Photosynthetic Affinity For CO2 Research Articles

Ocean acidification interacts with growth light to suppress CO2 acquisition efficiency and enhance mitochondrial respiration in a coastal diatom

Diatom responses to ocean acidification have been documented with variable and controversial results. We grew the coastal diatom Thalassiosira weissflogii under 410 (LC, pH 8.13) vs 1000 μatm (HC, pH 7.83) pCO2 and at different levels of light (80, 140, 220 μmol photons m−2 s−1), and found that light level alters physiological responses to OA. CO2 concentrating mechanisms (CCMs) were down-regulated in the HC-grown cells across all the light levels, as reflected by lowered activity of the periplasmic carbonic anhydrase and decreased photosynthetic affinity for CO2 or dissolved inorganic carbon. The specific growth rate was, however, enhanced significantly by 9.2% only at the limiting low light level. These results indicate that rather than CO2 “fertilization”, the energy saved from down-regulation of CCMs promoted the growth rate of the diatom when light availability is low, in parallel with enhanced respiration under OA to cope with the acidic stress by providing extra energy.

The physiological response of marine diatoms to ocean acidification: differential roles of seawater pCO2 and pH.

Although increasing the pCO2 for diatoms will presumably down-regulate the CO2 -concentrating mechanism (CCM) to save energy for growth, different species have been reported to respond differently to ocean acidification (OA). To better understand their growth responses to OA, we acclimated the diatoms Thalassiosira pseudonana, Phaeodactylum tricornutum, and Chaetoceros muelleri to ambient (pCO2 400μatm, pH 8.1), carbonated (pCO2 800μatm, pH 8.1), acidified (pCO2 400μatm, pH 7.8), and OA (pCO2 800μatm, pH 7.8) conditions and investigated how seawater pCO2 and pH affect their CCMs, photosynthesis, and respiration both individually and jointly. In all three diatoms, carbonation down-regulated the CCMs, while acidification increased both the photosynthetic carbon fixation rate and the fraction of CO2 as the inorganic carbon source. The positive OA effect on photosynthetic carbon fixation was more pronounced in C.muelleri, which had a relatively lower photosynthetic affinity for CO2 , than in either T.pseudonana or P.tricornutum. In response to OA, T.pseudonana increased respiration for active disposal of H+ to maintain its intracellular pH, whereas P.tricornutum and C.muelleri retained their respiration rate but lowered the intracellular pH to maintain the cross-membrane electrochemical gradient for H+ efflux. As the net result of changes in photosynthesis and respiration, growth enhancement to OA of the three diatoms followed the order of C.muelleri > P.tricornutum > T.pseudonana. This study demonstrates that elucidating the separate and joint impacts of increased pCO2 and decreased pH aids the mechanistic understanding of OA effects on diatoms in the future, acidified oceans.

Regulation of inorganic carbon acquisition in a red tide alga (<i>Skeletonema costatum</i>): the importance of phosphorus availability

Abstract. Skeletonema costatum is a common bloom-forming diatom and encounters eutrophication and severe carbon dioxide (CO2) limitation during red tides. However, little is known regarding the role of phosphorus (P) in modulating inorganic carbon acquisition in S. costatum, particularly under CO2 limitation conditions. We cultured S. costatum under five phosphate levels (0.05, 0.25, 1, 4, 10 µmol L−1) and then treated it with two CO2 conditions (2.8 and 12.6 µmol L−1) for 2 h. The lower CO2 reduced net photosynthetic rate at lower phosphate levels (< 4 µmol L−1) but did not affect it at higher phosphate levels (4 and 10 µmol L−1). In contrast, the lower CO2 induced a higher dark respiration rate at lower phosphate levels (0.05 and 0.25 µmol L−1) and did not affect it at higher phosphate levels (> 1 µmol L−1). The lower CO2 did not change relative electron transport rate (rETR) at lower phosphate levels (0.05 and 0.25 µmol L−1) and increased it at higher phosphate levels (> 1 µmol L−1). Photosynthetic CO2 affinity (1/K0.5) increased with phosphate levels. The lower CO2 did not affect photosynthetic CO2 affinity at 0.05 µmol L−1 phosphate but enhanced it at the other phosphate levels. Activity of extracellular carbonic anhydrase was dramatically induced by the lower CO2 in phosphate-replete conditions (> 0.25 µmol L−1) and the same pattern also occurred for redox activity of the plasma membrane. Direct bicarbonate (HCO3-) use was induced when phosphate concentration was more than 1 µmol L−1. These findings indicate P enrichment could enhance inorganic carbon acquisition and thus maintain the photosynthesis rate in S. costatum grown under CO2-limiting conditions via increasing activity of extracellular carbonic anhydrase and facilitating direct HCO3- use. This study sheds light on how bloom-forming algae cope with carbon limitation during the development of red tides.

