胞外CA活性.应用封闭系统中pH漂移技术和阴离子交换抑制剂4′4′-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS)等的研究表明,Coc. pelagicus和G. oceanica可通过阴离子交换机制进行HCO-3的直接利用.H. akashiwo没有潜在的HCO-3直接利用或细胞外CA催化的HCO-3利用.     

Comparison of the photosynthetic characteristics of three submersed aquatic plants

Light- and CO(2)-saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum, and Myriophyllum spicatum were 50 to 60 mumol O(2)/mg Chl.hr at 30 C. At air levels of CO(2), the rates were less than 5% of those achieved by terrestrial C(3) plants. The low photosynthetic rates correlated with low activities of the carboxylation enzymes. In each species, ribulose 1,5-diphosphate carboxylase was the predominant carboxylation enzyme. The apparent K(m)(CO(2)) values for photosynthesis were 150 to 170 mum at pH 4, and 75 to 95 mum at pH 8. The K(m)(CO(2)) of Hydrilla ribulose 1,5-diphosphate carboxylase was 45 mum at pH 8. Optimum temperatures for the photosynthesis of Hydrilla, Myriophyllum, and Ceratophyllum were 36.5, 35.0, and 28.5 C, respectively. The apparent ability of each species to use HCO(3) (-) ions for photosynthesis was similar, but at saturating free CO(2) levels, there was no indication of HCO(3) (-) use. Increasing the pH from 3.1 to 9.2 affected the photosynthetic rate indirectly, by decreasing the free CO(2). With saturating free CO(2) (0.5 mm), the maximum photosynthetic rates were similar at pH 4 and 8. Carbonic anhydrase activity, although much lower than in terrestrial C(3) plants, was still in excess of that required to support HCO(3) (-) utilization.Hydrilla and Ceratophyllum had CO(2) compensation points of 44 and 41 mul/l, respectively, whereas the value for Myriophyllum was 19. Relatively high CO(2) compensation points under 1% O(2) indicated that some "dark" respiration occurred in the light. The inhibition of photosynthesis by O(2) was less than with terrestrial C(3) plants. Glycolate oxidase activity was 12.3 to 27.5 mumol O(2)/mg Chl.hr, as compared to 78.4 for spinach. Light saturation of photosynthesis occurred at 600 to 700 mueinsteins/m(2).sec in each species grown under full sunlight. Hydrilla had the lowest light compensation point, and required the least irradiance to achieve the half-maximal photosynthetic rate.Field measurements in a Hydrilla mat indicated that in the afternoon, free CO(2) dropped to zero, and O(2) rose to over 200% air saturation. Most photosynthetic activity occurred in the morning when the free CO(2) was highest and O(2) and solar radiation lowest. The low light requirement of Hydrilla probably provides a competitive advantage under these field conditions.     

干出和沉水不同条件下羊栖菜光合作用碳源获得机制比较

经济海洋褐藻羊栖菜(Hizikia fusiforme(Harv.)Okamura)低潮时常常周期性地暴露于空气中.为了认识这种海藻在潮汐循环背景下的光合特征,对其在高潮沉水和低潮干出不同条件下的光合作用碳素获得机制进行了比较.沉水时,羊栖菜主要利用海水中HCO3-作为外源无机碳源驱动光合作用;而在干出条件下,其光合作用的主要碳源为空气中的CO2.在这两种不同环境条件下,光合作用与pH值的关系不同:沉水状态时,羊栖菜在高pH值(10.0)下光合活性很弱;而在干出条件下,羊栖菜在高pH值时仍有较高的光合活性.然而,光合作用无论是在沉水还是在干出条件下,对外源碳源的获得都表现出对胞外碳酸酐酶(CA)强烈的依赖性,并且其光合速率都受周围环境中无机碳源水平的限制.此外,在沉水和干出两种环境条件下,羊栖菜光合作用都表现出对氧气的敏感性.这表明,在羊栖菜中,依赖胞外CA的碳源获得机制不能使细胞内CO2浓度提高到阻碍其光呼吸的程度.增加空气中或海水中无机碳的浓度,能促进羊栖菜的光合作用,进而增加这种海藻的水产养殖产量.     

