Abstract
Aerobic anoxygenic phototrophs (AAPs) as being photoheterotrophs require organic substrates for growth and use light as a supplementary energy source under oxic conditions. We hypothesized that AAPs benefit from light particularly under carbon and electron donor limitation. The effect of light was determined in long-term starvation experiments with Dinoroseobacter shibae DFL 12T in both complex marine broth and defined minimal medium with succinate as the sole carbon source. The cells were starved over six months under three conditions: continuous darkness (DD), continuous light (LL), and light/dark cycle (LD, 12 h/12 h, 12 µmol photons m−2 s−1). LD starvation at low light intensity resulted in 10-fold higher total cell and viable counts, and higher bacteriochlorophyll a and polyhydroxyalkanoate contents. This coincided with better physiological fitness as determined by respiration rates, proton translocation and ATP concentrations. In contrast, LD starvation at high light intensity (>22 µmol photons m−2 s−1, LD conditions) resulted in decreasing cell survival rates but increasing carotenoid concentrations, indicating a photo-protective response. Cells grown in complex medium survived longer starvation (more than 20 weeks) than those grown in minimal medium. Our experiments show that D. shibae benefits from the light and dark cycle, particularly during starvation.
Highlights
Aerobic anoxygenic phototrophs (AAPs) are widespread in marine habitats [1,2,3] and occur in brackish [4,5] and fresh waters [6,7], saline lakes [8], soil [9], and hot springs [10]
Optimum Light Intensity for Starvation Experiments Cells pre-grown with succinate at different light intensities were harvested, washed, resuspended in medium without carbon source, and incubated under light and dark cycles (LD, 12 h/ 12 h) for four weeks
Bacteriochlorophyll a concentrations decreased, while carotenoid concentrations increased by a factor of 5 indicating their photo-protective function
Summary
Aerobic anoxygenic phototrophs (AAPs) are widespread in marine habitats [1,2,3] and occur in brackish [4,5] and fresh waters [6,7], saline lakes [8], soil [9], and hot springs [10]. It has been estimated that AAPs contribute up to 5% of the photosynthetic electron transport in oceanic surface waters [17]. Comparable to purple bacteria, AAPs are capable of lightdriven and respiratory electron transport for their energy metabolism. This activity is different in several aspects. AAPs use light energy under oxic conditions and contain much less bacteriochlorophyll a (BChl a), with sometimes higher carotenoid than BChl a concentrations [18]. Their photosynthetic activity is not used for CO2 fixation, but prevents the oxidation of organic substrates
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