Abstract
The extremophilic red alga Galdieria partita is a facultative heterotroph that occupies mostly low-light microhabitats. However, the exceptional detection of abundant populations of G. partita in sunlight-exposed soil raises the possibility that exogenous organic carbon sources protect cells from photo-oxidative damage. The present study aimed to identify the photoprotective process activated by exogenous glucose under photo-oxidative stress. We demonstrated that exogenous glucose mitigated the photo-oxidative damage of cells exposed to 300 μmol photons m–2 s–1 photosynthetic active radiation. Photosynthesis carbon assimilation scarcely contributed to the cell growth in the presence of glucose, but the photosynthetic apparatus was nevertheless maintained and protected by glucose in a concentration-dependent manner. Supplementation of glucose increased expression of the L-gulonolactone oxidase gene essential for ascorbic acid biosynthesis, whereas no enhanced expression of the genes involved in carotenoid or tocopherol biosynthesis was observed. Under the photo-oxidative stress condition, the ascorbic acid content was strongly enhanced by exogenous glucose. We propose that the biosynthesis of ascorbic acid is one of the major photoprotective processes induced by exogenous glucose. The elucidation of how ascorbic acid is involved in scavenging reactive oxygen species provides key insights into the photoprotective mechanism in red algae.
Highlights
Cyanidiophyceae is a unique group of red algae, most members of which occupy hot sulfur springs and are well-adapted to low pH, high temperature, and high concentrations of heavy metals (Doemel and Brock, 1971; Gross et al, 1998; Gross and Oesterhelt, 1999)
To determine light sensitivity of the isolated G. partita strain, cells were grown under three continuous light conditions: darkness, low light (LL), and high light (HL; equivalent to 0, 20, and 300 μmol photons m−2 s−1 photosynthetic active radiation (PAR), respectively)
This study characterized the extremophilic red alga G. partita under light stress and demonstrated that the photosynthetic apparatus was protected by glucose supplementation even though the cell grew heterotrophically
Summary
Cyanidiophyceae is a unique group of red algae, most members of which occupy hot sulfur springs and are well-adapted to low pH (pH 0–5), high temperature (up to 63◦C), and high concentrations of heavy metals (Doemel and Brock, 1971; Gross et al, 1998; Gross and Oesterhelt, 1999). The cellular content of the photosynthetic apparatus was largely reduced, photosynthetic pigments, such as chlorophyll (Chl) a and carotenoids, were still present in the light (Gross and Schnarrenberger, 1995; Stadnichuk et al, 1998; Oesterhelt et al, 2007) Once these pigments absorb excessive light, excitation energy influx outpaces the downstream photosynthetic processes, leading to the accumulation of reactive oxygen species (ROS) and subsequently photo-oxidative damage (Apel and Hirt, 2004). How these facultative heterotrophic cyanidiophytes react to strong light and their underlying acclimation strategies have not yet been thoroughly investigated
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