Biotransformation of coal by ligninolytic Streptomyces

Abstract

The chemistry of coal liquefaction by Streptomyces viridosporus, Streptomyces setonii and Streptomyces badius was studied. These ligninolytic actinomycetes solubilized lignite and bituminous coals when coal particles were placed on the surfaces of mycelial lawns on agar culture media. On agar, liquefaction proceeded with the formation of clear to black water-soluble products. Up to 40% of some coals was solubilized in 3 weeks. Lignite coals were also solubilized in liquid shake flask cultures. Coal solubilization by S. viridosporus was oxidative, as shown by Fourier transform infrared spectroscopy and elemental analysis. Compared to the starting coals, the soluble products were enriched in oxygen, nitrogen, sulfur, and ash, but were lower in hydrogen relative to carbon. Insoluble coal residues were less altered than the liquefied products, but they were higher in ash content than the starting coals. Chemical analysis of culture filtrates showed that some low molecular weight products were released, but most of the soluble products were acid precipitable and probably polymeric. Lignite coal solubilization by the S. viridosporus did not require induction and in liquid media solubilization was always accompanied by a significant increase in medium pH. When medium pH was not allowed to become basic, extracellular coal solubilizing activity was negligible. Extracellular solubilizing activity was heat stable, and ultrafiltration experiments showed that the solubilizing agent was of less than 10 000 molecular weight. Coal solubilizing activity of S. viridosporus culture supernatants could be eliminated by lowering their pH, but the activity could be recovered by raising the pH back up to above neutral. Phosphate and Tris buffers of varying strength and pH showed that nonbiological solubilization of lignite coal occurred under basic conditions. The combined data show that biological lignite coal solubilization by these Streptomyces is mediated by alkaline products that the cells release into the growth medium. These findings confirm and expand an earlier report by others that coal solubilization by Streptomyces setonii may be nonenzymatic.