Photosynthesis, photooxidation of chlorophyll and fluorescence of normal and manganese-deficient Chlorella with and without hydrogenase

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Abstract
Growth, chlorophyll content, chloroplast structure, photosynthesis, photooxidation of chlorophyll and fluorescence were studied in normal and manganese-deficient, closely related Chlorella strains with hydrogenase (Chlorella vulgaris f. tertia 211-8k) and without hydrogenase (Chlorella vulgaris 211-8m). Under Mn-deficient conditions, algae with hydrogenase grow slowly for several weeks without becoming chlorotic and without any major change in thylakoid structure, whereas those without hydrogenase show a rapid loss of most of their chlorophyll, followed by a breakdown of thylakoid structure. In algae without hydrogenase, the inhibition of photosynthesis by Mn deficiency is much less pronounced when photosynthesis is determined on a chlorophyll rather than on a cell-volume or dry-weight basis. Thus the little remaining chlorophyll of these algae is photosynthetically much more active than the chlorophyll of the Mn-deficient, non-chlorotic algae with hydrogenase. Photooxidation of chlorophyll under pure O2 in very strong light is always accelerated by Mn deficiency. Only in algae containing hydrogenase is there under aerobic conditions a pronounced peak of fluorescence at the beginning of illumination. This is indicative of a high degree of reduction of Q, the primary electron acceptor of System II of photosynthesis. The fluorescence peak can be eliminated by either 1 min preillumination with far red (λ=717 nm) or by treatment in the dark for 1 hr with pure O2. In algae without hydrogenase, in contrast, fluorescence intensity at the onset of illumination is lower than the steady-state level, indicating an oxidized state of Q. In these algae a high start of fluorescence can only be produced by prolonged anaerobic incubation. These results indicate that even under aerobic conditions algal hydrogenase, or a biochemical system very closely associated with it, has some residual activity which enables it to feed electrons into System II of photosynthesis, thereby keeping Q in a reduced state with correspondingly high fluorescence at the beginning of illumination. In organisms without hydrogenase, in contrast, this can only be achieved by prolonged anaerobiosis. In addition, the presence of hydrogenase seems to protect the chlorophyll against photooxidative destruction when the cells are under Mn deficiency.