From Lab to Farm: Elucidating the Beneficial Roles of Photosynthetic Bacteria in Sustainable Agriculture

Photosynthetic bacteria (PSB) possess versatile metabolic abilities and are widely applied in environmental bioremediation, bioenergy production and agriculture. In this review, we summarize examples of purple non-sulfur bacteria (PNSB) through biofertilization, biostimulation and biocontrol mechanisms to promote plant growth. They include improvement of nutrient acquisition, production of phytohormones, induction of immune system responses, interaction with resident microbial community. It has also been reported that PNSB can produce an endogenous 5-aminolevulinic acid (5-ALA) to alleviate abiotic stress in plants. Under biotic stress, these bacteria can trigger induced systemic resistance (ISR) of plants against pathogens. The nutrient elements in soil are significantly increased by PNSB inoculation, thus improving fertility. We share experiences of researching and developing an elite PNSB inoculant (Rhodopseudomonas palustris PS3), including strategies for screening and verifying beneficial bacteria as well as the establishment of optimal fermentation and formulation processes for commercialization. The effectiveness of PS3 inoculants for various crops under field conditions, including conventional and organic farming, is presented. We also discuss the underlying plant growth-promoting mechanisms of this bacterium from both microbial and plant viewpoints. This review improves our understanding of the application of PNSB in sustainable crop production and could inspire the development of diverse inoculants to overcome the changes in agricultural environments created by climate change.

Antibacterial and Toxicity Activities of Indonesian Herbal Medicine Extracts Used for Postpartum Treatment

Postpartum treatments have been used by local women in Indonesia for some time now. One commonly used postpartum treatment is the consumption of a traditional herbal medicine called PHM-1 during the first 40 days after childbirth. In addition, a second medicine known as PHM-2 is taken for days 41-80. However, sufficient scientific evidence about the benefits of this postpartum herbal medicine does not exist. In this study, the antibacterial abilities of PHM-1 and PHM-2 therefore was evaluated and showed positive potential for both. Low IC50 values were obtained against pathogenic bacteria such as Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Rhodopseudomonas palustris, Ralstonia pickettii, and Staphylococcus epidermidis and compared with chloramphenicol as a positive control. The antibacterial activity of both PHM-1 and PHM-2 against R. pickettii had the highest inhibitory activity as evidenced by the lowest IC50 values of 11.16 and 6.26 μg/ml, respectively. Furthermore, an in vivo acute toxicity test showed that PHM-1 produced a low LD50 value of 3.28 g/kg BW for both of male and female mice. These results support the use of both PHM-1 and PHM-2 as antibacterial treatments for postpartum women.

Investigating and modelling the effect of light intensity on Rhodopseudomonas palustris growth

Photosynthetic bacteria can be useful biotechnological tools – they produce a variety of valuable products, including high purity hydrogen, and can simultaneously treat recalcitrant wastewaters. However, while photobioreactors have been designed and modelled for photosynthetic algae and cyanobacteria, there has been less work on understanding the effect of light in photosynthetic bacterial fermentations. In order to design photobioreactors, and processes using these organisms, robust models of light penetration, utilisation and conversion are needed. This article uses experimental data from a tubular photobioreactor designed to focus in on light intensity effects, to model the effect of light intensity on the growth of Rhodopseudomonas palustris, a model photosynthetic bacterium. The work demonstrates that growth is controlled by light intensity, and that this organism does experience photoinhibition above 600 W/m2, which has implications for outdoor applications. Further, the work presents a model for light penetration in circular photobioreactors, which tends to be the most common geometry. The work extends the modelling tools for these organisms, and will allow for better photobioreactor design, and the integration of modelling tools in designing processes which use photosynthetic bacteria.

Iron oxidation is regulated by the two-component system, RegSR, and plays a role in photolithoheterotrophic growth in Rhodopseudomonas palustris

Purple nonsulfur bacteria (PNSB) are metabolically versatile organisms generate energy through both aerobic and anaerobic respiration as well as anoxygenic photosynthesis. In many PNSB, the redox-sensing, two-component system RegBA is a global regulator of energy generating and consuming pathways, such as photosynthesis, carbon fixation, and nitrogen fixation, when cells are shifted from an aerobic to an anaerobic environment. However, in the PNSB Rhodopseudomonas palustris, the role of the RegBA homolog, RegSR, was unclear since global regulation of these same pathways involves the oxygen-sensing signal transduction system, FixJL-K, in R. palustris. Using RNA-seq analysis, we found that RegSR plays a role in regulating the operon pioABC, which encodes genes required for Fe(II) oxidation. We found that transcript levels of pioABC under photoheterotrophic conditions was dependent on the oxidation state of the carbon substrate and whether the cells were fixing nitrogen. We also found that R. palustris can carry out photolithoheterotrophic growth using Fe(II) oxidation when grown with the oxidized carbon substrate, malate, requiring regSR and pioABC. We present a model in which RegSR regulates pioABC in response to a cellular redox signal, allowing R. palustris to use Fe(II) oxidation to access more electrons when there is an increased cellular demand for reducing equivalents.