The Challenges of Studying the Anaerobic Microbial World

Studies on strictly anaerobic microorganisms represent one of the most challenging areas of research, because anaerobic conditions (oxygen-free) need to be reconstructed to understand microbial activities and to obtain enrichments and pure culture. It is well-known that the micro/macro anaerobic environments are present everywhere on the Earth, and anaerobes comprise complex communities that play an important role in the carbon, nitrogen, and sulfur cycles on earth (49). Microbial community studies using 16S rRNA gene, as well as various functional genes, have offered new insights into anaerobic microbial ecology. Furthermore, numerous new lines of evidence offered by recent omics-driven and high-throughput sequencing studies provide a new vision of the anaerobic microbial world. In the current issue of Microbes and Environments, Cheng et al. (4) reported that different types of sulfate-reducing prokaryotes that can be grown under specific temperatures ranges were detected in sulfate-amended enrichment cultures of muddy fluids taken from a Taiwanese terrestrial hydro­ carbon seep, and that indigenous microbial communities might change based on the dynamic environmental fluctuations in volcanic mud ecosystems. Sulfate-reducing prokary-otes are frequently found in sulfate-supplied environment and are capable of growing on a variety of electron donors. In gas seeps and oil fields, the presence of various sulfate­reducing prokaryotes (9, 27, 30, 39) is associated with their potential to degrade anaerobic aromatic compounds and hydrocarbons. In fact, some sulfate-reducing bacteria are known to be decom-posers of these compounds (e.g., Desulfobacula toluolica, Desulfogloeba alkanexedens, Desulfosarcina sp.) (5, 11, 12, 48). In addition, a recent study reported that the hyperthermo-philic sulfate-reducing archaeon Archaeoglobus fulgidus oxidizes long-chain n­alkanes (24). Together with these findings, sulfate­reducing bacteria are also known to be an important microbial group as syntrophic partners in anaerobic ecosystems. Consortia of anaerobic methanotrophic archaea and sulfate-reducing bacteria contribute to the global methane consumption in methane-seeps (41). In addition, hydrogen and sulfur-compounds are syntrophically utilized by sulfate-reducing bacteria, sulfur-oxidizing bacteria, fermen-ters and anoxygenic photosynthetic bacteria in hot springs and hydrothermal fields showing the complexity and importance of synrtophic associations between organisms (10, 25, 36, 38). Methanogens play a key role in anaerobic ecosystems, and represent the most important member for the effective organic degradation and the recovery of methane as energy in anaerobic digesters treating various types of wastewater (3, 13, 34, 50). Due to their growth under very low redox conditions, their cultivation and physiological analyses require special laboratory techniques and apparatus (8, 22). Methanogens are phylogenetically widespread …