Photosynthetic responses of the marine diatom Thalassiosira pseudonana to CO2-induced seawater acidification

Ocean acidification due to atmospheric CO2 rise is expected to influence marine phytoplankton. Diatoms are responsible for about 40% of the total primary production in the ocean. In order to investigate the physiological response of marine diatom Thalassiosira pseudonana to ocean acidification, we grew the cells under ambient CO2 level (380 µatm) versus the elevated CO2 level (800 µatm) at a light level of 180 µmol m−2 s−1 for 30 generations. Our results showed that the elevated CO2 concentration caused a decrease of the effective photochemical efficiency of PSII $$\left( {{{F_{{\text{v}}}^{\prime } } \mathord{\left/ {\vphantom {{F_{{\text{v}}}^{\prime } } {F_{{\text{m}}}^{\prime } }}} \right. \kern-\nulldelimiterspace} {F_{{\text{m}}}^{\prime } }}} \right)$$ and increase of the dark respiration in T. pseudonana. The intracellular carbonic anhydrase activity was suppressed and the photosynthetic affinity for CO2 was lowered in the high CO2-grown cells, reflecting a downregulation of the CO2 concentrating mechanism (CCM). PSI activity was enhanced to support an increase in ATP synthesis by cyclic electron transfer as required for transport of inorganic carbon and regulation of intracellular pH. The energetic benefit from the downregulation of CCM to growth as reported in other diatom species was not observed here in T. pseudonana.

Elevated CO2 causes changes in the photosynthetic apparatus of a toxic cyanobacterium, Cylindrospermopsis raciborskii

We studied the physiological acclimation of growth, photosynthesis and CO2-concentrating mechanism (CCM) in Cylindrospermopsis raciborskii exposed to low (present day; L-CO2) and high (1300ppm; H-CO2) pCO2. Results showed that under H-CO2 the cell specific division rate (μc) was higher and the CO2- and light-saturated photosynthetic rates (Vmax and Pmax) doubled. The cells’ photosynthetic affinity for CO2 (K0.5CO2) was halved compared to L-CO2 cultures. However, no significant differences were found in dark respiration rates (Rd), pigment composition and light harvesting efficiency (α). In H-CO2 cells, non-photochemical quenching (NPQ), associated with state transitions of the electron transport chain (ETC), was negligible. Simultaneously, a reorganisation of PSII features including antenna connectivity (JconPSIIα), heterogeneity (PSIIα/β) and effective absorption cross sectional area (σPSIIα/β) was observed. In relation to different activities of the CCM, our findings suggest that for cells grown under H-CO2: (1) there is down-regulation of CCM activity; (2) the ability of cells to use the harvested light energy is altered; (3) the occurrence of state transitions is likely to be associated with changes of electron flow (cyclic vs linear) through the ETC; (4) changes in PSII characteristics are important in regulating state transitions.

