Rhodopseudomonas palustris and its role in bioremediation of soil pollutants

Introduction

Rhodopseudomonas palustris

Rhodopseudomonas palustris belongs to the group of phototrophic non-sulfur purple bacteria. It is a gram negative bacterium commonly found in soil and water. It can survive in both aerobic and anaerobic conditions. It is considered to be a “versatile bacteria” because it is able to fix carbon dioxide to form biomass, convert nitrogen to ammonia and produce hydrogen (hydrogen gas being the byproduct of nitrogen fixation). In the presence of oxygen it catabolizes a range of hydrocarbons from sugars to lignin; however it can switch to photosynthesis in the absence of oxygen. It is increasingly gaining research interest due to its abilities to produce hydrogen and degrade toxic waste. These two have very important applications in bioremediation and alternative energy. [1]

It is in the genes

The
R. palustris genome contains 5,459,213 base-pair circular chromosomes with 4,836 predicted genes and a plasmid of 8,427 bp. 1,514 genes, which is 31% of its genome, code for energy metabolism, biosynthesis, carbon & nitrogen metabolism and cellular processes.
R. palustris possess three nitrogenases, five benzene ring pathways and four light harvesting 2 systems which enable it to carry out its diverse metabolisms. It also has genes that enable it to oxidize hydrogen, thiosulfate and carbon monoxide and use them as energy sources.[2]

Rhodopseudomonas palustris genome Source:Nature

Degradation of Aromatic compounds

Hydrogen Production

R.palustris produces hydrogen as a byproduct of nitrogen fixation and it is possible to tap the hydrogen that is produced for human uses. Hydrogen is a far better fuel compared to coal or petroleum because the combustion of hydrogen only generates water.
R.palustris is unique compared to the other purple bacteria in that it codes for three different nitrogenases. It encodes a vanadium encoding nitrogenase that catalyzes the production of hydrogen three times as much as the common molybdenum-containing nitrogenases do. [1]
R.palustris has been coupled with other microbes such as cyanobacteria to form microreactors for hydrogen production. Cyanobacteria is also capable of nitrogen fixation. Cyanobacteria has three enzymes involved in hydrogen metabolism namely nitrogenase, membrane bound hydrogenase and soluble hydrogenase. Since the hydrogenases consume hydrogen in their course of action, mutants were produced by knocking off the gene encoding hydrogenase to maximize the production of hydrogen. The cyanobacteria mutants paired with
R.palustris had a 4.8 fold higher hydrogen production than the unmutated cyanobacteria/
R.palustris pair, 11.8mmol H2/h/mg protein vs. 3.9 mmol H2/h/mg protein. The reactions were monitored within reversed micelles made of N-ethyl hexyl sodium sulfosuccinate. [3]