Diurnal changes in the xanthophyll cycle pigments of freshwater algae correlate with the environmental XXXX concentration rather than non-photochemical quenching.

@entity5568 @entity7693 @entity26 @entity273 @entity505 @entity72131
BACKGROUND AND AIMS: In photosynthetic organisms exposure to high light induces the production of reactive @entity26 species (ROS), such as @entity273 ( @entity5568 ), which in part is prevented by non-photochemical quenching (NPQ). As one of the most stable and longest-lived ROS, @entity5568 is involved in key signalling pathways in development and stress responses, although in excess it can induce damage. A ubiquitous response to high light is the induction of the xanthophyll cycle, but its role in algae is unclear as it is not always associated with NPQ induction. The aim of this study was to reveal how diurnal changes in the level of @entity5568 are regulated in a freshwater algal community. METHODS: A natural freshwater community of algae in a temporary rainwater pool was studied, comprising photosynthetic Euglena species, benthic Navicula diatoms, Chlamydomonas and Chlorella species. Diurnal measurements were made of photosynthetic performance, concentrations of photosynthetic pigments and @entity5568 . The frequently studied model organisms Chlamydomonas and Chlorella species were isolated to study photosynthesis-related @entity5568 responses to high light. KEY RESULTS: NPQ was shown to prevent @entity5568 release in Chlamydomonas and Chlorella species under high light; in addition, dissolved organic @entity505 excited by UV-B radiation was probably responsible for a part of the @entity5568 produced in the water column. Concentrations of @entity5568 peaked at 2     m at midday and algae rapidly scavenged @entity5568 rather than releasing it. A vertical @entity5568 gradient was observed that was lowest next to diatom-rich benthic algal mats. The diurnal changes in photosynthetic pigments included the @entity7693 and @entity72131 cycles; the former was induced prior to the latter, but neither was strictly correlated with NPQ. CONCLUSIONS: The diurnal cycling of @entity5568 was apparently modulated by the organisms in this freshwater algal community. Although the community showed flexibility in its levels of NPQ, the diurnal changes in xanthophylls correlated with @entity5568 concentrations. Alternative NPQ mechanisms in algae involving proteins of the light-harvesting complex type and antioxidant protection of the thylakoid membrane by de-epoxidized carotenoids are discussed.
