Development of a saprophytic fungal inoculum for the biodegradation of sub-bituminous coal

Abstract

The rhizospheres of the weeds Ageratum conyzoides, Axonopus compressus, Emilia coccinea, Synedrella nodiflora, Urena lobata and Sida acuta from a sub-bituminous coal mining site and a control site, without coal discards, were screened for new fungi with ability to degrade sub-bituminous coal in the laboratory. The isolates were identified by cultural and molecular methods. Seventeen out of the sixty-one fungal isolates tested could utilize sub-bituminous coal as an energy source. Upon further evaluation, only seven of these were promising candidates for coal biodegradation, and they were assayed for their biosolubilization and depolymerization activities to determine their mechanisms of coal biodegradation. Based on the accumulation of humic acid (HA), which is the marker for biosolubilization, Mucor circinelloides and Aspergillus tubingensis were the most active. On the other hand, Cunninghamella bertholletiae, Simplicillium subtropicum, Penicillium daleae and Trichoderma koningiopsis were the highest producers of fulvic acid (FA), the indicator of depolymerization. Purpureocillium lilacinum produced the lowest yields of both HA and FA compared to the other six coal-degrading candidates. The presence of laccase in Trichoderma koningiopsis, Penicillium daleae and Simplicillium subtropicum suggests a role for this enzyme in the enhancement of the coal biodegradation process. However, the inability to amplify the laccase gene in Cunninghamella bertholletiae indicates that another enzyme probably aids its coal bioconversion. The current investigation highlights the potentials of these strains in harnessing biotechnological processes of sub-bituminous coal conversion into value-added products, which could be extended to the bioremediation of coal-polluted soils. The fungi with the highest coal bioconversion capabilities belonged to Ascomycota and Zygomycota and were found in the rhizospheres of the weeds Emilia coccinea, Ageratum conyzoides and Axonopus compressus.