title: Diurnal changes in the xanthophyll cycle pigments of freshwater algae correlate with the environmental hydrogen peroxide concentration rather than non-photochemical quenching.
abstract: OBJECTIVE
  In photosynthetic organisms exposure to high light induces the production of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), which in part is prevented by non-photochemical quenching (NPQ). As one of the most stable and longest-lived ROS, H2O2 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 H2O2 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 H2O2. The frequently studied model organisms Chlamydomonas and Chlorella species were isolated to study photosynthesis-related H2O2 responses to high light.
  
  
  RESULTS
  NPQ was shown to prevent H2O2 release in Chlamydomonas and Chlorella species under high light; in addition, dissolved organic carbon excited by UV-B radiation was probably responsible for a part of the H2O2 produced in the water column. Concentrations of H2O2 peaked at 2 µm at midday and algae rapidly scavenged H2O2 rather than releasing it. A vertical H2O2 gradient was observed that was lowest next to diatom-rich benthic algal mats. The diurnal changes in photosynthetic pigments included the violaxanthin and diadinoxanthin cycles; the former was induced prior to the latter, but neither was strictly correlated with NPQ.
  
  
  CONCLUSIONS
  The diurnal cycling of H2O2 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 H2O2 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.
authors: Institute of Botany, Leopold-Franzens-Universität-Innsbruck, Sternwartestraße 15, 6020 Innsbruck, Austria thomas.roach@uibk.ac.at.
authors: Thomas
authors: T
authors: Roach
authors: Institute of Botany, Leopold-Franzens-Universität-Innsbruck, Sternwartestraße 15, 6020 Innsbruck, Austria.
authors: Ramona
authors: R
authors: Miller
authors: Institute of Botany, Leopold-Franzens-Universität-Innsbruck, Sternwartestraße 15, 6020 Innsbruck, Austria.
authors: Siegfried
authors: S
authors: Aigner
authors: Institute of Botany, Leopold-Franzens-Universität-Innsbruck, Sternwartestraße 15, 6020 Innsbruck, Austria.
authors: Ilse
authors: I
authors: Kranner
mesh_terms: D001317:Austria; D000460:Chlorophyta / Q000378:metabolism*; D002940:Circadian Rhythm; D017377:Diatoms / Q000378:metabolism*; D005055:Euglena / Q000378:metabolism*; D005618:Fresh Water; D006861:Hydrogen Peroxide; D058086:Microalgae / Q000378:metabolism; D010788:Photosynthesis; D024341:Xanthophylls / Q000378:metabolism*
chemical_list: D024341:Xanthophylls; D006861:Hydrogen Peroxide
keywords: Chlamydomonas; Chlorella; Euglena; NPQ; ROS; algae; antioxidant; diatom; diurnal; hydrogen peroxide; oxidative stress; reactive oxygen species; xanthophyll cycle