Commercial applications of Rhodopseudomonas palustris

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Introduction

Figure 1. Rhodopseudomonas palustris photographed by Caroline Harwood [1].

Rhodopseudomonas palustris is an Alphaproteobacteria under the genus Rhodopseudomonas. R. palustris is a non-sulfur purple bacterium that is gram-negative with a rod shape. Cells are motile and the organism reproduces by means of budding. R. palustris is found in both aerobic and anaerobic environments including a wide variety of marine and soil ecosystems; coastal marine sediment and waste water treatment facilities for example. Typically non-sulfur purple bacteria are phototrophic, however R. palustris has the capability to switch between different forms of metabolism depending on environmental conditions: photoautotrophy, photoheterotrophy, chemoautotrophy, and chemoheterotrophy.

Rhodopseudomonas palustris is the focus of extensive research because it has a multitude of potential metabolic processes. Research focusing on bioremediation and removal of waste is interested in R. palustris for its potential to break down aromatic compounds and waste in polluted environments. Rhodopseudomonas also has the capability to produce hydrogen as a product of nitrogen fixation, which researchers are looking into as a form of alternate energy produced from biofuel. R. palustris has also displayed the capability to generate electricity further increasing the scope of research surrounding the microbe. Climate change induced by global warming and pollution of environments are some of the largest environmental problems facing the modern world. R. palustris is interesting in that it could hold the potential to solving not just one, but both of these serious environmental issues. The extensive research on this microbe presents a possibility for unique commercial applications of the microbe as a source of alternate energy and in waste management and removal.

Metabolism

Figure 2.Metabolic pathways of Rhodopseudomonas palustris. Image credit Larimer, F. et al. [2].

R. palustris has the capability to metabolize under aerobic and anaerobic conditions, meaning the organism can produce energy with or without oxygen present. Anaerobically, R. palustris can utilize photoheterotrophy and photoautotrophy, or photosynthesis . The cell has the capability to fix carbon dioxide as biomass through photosynthesis, similar to green plants, but also R. palustris has the capability to obtain energy from light and other organic carbon forms via photoheterotrophy . Under photoheterotrophic conditions, R. palustris utilizes organic compounds, most notably lignin monomers as a source of carbon . When the cell is in the presence of conditions conducive to phototrophy, lamellar intracytoplasmic membranes hold bacteriochlorophylls and carotenoids, the photosynthetic pigments of the cell that make up the light harvesting complex, next to the cell membrane, which allows sunlight to be absorbed by the photosynthetic pigments . The light energy absorbed is used to move electrons eventually resulting in the production of adenosine triphosphate (ATP). ATP is used as energy within the cell while the fixation of carbon dioxide or other organic compounds allows for the production of biomass and new cell material. Research has found that, R. palustris is able to respond to varying levels of light in its environment; in environments with lower light intensity the cell increases the amount of light harvesting complexes present to more effectively capture light energy . Like photosynthetic plants, R. palustris possesses genes, which encode for RuBisCo, the enzyme responsible for the fixation of carbon dioxide in plants . RuBisCo is used for the same fixation function in R. palustris photosynthesis, however unlike in plant photosynthesis oxygen is not a product of photosynthesis, as water is not split in the reaction and R. palustris utilizes thiosulfate, hydrogen and other inorganic electron donors .

Biodegredation

Figure 3. Formation and reductive dechlorination of 3-chlorobenzoyl-CoA into the product benezoyl-CoA. A process of R. palustris proposed by Egland, P. et al. [3].

Include some current research, with at least one figure showing data.

Electricity Generation

Include some current research, with at least one figure showing data.

Hydrogen Production

Figure 4. Effects of various nitrogen sources on H2 production, cell growth and acetate consumption. Symbols: No added nitrogen source (open circle), 5 mM glutamate (open square), 5mM NH4Cl (open triangle), and 3 g yeast extract (dark circle). Oh, Y. et al. [4].

References

Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2016, Kenyon College.