A comparison of photosynthetic and respiration rates in six aquatic carnivorous Utricularia species differing in morphology

The rootless carnivorous plant genus Utricularia includes about 50 submerged aquatic or amphibious species. They usually grow in standing humic waters rich in CO2 and are known to be strict photosynthetic CO2 users. In this study, oxygen-based net photosynthetic (PN) and dark respiration (RD) rates were measured in trap-free leaves of shoot segments from different aged sections of Utricularia australis and U. vulgaris and the photosynthetic CO2 affinity was estimated in adult shoot segments. PN and RD were compared in adult trap-free leaves in three species with linear shoots (U. australis, U. vulgaris, U. purpurea) and four populations of three rhizomatous/rosette species (U. dichotoma, U. resupinata, U. volubilis) under standard conditions (25°C, 400μmolm−2s−1 PAR, 0.25mM CO2). In U. australis and U. vulgaris shoots, dry-weight based RD was highest in the youngest segments (174–184mmolkg−1h−1) and gradually declined down to the progressively senescent segments (43–58mmolkg−1h−1). In U. vulgaris, PN increased from the shoot apex up to the 25th leaf nodes, while it was highest in the 3rd leaf nodes and decreased significantly down to the shoot bases in U. australis. Chlorophyll-based PN rates along the shoots in both species were almost constant. Dry-weight based PNmax in U. australis was significantly greater than that in U. vulgaris (1634±69 vs. 1077±56mmolkg−1h−1), while the CO2 affinity (Km) was opposite – 48±10 vs. 21±2μM. Although the mean photosynthetic parameters for individual species usually differed considerably from each other between both subgroups and supported the view that the linear-shoot species have higher PN rates than the other subgroup (732–1592 vs. 155–687mmolkg−1DWh−1), statistically significant difference between both subgroups was only found for chlorophyll a-based PN. The differences in photosynthetic characteristics found between both morphological subgroups of Utricularia presumably reflect the differences in growth rates: the linear-shoot species with high PN rates are known to grow very rapidly while those from the other subgroup with lower PN grow slowly. The high PN rates of the linear-shoot Utricularia species approach the upper limit of PN reported for other aquatic non-carnivorous plants and are a prerequisite for their very rapid growth. Therefore, to attain such high PN rates and rapid growth, these species demand high [CO2] >0.15mM in their habitats.

CO2-driven seawater acidification increases photochemical stress in a green alga

Liu Y., Xu J. and Gao K. 2012. CO2-driven seawater acidification increases photochemical stress in a green alga. Phycologia 51: 562–566. DOI: 10.2216/11-65.1Increased CO2 and associated acidification in seawater, known as ocean acidification, decreases calcification of most marine calcifying organisms. However, there is little information available on how marine macroalgae would respond to the chemical changes caused by seawater acidification. We hypothesized that down-regulation of bicarbonate acquisition by algae under increased acidity and CO2 levels would lower the threshold above which photosynthetically active radiation (PAR) becomes excessive. Juveniles of Ulva prolifera derived from zoospores were grown at ambient (390 µatm) and elevated (1000 µatm) CO2 concentrations for 80 days before the hypothesis was tested. Here, the CO2-induced seawater acidification increased the quantum yield under low levels of light, but induced higher nonphotochemical quenching under high light. At the same time, the PAR level at which photosynthesis became saturated was decreased and the photosynthetic affinity for CO2 or inorganic carbon decreased in the high-CO2 grown plants. These findings indicated that ocean acidification, as an environmental stressor, can reduce the threshold above which PAR becomes excessive.

The Antisense RNA As1_flv4 in the Cyanobacterium Synechocystis sp. PCC 6803 Prevents Premature Expression of the flv4-2 Operon upon Shift in Inorganic Carbon Supply

The functional relevance of natural cis-antisense transcripts is mostly unknown. Here we have characterized the association of three antisense RNAs and one intergenically encoded noncoding RNA with an operon that plays a crucial role in photoprotection of photosystem II under low carbon conditions in the cyanobacterium Synechocystis sp. PCC 6803. Cyanobacteria show strong gene expression dynamics in response to a shift of cells from high carbon to low levels of inorganic carbon (C(i)), but the regulatory mechanisms are poorly understood. Among the most up-regulated genes in Synechocystis are flv4, sll0218, and flv2, which are organized in the flv4-2 operon. The flavodiiron proteins encoded by this operon open up an alternative electron transfer route, likely starting from the Q(B) site in photosystem II, under photooxidative stress conditions. Our expression analysis of cells shifted from high carbon to low carbon demonstrated an inversely correlated transcript accumulation of the flv4-2 operon mRNA and one antisense RNA to flv4, designated as As1_flv4. Overexpression of As1_flv4 led to a decrease in flv4-2 mRNA. The promoter activity of as1_flv4 was transiently stimulated by C(i) limitation and negatively regulated by the AbrB-like transcription regulator Sll0822, whereas the flv4-2 operon was positively regulated by the transcription factor NdhR. The results indicate that the tightly regulated antisense RNA As1_flv4 establishes a transient threshold for flv4-2 expression in the early phase after a change in C(i) conditions. Thus, it prevents unfavorable synthesis of the proteins from the flv4-2 operon.