The role of the C4 pathway in carbon accumulation and fixation in a marine diatom

The role of a C(4) pathway in photosynthetic carbon fixation by marine diatoms is presently debated. Previous labeling studies have shown the transfer of photosynthetically fixed carbon through a C(4) pathway and recent genomic data provide evidence for the existence of key enzymes involved in C(4) metabolism. Nonetheless, the importance of the C(4) pathway in photosynthesis has been questioned and this pathway is seen as redundant to the known CO(2) concentrating mechanism of diatoms. Here we show that the inhibition of phosphoenolpyruvate carboxylase (PEPCase) by 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate resulted in a more than 90% decrease in whole cell photosynthesis in Thalassiosira weissflogii cells acclimated to low CO(2) (10 microm), but had little effect on photosynthesis in the C(3) marine Chlorophyte, Chlamydomonas sp. In 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate-treated T. weissflogii cells, elevated CO(2) (150 microm) or low O(2) (80-180 microm) restored photosynthesis to the control rate linking PEPCase inhibition with CO(2) supply in this diatom. In C(4) organic carbon-inorganic carbon competition experiments, the (12)C-labeled C(4) products of PEPCase, oxaloacetic acid and its reduced form malic acid suppressed the fixation of (14)C-labeled inorganic carbon by 40% to 50%, but had no effect on O(2) evolution in photosynthesizing diatoms. Oxaloacetic acid-dependent O(2) evolution in T. weissflogii was twice as high in cells acclimated to 10 microm rather than 22 microm CO(2), indicating that the use of C(4) compounds for photosynthesis is regulated over the range of CO(2) concentrations observed in marine surface waters. Short-term (14)C uptake (silicone oil centrifugation) and CO(2) release (membrane inlet mass spectrometry) experiments that employed a protein denaturing cell extraction solution containing the PEPCKase inhibitor mercaptopicolinic acid revealed that much of the carbon taken up by diatoms during photosynthesis is stored as organic carbon before being fixed in the Calvin cycle, as expected if the C(4) pathway functions as a CO(2) concentrating mechanism. Together these results demonstrate that the C(4) pathway is important in carbon accumulation and photosynthetic carbon fixation in diatoms at low (atmospheric) CO(2).     

PHOTOSYNTHETIC UTILIZATION OF INORGANIC CARBON IN THE ECONOMIC BROWN ALGA,HIZIKIA FUSIFORME (SARGASSACEAE) FROM THE SOUTH CHINA SEA1

The mechanism of inorganic carbon (C_i) acquisition by the economic brown macroalga, Hizikia fusiforme (Harv.) Okamura (Sargassaceae), was investigated to characterize its photosynthetic physiology. Both intracellular and extracellular carbonic anhydrase (CA) were detected, with the external CA activity accounting for about 5% of the total. Hizikia fusiforme showed higher rates of photosynthetic oxygen evolution at alkaline pH than those theoretically derived from the rates of uncatalyzed CO₂ production from bicarbonate and exhibited a high pH compensation point (pH 9.66). The external CA inhibitor, acetazolamide, significantly depressed the photosynthetic oxygen evolution, whereas the anion‐exchanger inhibitor 4,4′‐diisothiocyano‐stilbene‐2,2′‐disulfonate had no inhibitory effect on it, implying the alga was capable of using HCO₃^? as a source of C_i for its photosynthesis via the mediation of the external CA. CO₂ concentrations in the culture media affected its photosynthetic properties. A high level of CO₂ (10,000 ppmv) resulted in a decrease in the external CA activity; however, a low CO₂ level (20 ppmv) led to no changes in the external CA activity but raised the intracellular CA activity. Parallel to the reduction in the external CA activity at the high CO₂ was a reduction in the photosynthetic CO₂ affinity. Decreased activity of the external CA in the high CO₂ grown samples led to reduced sensitiveness of photosynthesis to the addition of acetazolamide at alkaline pH. It was clearly indicated that H. fusiforme, which showed CO₂‐limited photosynthesis with the half‐saturating concentration of C_i exceeding that of seawater, did not operate active HCO₃^? uptake but used it via the extracellular CA for its photosynthetic carbon fixation.