Light-Dependent Transformation of Aniline to Indole Esters by the Purple Bacterium Rhodobacter sphaeroides OU5

Anoxygenic phototrophic purple non-sulfur bacteria: tool for bioremediation of hazardous environmental pollutants

The extraordinary metabolic flexibility of anoxygenic phototrophic purple non-sulfur bacteria (PNSB) has been exploited in the development of various biotechnological applications, such as wastewater treatment, biohydrogen production, improvement of soil fertility and plant growth, and recovery of high-value compounds. These versatile microorganisms can also be employed for the efficient bioremediation of hazardous inorganic and organic pollutants from contaminated environments. Certain members of PNSB, especially strains of Rhodobacter sphaeroides and Rhodopseudomonas palustris, exhibit efficient remediation of several toxic and carcinogenic heavy metals and metalloids, such as arsenic, cadmium, chromium, and lead. PNSB are also known to utilize diverse biomass-derived lignocellulosic organic compounds and xenobiotics. Although biodegradation of some substituted aromatic compounds by PNSB has been established, available information on the involvement of PNSB in the biodegradation of toxic organic pollutants is limited. In this review, we present advancements in the field of PNSB-based bioremediation of heavy metals and organic pollutants. Furthermore, we highlight that the potential role of PNSB as a promising bioremediation tool remains largely unexplored. Thus, this review emphasizes the necessity of investing extensive research efforts in the development of PNSB-based bioremediation technology.

Contamination of N-poor wastewater with emerging pollutants does not affect the performance of purple phototrophic bacteria and the subsequent resource recovery potential

Propagation of emerging pollutants (EPs) in wastewater treatment plants has become a warning sign, especially for novel resource-recovery concepts. The fate of EPs on purple phototrophic bacteria (PPB)-based systems has not yet been determined. This work analyzes the performance of a photo-anaerobic membrane bioreactor treating a low-N wastewater contaminated with 25 EPs. The chemical oxygen demand (COD), N and P removal efficiencies were stable (76 ± 8, 62 ± 15 and 36 ± 8 %, respectively) for EPs loading rate ranging from 50 to 200 ng L^-1 d^-1. The PPB community adapted to changes in both the EPs concentration and the organic loading rate (OLR) and maintained dominance with >85 % of total 16S gene copies. Indeed, an increment of the OLR caused an increase of the biomass growth and activity concomitantly with a higher EPs removal efficiency (30 ± 13 vs 54 ± 11 % removal for OLR of 307 ± 4 and 590 ± 8 mgCOD L^-1 d^-1, respectively). Biodegradation is the main mechanism of EPs removal due to low EPs accumulation on the biomass, the membrane or the reactor walls. Low EPs adsorption avoided biomass contamination, resulting in no effect on its biological methane potential. These results support the use of PPB technologies for resource recovery with low EPs contamination of the products.

Diverse aromatic-degrading bacteria present in a highly enriched autotrophic nitrifying sludge

Biotransformation of refractory organics by ammonia-oxidizing microorganisms in nitrifying sludge have been widely reported, while the contribution of heterotrophic bacteria in nitrifying sludge in the biotransformation and degradation process might be overlooked. Here, we provide metagenomic and metatranscriptomic evidences showing that heterotrophic bacteria in a highly enriched autotrophic nitrifying sludge could significantly contribute to the aromatic biotransformation and biodegradation. Diverse genes encoding enzymes for aromatic degradation were observed in an enriched autotrophic nitrifying sludge. These genes are involved in the degradation of at least 15 complex aromatics. Genome binning results showed that these genes were mainly carried by species in Bacteroidetes (Flavobacteriaceae and Sphingobacteriales), Alphaproteobacteria (Rhodobacter) and Betaproteobacteria (Bordetella, Acidovorax, Ramlibacter and Pusillimonas). According to the metatranscriptomic analysis, the overall expression of the potential aromatic-degrading genes was significantly upregulated, and almost all genes involved in phenol degradation were over expressed after the nitrifying sludge was exposed to phenol. Overall, our results suggest that certain heterotrophs in nitrifying sludge are involved aromatic biotransformation and biodegradation and advance our knowledge of the underlying properties and metabolic mechanisms of the nitrifying sludge.

