Chlorophyll Photobleaching and Ethane Production in Dichlorophenyldimethylurea-(DCMU) or Paraquat-Treated Euglena gracilis Cells

Light dependent (35 Klux) chlorophyll bleaching in autotrophically grown Euglena gracilis cells at slightly acidic pH (6.5-5.4) is stimulated by the photosystem II blockers DCMU and DBMIB (both 10^–5 M) as well as by the autooxidizable photosystem I electron acceptor, paraquat (10^–3 M). Chlorophyll photobleaching is accompanied by the formation of thiobarbituric acid - sensitive material (“malondialdehyde”) and ethane. Both chlorophyll photobleaching and light dependent ethane formation are partially prevented by higher concentrations (10^–4 M) of the autooxidizable photosystem II electron acceptor DBMIB or by sodium bicarbonate (25 mM). In vitro studies with cell free extracts (homogenates) from E. gracilis suggest that α-linolenic acid oxidation by excited (reaction center II) chlorophyll represents the driving force for both ethane formation and chlorophyll bleaching. Ethane formation thus appears to be a sensitive and non-destructive “in vivo” marker for both restricted energy dissipation in photosystem II and, conditions yielding reactive oxygen species at the reducing side of photosystem I.