Metal contamination and methane oxidising bacteria around a formerly industrialised suburban river.

Heavy metal pollution is a very prominent problem persisting globally at the present time. Many technologies have been put forward to remediate such pollution. Bioremediation is a promising and eco-friendly tool to resolve environmental pollution. Microbes belonging to diverse phylogenetic groups have been investigated previously for remediation and immobilisation of pollutants. One among such groups are methanotrophs, which grow on methane as their sole source of carbon and energy and are able to remediate diverse hydrophobic organic pollutants due to the wide range of substrates utilised by their oxygenase enzymes. Recently, it was shown that the methanotroph Methylococcus capsulatus (Bath) was also able to remediate hexavalent chromium contamination. In the current study sediment samples were enriched from an urban river in Sheffield in Northern England that has previously suffered serious heavy metal pollution due to previous activity of the steel industry (17th to 19th century).The site selected for the present study was the River Sheaf located on south west of Sheffield in Northern England and approximately 3 miles from city centre, and which flows from Totley through Millhouses Park. Sediment samples were collected and characterised, according to various size fractions and then heavy metal analysis was carried out in the various fractions of the sediments. The most abundant heavy metals found at site were lead, chromium, nickel, arsenic and cobalt. The maximum concentrations of the heavy metals Pb, Cr, Ni, As and Co in the site were 412.80 mg/kg, 25.232 mg/kg, 25.196 mg/kg, 8.123 mg/kg and 7.66 mg/kg, respectively.Methanotrophs were enriched and isolated from the Sheaf sediments and then the isolated methanotrophs were investigated to determine their ability to reduce the hexavalent chromium. A strain of Methylomonas koyamae, which was given the strain designation SHU1, was isolated and found to remove hexavalent chromium across a range of concentrations in the range of 10-1000ppm after cultivation on methane as growth substrate. The Cr (VI) may be reduced to Cr (III) but the production of Cr (III) was not experimentally investigated and so there is the possibility that some or all of the chromium removal could be due to biosorption and uptake into the cells e.g. via the sulphate transport pathway. It was also found that the removal of Cr (VI) was inhibited by the addition of the metabolic inhibitor sodium azide, thus indicating that removal of chromium is largely a metabolic reaction mediated by enzymes rather than a passive biosorption process.It was speculated that methane monooxygenase (MMO) provides electrons from the oxidation of methane which may be used by other enzymes for removal (e.g. chemical reduction) of hexavalent chromium. Phenyl acetylene is a strong inhibitor of soluble MMO (sMMO) but inhibits particulate MMO (pMMO) less effectively. When cells expressing sMMO and pMMO were inhibited by phenyl acetylene the chromium removal reaction was completely inhibited compared in cells expressing sMMO. The above isolated organism produces a sMMO when there is copper deficiency in the media which is another distinguishing characteristic to potentially apply the organism in bioremediation of hydrophobic organic compounds, because sMMO generally has a wider substrate range than pMMO. The proteins encoded by available genome sequences of Methylomonas strains were compared with proteins from other microbes that are involved in chromium reduction, efflux systems and chromium uptake. Highly significantly similar proteins were found in the Methylomonas strains which resembled the proteins known to be involved in chromium removal, uptake and reduction. A number of strains of the Methylomonas genus are known to possess a gene for the sulphate transporter systems which could also play a major role in transportation of chromium (VI) into the cells. To the author's knowledge this is the first description of a strain of the widely environmentally distributed genus Methylomonas that is capable of remediating hexavalent chromium.