Biotransformation of low rank coal by bacteria isolated from environments influenced by coal residue Titulo corto : Biotransformacion de carbon por bacterias Resumen Se describio la capacidad de cinco cepas bacterianas para transformar un carbon de bajo rango (CBR), para ello se evaluaron cepas aisladas de microhabitats con presencia de particulas procedentes de los procesos de almacenamiento y lavado de carbon en la mina El Cerrejon (Colombia). Se realizaron ensayos de solubilizacion de CBR en medio de cultivo solido y liquido, ademas de la decoloracion de sustancias humicas (SH) extraidas del CBR. Todas las bacterias evaluadas presentaron capacidad para biotransformar CBR en medio solido, esta actividad es mayor cuando el CBR ha sido pretratado con acido nitrico ; en medio liquido se alcanzo una perdida de peso de CBR hasta del 37% por accion de una cepa de Acinetobacter baumannii , acompanada de la aparicion de hasta 8,06 mg/ml -1 de sustancias solubles con absorbancia a 465 nm; los cambios en el pH del medio sugieren que la actividad biotransformadora de CBR esta asociada a la liberacion de sustancias alcalinas; finalmente, se encontro evidencia para sugerir que l as SH presentes en el CBR son transformadas por cometabolismo, posiblemente mediante reacciones de depolimerizacion, decoloracion y repolimerizacion. Palabras clave: solubilizacion de carbon, sustancias humicas, El Cerrejon, Acinetobacter baumannii . Abstract in this study were evaluated five bacterial strains isolated from microhabitats with high content of coal particles generated from storage and washing processes, in The Cerrejon” open coal mine ( Colombia), their ability to biotransform a low rank coal ( LRC) was described by testing solubilization on solid and liquid culture medium , as well as bleaching of humic substances ( HS) extracted from LRC . All tested bacteria showed ability to biotransform LRC on solid medium , this activity is greater when the LRC has been pretreated with nitric acid; in liquid medium LRC reached 37% of weight loss by Acinetobacter baumannii strain , accompanied by the appearance of up to 6.8 mg/ml -1 of soluble substances with absorbance at 465 nm , changes in culture medium pH suggest that LRC biotransformations activity is associated with alkaline substances release , finally found evidence to suggest that HS contained within LRC are transformed by cometabolism , possibly by depolymerization reactions , bleaching and repolimerization . Key words: Coal solubilization , humic substances , El Cerrejon, by Acinetobacter baumannii .