Photorespiration in Air and High CO(2)-Grown Chlorella pyrenoidosa

The Water to Water Cycles in Microalgae

In oxygenic photosynthesis, light produces ATP plus NADPH via linear electron transfer, i.e. the in-series activity of the two photosystems: PSI and PSII. This process, however, is thought not to be sufficient to provide enough ATP per NADPH for carbon assimilation in the Calvin-Benson-Bassham cycle. Thus, it is assumed that additional ATP can be generated by alternative electron pathways. These circuits produce an electrochemical proton gradient without NADPH synthesis, and, although they often represent a small proportion of the linear electron flow, they could have a huge importance in optimizing CO₂ assimilation. In Viridiplantae, there is a consensus that alternative electron flow comprises cyclic electron flow around PSI and the water to water cycles. The latter processes include photosynthetic O₂ reduction via the Mehler reaction at PSI, the plastoquinone terminal oxidase downstream of PSII, photorespiration (the oxygenase activity of Rubisco) and the export of reducing equivalents towards the mitochondrial oxidases, through the malate shuttle. In this review, we summarize current knowledge about the role of the water to water cycles in photosynthesis, with a special focus on their occurrence and physiological roles in microalgae.

Photorespiration and Internal CO(2) Accumulation in Chara corallina as Inferred from the Influence of DIC and O(2) on Photosynthesis

An O(2) electrode system with a specially designed chamber for ;whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O(2) concentrations on photosynthetic O(2) evolution. At pH = 5.5 and 20% O(2), cells grown in HCO(3) (-) medium (low CO(2), pH >/= 9.0) exhibited a higher affinity for external CO(2) (K((1/2))(CO(2)) = 40 +/- 6 micromolar) than the cells grown for at least 24 hours in high-CO(2) medium (pH = 6.5), (K((1/2))(CO(2)) = 94 +/- 16 micromolar). With O(2) </= 2% in contrast, both types of cells showed a high apparent affinity (K((1/2))(CO(2)) = 50 - 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO(2) medium (pH = 6.5). The high-pH, HCO(3) (-)-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO(2) which exceeded the CO(2) externally supplied, and the reverse situation has been observed in high-CO(2)-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO(3) (-) (K((1/2))[HCO(3) (-)]) was 0.6 +/- 0.2 millimolar, at 20% O(2). But under low O(2) concentrations (</=2%), surprisingly, an inhibition of net O(2) evolution was elicited, which was maximal at low HCO(3) (-) concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO(2) medium, (b) Chara cells are able to accumulate CO(2) internally by means of a process apparently independent of the plasmalemma HCO(3) (-) transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO(3) (-): pseudocyclic electron flow, sustained by O(2) photoreduction, may produce the additional ATP needed for the HCO(3) (-) transport.

Oxygen Uptake and Photosynthesis of the Red Macroalga, Chondrus crispus, in Seawater: Effects of Oxygen Concentration

With an experimental system developed for aquatic plants using the mass spectrometry technique and infrared gas analysis of CO(2), we studied the responses to various O(2) concentrations of gas exchanges with the red macroalga Chondrus crispus S. The results were as follows. (a) Irrespective of the CO(2) concentration, net photosynthesis was O(2) sensitive with a 45 to 70% stimulation at 2% O(2). Even with high CO(2), a significant Warburg effect was detected. (b) Although photosynthesis was CO(2) sensitive, O(2) photoconsumption was only weakly affected by CO(2) even at high CO(2) where it was still photodependent. (c) O(2) photoconsumption was always sensitive to O(2) concentration whatever the CO(2) concentration, but with O(2) exceeding 20% the kinetics disagreed with the Michaelis-Menten model, with saturation being reached more rapidly. With various CO(2) concentrations, the apparent K(m) (O(2)) ranged from 4 to 16% O(2) with a relatively constant V(max) (O(2)) of about one-third the V(max) (CO(2)). (d) Dark respiration seemed to be O(2) insensitive. These results are discussed in relation to the nature of the processes able to consume O(2) in the light, and seem to be consistent with a significant involvement of a Mehlertype reaction.

Ethoxyzolamide repression of the low photorespiration state in two submersed angiosperms

Net photosynthesis in the submersed angiosperms Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royal was inhibited by 21% O2, but the degree of inhibition was greater for plants in the high than in the low photorespiratory state. Increasing the CO2 concentration from 50 through 2,500 μl l(-1) decreased the O2 inhibition of the high-photorespiration plants in a competitive manner, but it had no effect on the O2 inhibition of plants in the low photorespiratory state. Carbonic-anhydrase activity increased by almost threefold with the induction of the low photorespiratory state. Ethoxyzolamide, an inhibitor of carbonic anhydrase, reduced the net photosynthesis of low-photorespiration Myriophyllum and Hydrilla plants by 40%, but their dark respiration was unaffected. This ethoxyzolamide inhibition of net photosynthesis exhibited a competitive response to CO2 concentration, resulting in a decrease in the apparent affinity of photosynthesis for CO2. The net photosynthesis of plants in the high photorespiratory state was inhibited only slightly by ethoxyzolamide, and this inhibition was independent of the CO2 level. Ethoxyzolamide treatment caused an increase in the O2 inhibition of net photosynthesis of plants in the low photorespiratory state. Ethoxyzolamide increased the low CO2 compensation points of low-photorespiration Myriophyllum and Hydrilla, but the values for the high-photorespiration plants were unchanged. In comparison, the CO2 compensation points of the terrestrial plants Sorghum bicolor (C4), Moricandia arvensis (C3-C4 intermediate) and Nicotiana tabacum (C3) were unaltered by ethoxyzolamide treatment. These data indicate that the low photorespiratory state in Myriophyllum and Hydrilla is repressed by ethoxyzolamide treatment, thus implicating carbonic anhydrase as a component of the photorespiration-reducing mechanism in these plants. The competitive interaction of CO2 with ethoxyzolamide provides evidence that the low photorespiratory state in submersed angiosperms is the result of some type or types of CO2 concentrating mechanism. In Myriophyllum it may be via bicarbonate utilization, but in Hydrilla it probably takes the form of an inducible C4-type system.

Adaptation of the Cyanobacterium Anabaena variabilis to Low CO(2) Concentration in Their Environment

The rate of adaptation of high CO(2) (5% v/v CO(2) in air)-grown Anabaena to a low level of CO(2) (0.05% v/v in air) was determined as a function of O(2) concentration. Exposure of cells to low (2.6%) O(2) concentration resulted in an extended lag in the adaptation to low CO(2) concentration. The rate of adaptation following the lag was not affected by the concentration of O(2). The length of the lag period is markedly affected by the O(2)/CO(2) concentration ratio, indicating that the signal for adaptation to low CO(2) may be related to the relative rate of ribulose-1,5-bisphosphate carboxylase/oxygenase activities, rather than to CO(2) concentration proper. This suggestion is supported by the observed accumulation of phosphoglycolate following transfer of cells from high to low CO(2) concentration.