Physiological responses of the marine diatom Thalassiosira pseudonana to increased pCO2 and seawater acidity

We studied the effects of elevated CO2 concentration and seawater acidity on inorganic carbon acquisition, photoinhibition and photoprotection as well as growth and respiration in the marine diatom Thalassiosira pseudonana. After having grown under the elevated CO2 level (1000 μatm, pH 7.83) at sub-saturating photosynthetically active radiation (PAR, 75 μmol photons m−2 s−1) for 20 generations, photosynthesis and dark respiration of the alga increased by 25% (14.69 ± 2.55 fmol C cell−1 h−1) and by 35% (4.42 ± 0.98 fmol O2 cell−1 h−1), respectively, compared to that grown under the ambient CO2 level (390 μatm, pH 8.16), leading to insignificant effects on growth (1.09 ± 0.08 d−1v 1.04 ± 0.07 d−1). The photosynthetic affinity for CO2 was lowered in the high-CO2 grown cells, reflecting a down-regulation of the CO2 concentrating mechanism (CCM). When exposed to an excessively high level of PAR, photochemical and non-photochemical quenching responded similarly in the low- and high-CO2 grown cells, reflecting that photoinhibition was not influenced by the enriched level of CO2. In T. pseudonana, it appeared that the energy saved due to the down-regulated CCM did not contribute to any additional light stress as previously found in another diatom Phaeodactylum tricornutum, indicating differential physiological responses to ocean acidification between these two diatom species.

Solar ultraviolet radiation and CO2-induced ocean acidification interacts to influence the photosynthetic performance of the red tide alga Phaeocystis globosa (Prymnesiophyceae)

Future CO2-induced ocean acidification may interact with solar UV radiation to affect physiological performance of microalgae. Therefore, CO2/pH perturbation experiments were carried out under solar radiation with or without UV radiation (295–400 nm) to evaluate the combined effects of seawater acidification (pH 7.7 at 101.3 Pa CO2) and UV on Phaeocystis globosa that forms harmful algal blooms. Under high levels of solar radiation, the acidification reduced the growth rate and photochemical efficiency either under PAR alone or with the presence of UVR radiation. Under reduced levels of solar radiation (cloudy days), however, the CO2-enrichment and UVA acted synergistically to stimulate the photochemical yield and enhanced the growth rate. That is, the effects of CO2-induced acidification were reversed from the negative (sunny days) to positive (cloudy days). CO2 concentrating mechanism f P. globosa was not affected by the elevated pCO2 in view of unchanged photosynthetic affinity for CO2 and stable activity of both intracellular and extracellular carbonic anhydrase. The increased acidity induced higher UVB-related photoinhibition of growth and non-photochemical quenching, and increased the dark respiration and the contents of Chl a, Chl c, and carotenoids, causing the cells to increase their energy demand against the combined stress. Overall, the findings imply that net or balanced effects of ocean acidification on phytoplankton would depend on the depth or mixing that alters the exposures of the cells in water columns to solar radiation.

Photosynthetic CO2 affinity of aquatic carnivorous plants growing under nearly-natural conditions and in vitro

Net photosynthetic rate of aquatic carnivorous plants in standing waters can sometimes be limited by low concentration of free CO2. As net photosynthetic rate of terrestrial plants growing in vitro is greatly reduced, as compared to the same plants grown naturally, it could be assumed that photosynthetic CO2 affinity in aquatic carnivorous plants growing in vitro will be reduced. The aim of this study was to compare values of CO2 compensation point of photosynthesis in several strains of Aldrovanda vesiculosa and in 13 aquatic Utricularia species, both in plants growing under nearly-natural conditions in containers or aquaria and in vitro. The dependence of CO2 compensation point on growth conditions is discussed.

