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- PALMQVIST, K, et al.
(författare)
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MECHANISMS OF ADAPTATION OF MICROALGAE TO CONDITIONS OF CARBON-DIOXIDE LIMITATION OF PHOTOSYNTHESIS - POSSIBLE ROLE OF CARBONIC-ANHYDRASE
- 1990
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Ingår i: SOVIET PLANT PHYSIOLOGY. - 0038-5719. ; 37:5, s. 680-686
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Tidskriftsartikel (refereegranskat)abstract
- We studied the rate of assimilation of inorganic carbon (C(in)) in relation to CO2 concentration in Dunaliella salina. Chlorella vulgaris, Scenedesmus obliguus, and Chlamydomonas reinhardtii WT grown at 5 and 0.03% CO2. It is demonstrated that K(m) (CO2) of photosynthesis reached 50-60-mu-M in algae grown at 5% CO2, but 2-5-mu-M at 0.03% CO2. Carbonic anhydrase (CA) activity of intact cells and in the homogenate was significantly higher in algae grown at 0.03% CO2 than at 5% CO2. Several forms of CA are present, viz., soluble (sCA), membrane-bound CA of cytoplasmic (cCA) and thylakoid (tCA) membranes, and CA of intact cells (iCA). Acetoazolamide lowered the rate of C(in) assimilation in algae grown at 0.03% CO2, whereas ethoxyzolamide totally suppressed the ability of the algae to adapt to low CO2 concentrations. The inhibitor of plasmalemma ATPase vanadate likewise suppressed the rate of C(in) assimilation in the algae. It is concluded that enzymatic reactions catalyzed by CA and plasmalemma ATPase are involved in adaptation of algae to low CO2 concentrations and in mechanisms of C(in) concentration. Mechanisms governing adaptation of algae to conditions of carbon dioxide limitation of photosynthesis are discussed in the paper.
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| 39. |
- PALMQVIST, K, et al.
(författare)
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PHOTOBIONT-RELATED DIFFERENCES IN CARBON ACQUISITION AMONG GREEN-ALGAL LICHENS
- 1994
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Ingår i: Planta. - 0032-0935 .- 1432-2048. ; 195:1, s. 70-79
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Tidskriftsartikel (refereegranskat)abstract
- The photosynthetic properties of a range of lichens (eight species) containing green algal primary photobionts of either the genus Coccomyxa, Dictyochloropsis or Trebouxia were examined with the aim of obtaining a better understanding for the different CO2 acquisition strategies of lichenized green algae. Fast transients of light/dark-dependent CO2 uptake and release were measured in order to screen for the presence or absence of a photosynthetic CO2-concentrating mechanism (CCM) within the photobiont. It was found that lichens with Trebouxia photobionts (four species) were able to accumulate a small pool of inorganic carbon (DIC; 70-140 nmol per mg chlorophyll (Chl)), in the light, which theoretically may result in, at least, a two- to threefold increase in the stromal CO2 concentration, as compared to that in equilibrium with ambient air. The other lichens (four species), which were tripartite associations between a fungus, a cyanobacterium (Nostoc) and a green alga (Coccomyxa or Dictyochloropsis) accumulated a much smaller pool of DIC (10-30 nmol.(mg Chl)(-1)). This pool is most probably associated with the previously documented CCM of Nostoc, inferred from the finding that free-living cells of Coccomyxa did not show any signs of DIC accumulation. In addition, the kinetics of fast CO2 exchange for free-living Nostoc were similar to those of intact tripartite lichens, especially in their responses to the CCM and the carbonic anhydrase (CA) inhibitor ethoxyzolamide. Trebouxia lichens had a higher photosynthetic capacity at low and limiting external CO2 concentrations, with an initial slope of the CO2-response curve of 2.6-3.9 mu mol.(mg Chl)-1.h(-1).Pa-1, compared to the tripartite lichens which had an initial slope of 0.5-1.1 mu mol.(mg Chl)(-1).h(-1).Pa-1, suggesting that the presence of a CCM in the photobiont affects the photosynthetic performance of the whole lichen. Regardless of these indications for the presence or absence of a CCM, ethoxyzolamide inhibited the steady-state rate of photosynthesis at low CO2 in all lichens, indicating a role of CA in the photosynthetic process within all of the photobionts. Measurements of CA activity in photobiont-enriched homogenates of the lichens showed that Coccomyxa had by far the highest activity, while the other photobionts displayed only traces or no activity at all. As the CCM is apparently absent in Coccomyxa, it is speculated that this alga compensates for this absence with high internal CA activity, which may function to reduce the CO2-diffusion resistance through the cell.
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| 40. |
- Palmqvist, K, et al.
(författare)
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Photosynthetic carbon acquisition in the lichen photobionts Caccomyxa and Trebouxia (Chlorophyta)
- 1997
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Ingår i: Physiologia Plantarum. - 0031-9317 .- 1399-3054. ; 101:1, s. 67-76
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Tidskriftsartikel (refereegranskat)abstract
- Processes involved in photosynthetic CO2 acquisition were characterised for the isolated lichen photobiont Trebouxia erici (Chlorophyta, Trebouxiophyceae) and compared with Coccomyxa (Chlorophyta), a lichen photobiont without a photosynthetic CO2-concentrating mechanism. Comparisons of ultrastructure and immune-gold labelling of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco; EC 4.1.1.39) showed that the chloroplast was larger in T. erici and that the majority of Rubisco was located in its centrally located pyrenoid. Coccomyxa had no pyrenoid and Rubisco was evenly distributed in its chloroplast. Both species preferred CO2 rather than HCO3- as an external substrate for photosynthesis, but T. erici was able to use CO2 concentrations below 10-12 mu M more efficiently than Coccomyxa. In T. erici, the lipid-insoluble carbonic anhydrase (CA; EC 4.2.1.1) inhibitor acetazolamide (AZA) inhibited photosynthesis at CO2 concentrations below 1 mu M, while the lipid-soluble CA inhibitor ethoxyzolamide (EZA) inhibited CO2-dependent O-2 evolution over the whole CO2 range. EZA inhibited photosynthesis also in Coccomyxa, but to a much lesser extent below 10-12 mu M CO2. The internal CA activity of Trebouxia, per unit chlorophyll (Chi), was ca 10% of that of Coccomyxa. Internal CA activity was also detected in homogenates from T. erici and two Trebouxia-lichens (Lasallia hispanica and Cladina rangiferina). In all three, the predominating CA had a-type characteristics and was significantly inhibited by low concentrations of AZA, having an I-50 below 10-20 ruM. In Coccomyxa a beta-type CA predominates, which is much less sensitive to AZA. Thus, the two photobionts differed in three major characteristics with respect to CO2 acquisition, the subcellular location of Rubisco, the relative requirement of CA and the biochemical characteristics of their predominating internal CA. These differences may be linked to the ability of Trebouxia to accumulate dissolved inorganic carbon internally, enhancing their CO2 use efficiency at and below air-equilibrium concentrations (10-12 mu M CO2) in comparison with Coccomyxa.
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