Biotechnology of Microorganisms from Coal Environments: From Environmental Remediation to Energy Production

Abstract

It was generally believed that coal sources are not favorable as live-in habitats for microorganisms due to their recalcitrant chemical nature and negligible decomposition. However, accumulating evidence has revealed the presence of diverse microbial groups in coal environments and their significant metabolic role in coal biogeochemical dynamics and ecosystem functioning. The high oxygen content, organic fractions, and lignin-like structures of lower-rank coals may provide effective means for microbial attack, still representing a greatly unexplored frontier in microbiology. Coal degradation/conversion technology by native bacterial and fungal species has great potential in agricultural development, chemical industry production, and environmental rehabilitation. Furthermore, native microalgal species can offer a sustainable energy source and an excellent bioremediation strategy applicable to coal spill/seam waters. Additionally, the measures of the fate of the microbial community would serve as an indicator of restoration progress on post-coal-mining sites. This review puts forward a comprehensive vision of coal biodegradation and bioprocessing by microorganisms native to coal environments for determining their biotechnological potential and possible applications.

Keywords: bacteria; biodegradation; bioremediation; coal; fungi; humic substances; microalgae; microbial community; microorganisms.

Figure 1

Keyword co-occurrence network visualization map for publications involving “coal* OR low-rank coal* OR lignite*” and “microorganisms* OR microbes*”. Only publications from 1980 to June 2022 were considered. After exporting the publications from the Scopus platform in RIS form, they were analyzed using the keyword co-occurrence function of the VOSviewer software. Here, the top 30 items in terms of the number of occurrences are shown; different circles in the figure represent keywords, and their size indicates the number of times the keywords appear. The lines between the circles indicate that two keywords have appeared together in an article, and the more times they appear, the thicker the line is. On this basis, the main aspects of coal microbiology involve substrates (coal, lignite, low-rank coal, soil, flue gas, and heavy metals), microorganisms (bacteria, microalgae, fungi, and microbial diversity), processes (biodegradation, bioremediation, and biodesulfurization), environments (acid mine drainage, coal mining, wastewater, and sewage sludge), and products (methane, biofuel, and organic matter). By selecting the keyword “microalgae”, as an example, we could observe the connection between this and other keywords, such as coal, flue gas, carbon dioxide, biodiesel, biofuel, etc.

Figure 2

Number of yearly publications about coal microbiology.

Figure 3

Exploiting indigenous “microbial cocktails” to degrade/convert coal from coal-impacted sites may have multi-faceted advantages in agricultural productivity and environmental sustainability, i.e., the bioremediation of post-coal mining sites, production of humified organic substances, and development of plant growth-stimulating bacteria. More detailed information is provided in the following sections.

Figure 4

Native microalgae from the aquatic coal environment possess enormous biotechnological potential: they capture and sequester carbon to offset flue gas emissions from power plants, and the easily obtained microalgal biomass can be used for lipid/biofuel production. Coal mine drainage and power plant effluents can be decontaminated with microalgae. Finally, coal discards/fines can be burned with microalgae as a heat-efficient coal–microalgae composite. Microalgal bioremediation becomes even more attractive when the biomass cultivated in wastewater treatment systems is used as a feedstock.