Photosynthetic inorganic carbon utilization of gametophytes and sporophytes of Undaria pinnatifida (Phaeophyceae)

X. Zhang, H. Hu and T. Tan. 2006. Photosynthetic inorganic carbon utilization of gametophytes and sporophytes of Undaria pinnatifida (Phaeophyceae). Phycologia 45: 642–647. DOI: 10.2216/05-28.1The characteristics of inorganic carbon assimilation by photosynthesis were investigated in male and female gametophytes and juvenile sporophytes of Undaria pinnatifida. Gametophytes and sporophytes have detectable extracellular and intracellular carbonic anhydrase (CA) activity, and the CA inhibitor, acetazolamide (AZ), significantly inhibited their photosynthesis O2 evolution. In pH-drift experiments, it was found that gametophytes did not raise the final pH of seawater above 9.00 (CO2 concentrations of about 2.2 μM), indicating a low ability to utilize inorganic carbon. In contrast, sporophytes rapidly raised pH to over 9.53 and depleted the free CO2 concentration to less than 0.16 μM. The apparent photosynthetic affinity for CO2 was almost the same for gametophytes and sporophytes, whereas gametophytes had a much lower affinity for HCO3− than sporophytes. Two inhibitors of band 3 anion exchange protein (DIDS and SITS) inhibited the photosynthesis of gametophytes but not that of sporophytes. It was indicated that both gametophytes and sporophytes were capable of using HCO3−, which involved the external CA activity, and a direct HCO3− use also occurred in the former, but the latter showed a greater capacity of HCO3− use than the former. In addition, male and female gametophytes did not show great differences in the inorganic carbon uptake mechanism underlying photosynthesis.

Resistance to CO2 diffusion in cuticular membranes of amphibious plants and the implication for CO2 acquisition

Cuticular membranes (CMs) were isolated from leaves of amphibious and submerged plants and their CO2 resistances were determined as a contribution to establish quantitatively the series of resistances met by CO2 diffusing from bulk water to the chloroplasts of submerged leaves. The isolation was performed enzymatically; permeabilities were determined and converted to resistances. The range of permeance values was 3 to 43 x 10(-6) m s(-1) corresponding to resistance values of 23 to 295 x 10(3) s m(-1), i.e. of the same order of magnitude as boundary layer resistances. The sum of boundary layer, CM, leaf cell and carboxylation resistances could be contained within the total diffusion resistance as determined from the photosynthetic CO2 affinity of the leaf. From the same species, the aerial leaf CM resistance was always higher than the aquatic leaf CM resistance. In a terrestrial plant, the CM resistance to CO2 diffusion was found lower in leaves developed submerged.

Adaptation of a Chlamydomonas mutant with reduced rate of photorespiration to different concentrations of CO2

It has been widely accepted that Chlamydomonas reinhardtii cells utilize inorganic carbon very efficiently for photosynthesis by operating a CO2-concentrating mechanism (CCM) under conditions of limited CO2. To help define the mechanism, 7FR2N, one of the suppressor double mutants of phosphoglycolate phosphatase-deficient (pgp1) mutants that have a reduced photorespiration rate (RPR) was crossed with wild-type strains to generate the strain N21 as a single RPR mutant. The comparison of photosynthetic characteristics with wild-type strains after the cells adapted to different concentrations of CO2 revealed that photosynthetic affinity for inorganic carbon was higher than that in wild-type strains after adaptation to concentrations between 50 µL·L–1 CO2 and 5% CO2. Chlorophyll fluorescence parameters were also compared, and the biggest difference between N21 and the wild-type strains was observed in the photochemical quenching and effective quantum yield of photosystem II (ΔF/Fm′) at the CO2 compensation point. These values in N21 increased in a similar manner to the photosynthetic affinity for CO2, and increased significantly when the cells adapted to low-CO2 levels, whereas the values in the wild-type strains were apparently lower without any significant changes, regardless of the CO2 concentrations to which they were adapted. Although it was not clear if a nonphotochemical quenching parameter (NPQ) in N21 was higher than that in wild-type strains, NPQ increased coincidentally with the increase in photosynthetic affinity for inorganic carbon when the CO2 concentrations to which the strains were adapted decreased, in both the mutant and wild-type strain, suggesting that this form of NPQ reflects the operation of CCM in certain conditions. Possible candidates for the RPR mutation and the relationship between CCM and photosynthetic electron flow are discussed.Key words: Chlamydomonas reinhardtii, chlorophyll fluorescence, CO2-concentrating mechanism, low-CO2 responsive gene, phosphoglycolate phosphatase, photorespiration.

