Effects of light/dark diel cycles on the photoorganoheterotrophic metabolism of Rhodopseudomonas palustris for differential electron allocation to PHAs and H2

Abstract

Light/dark cycles can impact the electron distribution in Rhodopseudomonas palustris, a hyperversatile photoorganoheterotrophic purple non-sulfur bacterium (PNSB). Dynamic conditions during diel cycles are important for the physiology of PNSB, but the coupling between illumination patterns and redox balancing has not been extensively studied. For survival and growth, Rhodopseudomonas has developed different mechanisms to allocate electrons under dynamic growth conditions. Products such as hydrogen and poly-{beta}-hydroxyalkanoates (PHAs) can form alternative electron sinks. A continuous culture, fed with a balanced nutrients medium, was exposed to three different conditions: 24 h continuous infrared illumination, 16h light/8h dark, and 8h light/16h dark. Light and dark phase durations in a cycle determined the energy availability level (light) and the attainment of a stationary state. Under long dark phases, the acetate substrate accumulated to levels that could not be depleted by growth in the light. Under short dark phases, acetate was rapidly consumed in the light with most of the phototrophic growth occurring under acetate-limiting conditions. Under diel cycles, substrate uptake and growth were unbalanced and Rhodopseudomonas shunted the excess of carbon and electron flow first toward PHAs production. Only secondarily, when PHA storage got saturated, the electron excess was redirected toward H2. A numerical model described well the dynamics of biomass and nutrients during the different light/dark cycle regimes. The model simulations allowed determination of stoichiometric and kinetic parameters for conversion by Rhodopseudomonas. Understanding the inherent process dynamics of diel light cycles in purple sulfur bacteria cultures would enable optimization procedures for targeted bioproduct formation. ImportancePurple non-sulfur bacteria (PNSB) are important anoxygenic phototrophic microorganisms that take part in numerous environmental processes, based on their metabolic versatility. Rhodopseudomonas palustris is a model photosynthetic bacterium of the PNSB guild. Light cycles influence deeply its physiology. Poly-{beta}-hydroxyalkanoates (PHAs) and biohydrogen are two of the most studied metabolic products of Rhodopseudomonas, because of their biotechnology potential besides involvement in carbon and electron allocations in its metabolism. Their production mechanisms have often been described as competitive, but the rationale behind the production of one or the other compound has not been elucidated. Here, we found that under light / dark cycles an excess of organic substrate was first directed toward PHAs production, and only when this pathway was saturated H2 was produced. Understanding the dynamics of carbon and electron allocation under intermittent light cycles enhances our knowledge on PNSB metabolisms and paves ways to manage the formation of targeted bioproducts.