Selective repression of light harvesting complex 2 formation in Rhodobacter azotoformans by light under semiaerobic conditions

Abstract

Photosystem formation in anaerobic anoxygenic phototrophic bacteria (APB) is repressed by oxygen but is de-repressed when oxygen tension decreases. Under semiaerobic conditions, the synthesis of photopigments and pigment protein complexes in Rhodobacter (Rba.) sphaeroides are repressed by light. AppA, a blue-light receptor, mediates this regulation. In the present study, it was showed that the synthesis of bacteriochlorophyll, carotenoid, and pigment protein complexes in Rba. azotoformans 134K20 was significantly repressed by oxygen. Oxygen exposure also led to a conversion of spheroidene to spheroidenone. In semiaerobically growing cells, light irradiation resulted in a decrease in the formation of photosystem, and blue light was found to be the most effective light source. Blue light reduced the contents of bacteriochlorophyll and carotenoid slightly, but had negligible effects on light harvesting complex (LH) 1 content, whereas the content of LH2 was significantly decreased indicating that blue light selectively repressed the synthesis of LH2 in semiaerobically growing 134K20. It was concluded that, similar to Rba. sphaeroides, a blue light receptor presented in strain 134K20 played important roles in its light-dependent repression. A possible mechanism involved in controlling the differential inhibitory of blue light on the synthesis of photosystem was discussed.

Purple bacteria screening for photoautohydrogenotrophic food production: Are new H2-fed isolates faster and nutritionally better than photoheterotrophically obtained reference species?

“…Finally, PB are mostly known for their metabolic versatility, yet the effect of switching between metabolic conditions on the performance remains a gap in literature. Thus far, metabolic switching has been focused on heterotrophic conditions in which switches between the energy source were made (photo-and chemotrophic) to study the gene regulation of the photosystem [22,23] or the possible reduction in production cost [24]. The effect of switching between autotrophic and heterotrophic conditions (and thereby also switching electron source) on the performance of PB, while remaining phototrophic, has not been studied previously.…”

Purple phototrophic bacteria are outcompeted by aerobic heterotrophs in the presence of oxygen

“…It has been hypothesized that repressing gene expression is a protection mechanism of the cells to avoid photooxidative damage (Zhu et al, 1986). This suppression results in the inhibition of photoheterotrophy and the shift towards chemoheterotrophy, which results in reduced biomass yields and COD:N:P substrate ratios (Izu et al, 2001;Jiao et al, 2003;Siefert et al, 1978;Yue et al, 2015). Both quantitative studies and theoretical analyses have confirmed this phenomenon under micro-aerobic conditions, with oxidative phosphorylation, rather than cyclic photophosphorylation, being the predominant ATP-production route in PPB under these conditions (Lu et al, 2011a(Lu et al, , 2011b.…”