Viruses in anaerobic digestion systems: Diversity, role and future prospects

Anaerobic digestion (AD) has received continuous attention over the past few decades as an efficient biological process for converting waste materials into biogas, biohydrogen, and volatile fatty acids (VFAs). Given the intricacy of the AD microbiome, it is important to explore the microorganisms carrying out the AD process, especially the viruses, which have received insufficient attention thus far. With the advent of novel molecular techniques and meta-omics approaches, it has become more accessible to ascertain the details of viral communities involved in AD systems. This is important as it could help us in monitoring and regulating the factors for improving AD process stability to achieve a selective bio-product like methane. This review highlights the distinctive viral communities in AD systems and their possible interactions with other microbial communities. It also highlights how these viral communities, particularly phages, are shaping the AD microbial communities. Furthermore, the role of auxiliary metabolic genes in AD viruses is discussed. Finally, the review provides insights into the effect of viruses on methanogenesis and highlights the challenges and future perspectives of studying viruses in AD.