Effects of Acetazolamide (Diamox), Ethoxzolamide and High Levels of CO₂ on Carbonic Anhydrase, Photosystem Activity, and Oxygen Evolution in Euglena gracilis

Abstract

Investigations using steady‐state culture conditions indicate that carbonic anhydrase activity is correlated to the photosynthetic rate in Euglena in some but not all circumstances. When cultures grown with 5% CO2 were changed to air growth, the photosynthetic rate was independent of the carbonic anhydrase activity. While experiments using the inhibitor acetazolamide indicated a close correlation between photosynthetic capacity and carbonic anhydrase activity, the inhibitor was found to be nonspecific. Acetazolamide altered photosystem activities directly as measured by the photoreduction of DCPIP in chloroplast preparations, whole‐cell fluorescence transients of chlorophyll a, and by whole chain photoelectron flow. Ethoxzolamide, another inhibitor of carbonic anhydrase, was also found to inhibit photosystem activities, i.e., the photoreduction of DCPIP, and in vivo photoelectron flow, at high concentrations. Cells grown in 5% CO2 were less sensitive to the effects of acetazolamide than cells exposed to air. The rate of electron flow in chloroplasts from cells grown with 5% CO2 and exposed to 10 mM acetazolamide was 2.5‐fold faster than that of chloroplasts from air‐grown cells exposed to the same concentration of inhibitor. The whole cell chlorophyll a fluorescence transients of cultures grown with high CO2 were completely different from those of air‐grown cells and also showed fewer effects on exposure to acetazolamide. These results suggest a reevaluation of the hypothesis that carbonic anhydrase activity regulates photosynthesis. It is also apparent that results from air‐grown and 5% CO2‐grown cultures cannot be directly compared in such studies.

Regulation of the Photosynthesis Rhythm in Euglena gracilis

“…The fluorescence transients obtained have fast and slow components which are hypothesized to represent transitory stages from photosynthetic induction to steady-state photosynthesis (27). With Euglena, however, none of the transient components exhibited rhythmic behavior, and thusall of the traces of the transients were similar to those previously published (21) State I-State II Transitions. A measure of the efficiency of electron transport between PSII and PSI is the ratio of the low temperature fluorescence from the two photosystems, and the ability to poise the systems in one direction or another with preillumination (5,7,39).…”

Carbonic anhydrase in Ranunculus penicillatus spp. pseudofluitans: activity, location and implications for carbon assimilation

“…The inhibition of the photosynthetic rate of protoplasts by a constant 30% may be explained by the possibility that AZA is membrane permeable and inhibits intracellular CA (Williams & Turpin 1987), or photosynthetic electron flow (Lonergan & Sargent 1978). Neither of these two effects has been investigated in this study, and speculation as to the cause of the depression of photosynthetic rate cannot distinguish between these two possibilities.…”

The case for chloroplast thylakoid carbonic anhydrase

“…) In fact, by the eariy 1980s there was ample indirect evidence that PSn indeed possessed CA activity. For example, it had been shown by Swader and lacobson (1978) and by Lonergan and Sargent (1978) that the specific CA inhibitor acetazolamide, was also an effective inhibitor of PSn. Moreover, all of the classic inhibitors of CA were effective against PSII (Stemler andJursinic 1983, Stemler et al 1986).…”