[Light-dependent carotenoid Synthesis : IV. The role of oxygen in photoinduction]

The purpose of these studies was to investigate wether the primary step in the reaction chain of the light-dependent carotenoid synthesis in heterotrophic organisms, the photochemical reaction of the photoreceptor, requires oxygen. Submerged cultures of Fusarium aquaeductuum and Neurospora crassa were used.Biosynthesis of carotenoids following a photoinduction occurred only under aerobic conditions. When the mycelia were illuminated in the absence of oxygen and subsequently transferred to aerobic conditions in the dark, they produced carotenoids; however, the amount of pigments produced was lower in comparison to that synthesized after photoinduction in O2 atmosphere. Under anaerobic conditions light saturation of photoinduction occurred at a dosage of approximately 5.10(3) Lux. min (white light) in both organisms; this dosage was independent of light intensity and time of illumination. The photoeffect followed the reciprocity law even in the absence of oxygen. The amount of pigments synthesized following a saturating photoinduction under anaerobic conditions was 10% in Fusarium and 75% in Neurospora of that after photoinduction under aerobic conditions. When mycelia were illuminated in N2 atmosphere with saturating dosages, subsequently supplied with oxygen in the dark and then illuminated a second time with or without oxygen, an additional photoinduction took place. This result indicates that the photoreceptor can be reactivated by oxygen. This reactivation is completed in approximately 10 min; it is independent of temperature and does not occur in the absence of oxygen.From these results it is concluded that oxygen does not directly participate in the primary photochemical event but functions as an electron acceptor to keep the photoreceptor (possibly a flavin) in the proper oxidation state.