Growth, photosynthesis, and gene expression in Chlamydomonas over a range of CO2 concentrations and CO2/O2 ratios: CO2 regulates multiple acclimation states

Growth, photosynthesis, and induction of two low CO2-inducible genes of Chlamydomonas reinhardtii Dangeard strain CC125 were quantified in a range of physiologically relevant CO2 and O2 concentrations (5%–0.005% CO2 and 20% or 2% O2) using airlift bioreactors to facilitate the simultaneous measurement of both growth and in situ photosynthetic rates. Within these CO2 concentration ranges, O2 concentrations (20% vs. 2%) had no discernable effect on growth, photosynthetic rate, or induction of the periplasmic carbonic anhydrase (Cah1) and glycolate dehydrogenase (Gdh) genes in wild-type C. reinhardtii. These results failed to support the hypothesis that the CO2/O2 ratio plays any role in signaling for the up-regulation of limiting CO2-induced genes and (or) of the CO2-concentrating mechanism (CCM). The mRNA abundance of the Cah1 and Gdh genes appeared to be regulated in concert, suggesting co-regulation by the same signaling pathway, which, because of a lack of an O2 effect, seems unlikely to involve photorespiration or a photorespiratory metabolite. Instead, it appeared that the CO2 concentration alone was responsible for regulation of limiting CO2 acclimation responses. Based on growth, photosynthesis, and gene expression characteristics, three distinct CO2-regulated physiological states were recognized within the studied parameters, a high CO2 (5%–0.5%) state, a low CO2 (0.4%–0.03%) state, and a very low CO2 (0.01%–0.005%) state. Induction of Cah1 expression and Gdh up-regulation occurred at a CO2 concentration between 0.5% and 0.4% CO2, delineating the high from the low CO2 states. Photosynthetic characteristics also were distinct in the three CO2-regulated physiological states, e.g., the estimated K0.5(CO2) of the high CO2, low CO2, and very low CO2 states were 72, 10, and 0.9 µmol·L–1 CO2, respectively. In addition to a greater photosynthetic CO2 affinity, the very low CO2 state could be distinguished from the low CO2 state by an increased cell-doubling time and a smaller cell size.Key words: algae, Chlamydomonas, CO2, gene expression, induction, photorespiration, photosynthesis.

Photosynthetic utilisation of inorganic carbon and its regulation in the marine diatom Skeletonema costatum.

Photosynthetic uptake of inorganic carbon and regulation of photosynthetic CO2 affinity were investigated in Skeletonema costatum (Grev.) Cleve. The pH independence of K1/2(CO2) values indicated that algae grown at either ambient (12 μmol L-1) or low (3 μmol L-1) CO2 predominantly took up CO2 from the medium. The lower pH compensation point (9.12) and insensitivity of photosynthetic rate to di-isothiocyanatostilbene disulfonic acid (DIDS) indicated that the alga had poor capacity for direct HCO3- utilisation. Photosynthetic CO2 affinity is regulated by the concentration of CO2 rather than HCO3-, CO32- or total dissolved inorganic carbon (DIC) in the medium. The response of photosynthetic CO2 affinity to changes in CO2 concentration was most sensitive within the range 3-48 μmol L-1 CO2. Light was required for the induction of photosynthetic CO2 affinity, but not for its repression, when cells were shifted between high (126 μmol L-1) and ambient (12 μmol L-1) CO2. The time needed for cells grown at high CO2 (126 μmol L-1) to fully develop photosynthetic CO2 affinity at ambient CO2 was approximately 2 h, but acclimation to low or very low CO2 levels (3 and 1.3 μmol L-1, respectively) took more than 10 h. Cells grown at low CO2 (3 μmol L-1) required approximately 10 h for repression of all photosynthetic CO2 affinity when transferred to ambient or high CO2 (12 or 126 μmol L-1, respectively), and more than 10 h at very high CO2 (392 μmol L-1).

