Single-particle genomics uncovers abundant non-canonical marine viruses from nanolitre volumes

Viruses and other extracellular genetic elements play essential roles in marine communities. However, methods to capture their full diversity remain limited by the constraints of bulk sequencing assemblers or pre-sorting throughput. Here we introduce environmental micro-compartment genomics (EMCG), which vastly improves the throughput and efficiency of single-particle genomic sequencing obtained from nanolitre volumes by compartmentalizing particles of a sample into picolitre-sized, semi-permeable capsules for in-capsule DNA amplification and barcoding. From 300 nanolitres of seawater, EMCG obtained genomic sequences of 2,037 particles. The microbiome composition agreed with other methods, and the virus-like assembly lengths indicated that most were near complete. Many viral assemblies belonged to the Naomiviridae, lacked metagenomic representation and aligned to outlier contigs of abundant, putative host lineages, suggesting their use of non-canonical DNA and overlooked ecological importance. This approach provides opportunities for high-throughput, quantitative and cost-effective genome analyses of individual cells and extracellular particles across complex microbiomes.