title: The role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L.
abstract: This study investigates the role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L. Electron transport rates (ETRs) and non-photochemical quenching (NPQ) of Z. marina were measured under saturating irradiance in synthetic seawater containing 2.2 mM DIC and no DIC with different O2 levels (air-equilibrated levels, 3 % of air equilibrium and restored air-equilibrated levels). Lowering O2 did not affect ETR when DIC was provided, while it caused a decrease in ETR and an increase in NPQ in DIC-free media, indicating that O2 acted as an alternative electron acceptor under low DIC. The ETR and NPQ as a function of irradiance were subsequently assessed in synthetic seawater containing (1) 2.2 mM DIC, air-equilibrated O2; (2) saturating CO2, no O2; and (3) no DIC, air-equilibrated O2. These treatments were combined with glycolaldehyde pre-incubation. Glycolaldehyde caused a marked decrease in ETR in DIC-free medium, indicating significant electron flow supported by photorespiration. Combining glycolaldehyde with O2 depletion completely suppressed ETR suggesting the operation of the Mehler reaction, a possibility supported by the photosynthesis-dependent superoxide production. However, no notable effect of suppressing the Mehler reaction on NPQ was observed. It is concluded that during DIC-limiting conditions, such as those frequently occurring in the habitats of Z. marina, captured light energy exceeds what is utilised for the assimilation of available carbon, and photorespiration is a major alternative electron acceptor, while the contribution of the Mehler reaction is minor.
authors.affiliation: Department of Biology, Faculty of Science, Prince of Songkla University, Hat Yai, 90112, Songkhla, Thailand. pimchanok.b@psu.ac.th.
authors.forename: Pimchanok
authors.initials: P
authors.lastname: Buapet
authors.affiliation: Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, SE-106 91, Sweden.
authors.forename: Mats
authors.initials: M
authors.lastname: Björk
mesh_terms: D002244:Carbon / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism; D004579:Electron Transport; D005453:Fluorescence; D008027:Light; D010100:Oxygen / Q000378:metabolism*; D010788:Photosynthesis / Q000502:physiology* / Q000528:radiation effects; D012623:Seawater; D035962:Zosteraceae / Q000502:physiology* / Q000528:radiation effects
chemical_list: D002734:Chlorophyll; D002245:Carbon Dioxide; D002244:Carbon; D010100:Oxygen
keywords: Chlorophyll fluorescence; Mehler reaction; Photorespiration; Zostera marina