Light-Driven Oxygen Consumption in the Water-Water Cycles and Photorespiration, and Light Stimulated Mitochondrial Respiration

Springer Nature
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Photosynthesis in Algae: Biochemical and Physiological Mechanisms, 2020, pp. 161-178
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Oxygen uptake in illuminated photosynthetic algae can be measured using 18O2 or (with some reservations) photosystem II electron flux measured by fluorescence corrected for electron flux to sinks other than O2. Water-water cycles, involve photosystem II removing electrons from water to produce O2. Water-water cycles involving photosystem I as well as photosystem II use the Mehler-peroxidase reaction, or flavodiiron protein, in the O2 reduction reactions, with pumping of 3 H+ per electron. The other known water-water cycle involves only photosystem II, with the plastid (plastoquinol) terminal oxidase transferring electrons to O2, pumping of 1 H+ per electron. Return flux of the pumped H+ through the CFOCF1 ATP synthase can supplement or replace ATP from cyclic electron flow and oxidative phosphorylation in equating the supply of NADPH and ATP to the metabolic need. A further light-dependent O2 process in algae is the energy-consuming process of photorespiration metabolising 2-phosphoglycolate from Rubisco oxygenase activity. The rate of photorespiration relative to that of photosynthesis depends on the CO2:O2 selectivity of Rubisco, and the steady-state CO2:O2 ratio at the site of Rubisco. In most algae a CO2 concentrating mechanism limits photorespiration by increasing the CO2:O2 ratio at the site of Rubisco, though, in cyanobacteria at least, some photorespiratory flux is essential. The minimum rate of mitochondrial respiration in the light is set by the requirement in biosynthesis of carbon skeletons produced only in the tricarboxylic acid cycle; higher rates may occur to supply ATP. There are problems in allocating O2 uptake in the light among the various pathways; the use of inhibitors, mutants or knockouts of a particular pathway is undermined by the possibility, or even likelihood, of compensatory reactions.
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