The raphidophyte Chattonella marina complex (hereafter Chattonella) consists of noxious red-tide-forming algae that are damaging to fish farms. Chattonella produces and secretes large amounts of the superoxide anion (•O2-), and the production of extracellular •O2- has been associated with fish mortality. We reported previously that photosynthetic electron transport is correlated with the production of •O2- in the genus Chattonella. However, the physiological roles of the production of extracellular •O2- remain to be clarified. In the present study, we examined the effects of the production of extracellular •O2- on photosynthesis and cell proliferation in two strains of Chattonella, namely, Ago03, a highly toxic strain that produces large amounts of •O2- externally, and Ago04, a low-toxicity strain that produces very small amounts of •O2-. Both the growth rate and the net photosynthetic activity of Ago04 were higher than those of Ago03. In Ago04, levels of Rubisco and 3-phosphoglycerate, the product of the reaction catalyzed by Rubisco, were 4-fold higher than those in Ago03, suggesting the higher photosynthetic activity of Ago04. In the presence of glycolaldehyde, a specific inhibitor of the Calvin-Benson cycle, the levels of NADP+ and the photosynthetic parameter qP declined under strong light in Ago04. By contrast, levels of NADP+ and qP in Ago03 changed less significantly than those in Ago04. Given that •O2- is produced by a putative NADPH oxidase that converts O2 to •O2- in Chattonella, it seems likely that the production of •O2- might play a role not only in the elimination of excess reducing power of NADPH from the cell, via •O2-, but also in the regeneration of NADP+, as a result of the action of NADPH oxidase, which oxidizes NADPH, to maintain photosynthetic electron transport.
Keywords: Calvin-benson cycle; Electron transfer; NADPH oxidase; Photosynthesis; Strong light tolerance; Superoxide.
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