Exploring Viral Communities Associated With Terrestrial Cyanobacteria Metagenomes

Viruses are exceedingly common, but little is known about how they behave in extreme environments, and whether viruses facilitate adaptation of their hosts to harsh conditions. To understanding of these understudied viral-host interactions, we analyzed metagenomes from 50 unialgal but nonaxenic Cyanobacteria cultures with 47 representing various terrestrial habitats, including desert soil and rock surfaces, tropical soil, and vernal pools. These cultures represent low diversity microbial consortia dominated by the terrestrial Cyanobacteria and its associated cyanosphere microbiome containing heterotrophic microbes. We identified viral sequences in metagenomes, grouped these into viral operational taxonomic units (vOTUs) and then placed vOTUs into viral clusters (VCs). We also calculated vOTU relative abundance and predicted possible bacterial hosts In total we predicted 814 viral sequences representing 726 vOTUs. We assigned putative taxonomy to 72 of the 814 putative viral sequences; these were distributed into 15 VCs — mostly assigned to the Caudovirales order of viruses. We found that metagenomes were dominated by unclassified and unclustered viral sequences. Furthermore, we predicted bacterial hosts for 211 vOTUs, with the majority of viruses predicted to infect a Proteobacteria host, and only 3 vOTUs predicted to infect a Cyanobacteria host. Overall, these results are consistent with the notion that viruses in extreme environments are underrepresented in reference datasets. This work increases knowledge of viral diversity and sets a foundation for future work on viruses associated with terrestrial Cyanobacteria and their heterotroph associates, such as connecting specific viruses to critical cycling processes and investigating their metabolic functions. Importance Relatively little is known about Cyanobacteria that live on land, or their associated viral communities. To begin to address this knowledge gap, we identified viruses from metagenomes of multi-species cultures which contain mainly Cyanobacteria, but also associated heterotrophic bacteria. The majority of surveyed viruses did not share similarity with reference viral genomes, potentially indicating some viral novelty. Further, we found that for the viruses where we could predict a putative bacterial host, most were not associated with the Cyanobacteria members in these cultures. These results highlight how little is known about land-based Cyanobacteria or their associated viruses. Given the role of some viruses in influencing ecosystem services and enhancing host fitness, surveying viruses is foundational to understanding how Cyanobacteria may have adapted to and survive in some of the most extreme environments on land.