Photobiological transformation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) using Rhodobacter sphaeroides

Pump-and-treat strategies for groundwater containing explosives may be necessary when the contaminated water approaches sensitive receptors. This project investigated bacterial photosynthesis as a strategy for ex situ treatment, using light as the primary energy source to facilitate RDX transformation. The objective was to characterize the ability of photosynthetic Rhodobacter sphaeroides (strain ATCC(®) 17023 ™) to transform the high-energy explosive RDX. R. sphaeroides transformed 30 μM RDX within 40 h under light conditions; RDX was not fully transformed in the dark (non-photosynthetic conditions), suggesting that photosynthetic electron transfer was the primary mechanism. Experiments with RDX demonstrated that succinate and malate were the most effective electron donors for photosynthesis, but glycerol was also utilized as a photosynthetic electron donor. RDX was transformed irrespective of the presence of carbon dioxide. The electron shuttling compound anthraquinone-2,6-disulfonate (AQDS) increased transformation kinetics in the absence of CO2, when the cells had excess NADPH that needed to be re-oxidized because there was limited CO2 for carbon fixation. When CO2 was added, the cells generated more biomass, and AQDS had no stimulatory effect. End products indicated that RDX carbon became CO2, biomass, and a soluble, uncharacterized aqueous metabolite, determined using (14)C-labeled RDX. These data are the first to suggest that photobiological explosives transformation is possible and will provide a framework for which phototrophy can be used in environmental restoration of explosives contaminated water.

Aniline is an inducer, and not a precursor, for indole derivatives in Rubrivivax benzoatilyticus JA2

Rubrivivax benzoatilyticus JA2 and other anoxygenic photosynthetic bacteria produce indole derivatives when exposed to aniline, a xenobiotic compound. Though this phenomenon has been reported previously, the role of aniline in the production of indoles is still a biochemical riddle. The present study aims at understanding the specific role of aniline (as precursor or stimulator) in the production of indoles and elucidating the biochemical pathway of indoles in aniline-exposed cells by using stable isotope approaches. Metabolic profiling revealed tryptophan accumulation only in aniline exposed cells along with indole 3-acetic acid (IAA) and indole 3-aldehyde (IAld), the two major catabolites of tryptophan. Deuterium labelled aniline feeding studies revealed that aniline is not a precursor of indoles in strain JA2. Further, production of indoles only in aniline-exposed cells suggests that aniline is an indoles stimulator. In addition, production of indoles depended on the presence of a carbon source, and production enhanced when carbon sources were added to the culture. Isotope labelled fumarate feeding identified, fumarate as the precursor of indole, indicating de novo synthesis of indoles. Glyphosate (shikimate pathway inhibitor) inhibited the indoles production, accumulation of tryptophan, IAA and IAld indicating that indoles synthesis in strain JA2 occurs via the de novo shikimate pathway. The up-regulation of anthranilate synthase gene and induction of anthranilate synthase activity correlated well with tryptophan production in strain JA2. Induction of tryptophan aminotransferase and tryptophan 2-monooxygenase activities corroborated well with IAA levels, suggesting that tryptophan catabolism occurs simultaneously in aniline exposed cells. Our study demonstrates that aniline (stress) stimulates tryptophan/indoles synthesis via the shikimate pathway by possibly modulating the metabolic pathway.

Aniline-induced tryptophan production and identification of indole derivatives from three purple bacteria

Growth on aniline by three purple non-sulfur bacteria (Rhodospirillum rubrum ATCC 11170, Rhodobacter sphaeroides DSM 158, and Rubrivivax benzoatiliticus JA2) as nitrogen, or carbon source could not be demonstrated. However in its presence, production of indole derivatives was observed with all the strains tested. At least 14 chromatographically (HPLC) distinct peaks were observed at the absorption maxima of 275-280 nm from aniline induced cultures. Five major indoles were identified based on HPLC and LC-MS/MS analysis. While tryptophan was the major common metabolite for all the three aniline induced cultures, production of indole-3-acetic acid was observed with Rvi. benzoatilyticus JA2 alone, while indole-3-aldehyde was identified from Rvi. benzoatilyticus JA2 and Rba. sphaeroides DSM 158. Indole-3-ethanol was identified only from Rsp. rubrum ATCC 1170 and anthranilic acid was identified from Rba. sphaeroides DSM 158.