Soil viruses: Understudied agents of soil ecology.

Over the past couple decades there has been intense exploration of the soil microbiome using modern sequencing and bioinformatics approaches. These studies have revealed a vast diversity of microorganisms across a variety of habitats (Thompson et al., 2017) and how differences in the environment, for example with climate change, influence the soil microbial community composition (Jansson and Hofmockel, 2020). The focus of these studies has primarily been on microbial cellular organisms, including bacteria, archaea and fungi. By contrast, in depth study of soil viruses has been largely neglected, until recently. There are several reasons that soil viruses are less studied, including difficulties with extraction of viruses from soil particles, difficulties with classification of soil viruses based on microscopic imaging, and reliance on the small proportion of microbial hosts that can be cultivated. Now that comprehensive soil metagenomes are available the current trend has been to survey the metagenomes for viral sequences. This approach has revealed that soil viruses are incredibly abundant, highly diverse, and largely uncharacterized (Paez-Espino et al., 2016). This so-called 'viral dark matter' presents an incredible research opportunity for the future. Here I will highlight some of these potential research avenues based on what is known to date.