Impact of light/dark regimen on growth rate, biomass formation and bacteriochlorophyll synthesis in Erythromicrobium hydrolyticum

Abstract

The impact of illumination on specific growth rate, biomass formation, and synthesis of photopigment was studied in Erythromicrobium hydrolyticum, an obligately aerobic heterotrophic bacterium having the ability to synthesize bacteriochlorophyll a. In dark-grown continuous cultures the concentration of protein increased with increasing dilution rate, the concentration of bacteriochlorophyll a showed the opposite effect. At a dilution rate of 0.08 h^-1 (68% of μ_max in the dark) and S_R-acetate of 11.8 mM, the concentration of BChla of illuminated cultures in steady-state was 11–22 nM, compared to 230–241 nM in cultures incubated in darkness. No significant differences were observed in the concentration of protein. A shift from darkness to light conditions resulted in increased specific growth rates resulting in increased biomass formation, thus showing that light enhances growth by serving as an additional energy source. This phenomenon, however, was temporary because bacteriochlorophyll synthesis is inhibited by light. In contrast to incubation in continuous light or dark, incubation under light/dark regimen resulted in permanently enhanced biomass formation. In the dark periods, bacteriochlorophyll was synthesized at elevated rates (compared to constant darkness), thus compensating the inhibitory effect of light in the preceding period. It thus appears that the organism is well-adpated to life in environments with alternating light/dark conditions. The ecological relevance of the observations is discussed.