Characterization of diurnal photosynthetic rhythms in the marine diatom Skeletonema costatum grown in synchronous culture under ambient and elevated CO2.

Photosynthetic performance was examined in Skeletonema costatum (Greville) Cleve. under 12 : 12-h light : dark (LD) cycle at ambient CO2 (350 μL L-1) and elevated CO2 (1000 μL L-1). At ambient CO2, the cellular chlorophyll a content, the light-saturated photosynthetic rate (Pm), the initial slope of the light saturation curves (α), the photochemical efficiency of PSII (Fv / Fm), the apparent carboxylating efficiency (ACE) and the photosynthetic affinity for CO2 [1 / Km(CO2)] all showed rhythmical changes with different amplitudes during the light period. The Pm had similar changing pattern in the light period with the ACE and 1 / Km(CO2) rather than with the α and Fv / Fm, indicating that rhythmical changes of photosynthetic capacity may be mainly controlled by the activity of C-reduction associated with CO2 uptake during the light period. The CO2 enrichment reduced the ACE and the affinity to CO2, and increased the α, cellular chlorophyll a content and Pm based on cell number. By contrast, the changing patterns of all photosynthetic parameters examined here during the light period had almost the same for cells grown at ambient CO2 and elevated CO2, suggesting that the photosynthetic rhythms of S. costatum are not affected by CO2 enrichment.

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

The mechanism of inorganic carbon (Ci) 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 Ci 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 Ci exceeding that of seawater, did not operate active HCO3− uptake but used it via the extracellular CA for its photosynthetic carbon fixation.

Effect of CO2 concentrations on the activity of photosynthetic CO2 fixation and extracelluar carbonic anhydrase in the marine diatom Skeletonema costatum

The growth and activity of photosynthetic CO2 uptake and extracellular carbonic anhydrase (CAext) of the marine diatom Skeletonema costatum were investigated while cultured at different levels of CO2 in order to see its physiological response to different CO2 concentrations under either a low (30 μmol · m−2 · s−1) or high (210 μmol · m−2 · s−1) irradiance. The changes in CO2 concentrations (4–31 μmol/L) affected the growth and net photosynthesis to a greater extent under the low than under the high light regime. CAext was detected in the cells grown at 4 μmol/L CO2 but not at 31 and 12 μmol/L CO2, with its activity being about 2.5-fold higher at the high than at the low irradiance. Photosynthetic CO2 affinity (1/ K1/2(CO2)) of the cells decreased with increased CO2 concentrations in culture. The cells cultured under the high-light show significantly higher photosynthetic CO2 affinity than those grown at the low-light level. It is concluded that the regulations of CAext activity and photosynthetic CO2 affinity are dependent not only on CO2 concentration but also on light availability, and that the development of higher CAext activity and CO2 affinity under higher light level could sufficiently support the photosynthetic demand for CO2 even at low level of CO2.

Daily production and photosynthetic characteristics of Nostoc flagelliforme grown under ambient and elevated CO2 conditions

Diurnal photosynthesis of Nostoc flagelliforme was investigated at varied levels of CO2 concentrations and desiccation in order to estimate the effects of enriched CO2 and watering on its daily production. Photosynthetic activity was closely correlated with the desiccated status of the algal mats, increased immediately after watering, reached a maximum at moderate water loss, and then declined with further desiccation. Increased CO2 concentration enhanced the diurnal photosynthesis and raised the daily production. Watering twice per day enhanced the daily production due to prolonged period of active photosynthesis. The values of daily net production were 1321280 mumol CO2 g (d. wt)(-1) d(-1), corresponding to about 0.6-6.1% daily increase in dry weight. High-CO2-grown mats required higher levels of photon flux density to saturate the alga's photosynthesis in air. Air-grown mats showed higher photosynthetic affinity for CO2 and higher levels of dark respiration compared with high-CO2-grown samples.