Mitigation of the effect of high light on the photosynthetic apparatus of Rhodobacter alkalitolerans when grown in an alkaline environment

In the phototrophic alphaproteobacteria, photosynthesis is performed by pigment-protein complexes, including the light-harvesting complexes known as LH1 and LH2. The photosystem also encompasses carotenoids to assist in well-functioning of photosynthesis. Most photosynthetic bacteria are exposed to various abiotic stresses, and here, the Rhodobacter (R.) alkalitolerans were extracted from the alkaline pond. We report the comparative study of the photosynthetic apparatus of R. alkalitolerans in various light intensities in relation to this bacteriums high pH tolerance ability. We found that as the light intensity increased, the stability of photosystem complexes decreased in normal pH (npH pH 6.8{+/-}0.05) conditions, whereas in high pH (hpH pH 8.6{+/-}0.05) acclimation was observed to high light. The content of bacteriochlorophyll a, absorbance spectra, and circular dichroism data shows that the integrity of photosystem complexes is less affected in hpH compared to npH conditions. LP-BN of photosystem complexes also shows that LH2 is more affected in npH than hpH, whereas RC-LH1 monomer or dimer has shown interplay between monomer and dimer in hpH although the dimer and monomer both increased in npH. The sucrose density gradient of {beta}-DM solubilized intracytoplasmic membranes, further evidences the pattern of monomer-dimer conversion. Additionally, thin layer chromatographic separation of isolated membrane lipids shows that phosphatidylcholine (PC) levels have increased in hpH conditions which further confirms the integrity of photosystem complexes in hpH conditions. Moreover, qPCR data showed that the subunit -c of ATPase levels was overexpressed in hpH. Consequently, the P515 measurement shows that more ATP production is required in hpH, which dissipates the protons from the chromatophore lumen. This could be the reason the photosystem protein complex destabilized due to more lumen acidification. To maintain homeostasis in hpH, the antiporter NhaD expressed more than in the npH condition. IMPORTANCER. alkalitolerans is an alkaline tolerant species discovered from an alkaline pond in Gujrat India. Being a photoautotrophic photosynthetic organism, it serves as a good model organism to study the photosynthetic apparatus among phototrophic alphaproteobacteria. In nature organisms not only tackle a single abiotic stress but many including temperature, light, salinity, and many other abiotic stresses. Here we investigate how two different abiotic factors light and alkaline conditions modulate the growth and photosynthetic apparatus in a phototrophic alphaproteobacterium, R. alkalitolerans. Our results of this study will give leads in developing alkali-tolerant algae and higher plants.