## 3 Results
### Viral Similarity to Previously Published Databases  
Genes from the 1770 contigs were screened against the IMG/VR [36; 46] databases using BLASTn to assess the degree of similarity of the viral contigs with previously sequenced and assembled viral genomic fragments from other metavirome or metagenomic studies (Figure 2). Approximately half of the contigs shared little similarity (\(<\)60% of shared genes) to previously sequenced bacteriophage and could be considered as novel bacteriophage (Figure 2), suggesting that phage that inhabit the aphotic zone are highly underrepresented in current databases. However, a considerable number of the contigs analysed, 202 out of the 1770, were highly similar to previously sequenced bacteriophage (\(>\)80% ofshared genes) that had been found in different oceanic regions. These widely distributed phages are likely to play an important role in the microbial assemblages and marine ecosystems in general. Viral Diversity in the Different Oceanic Provinces, Depth Distribution and Comparison between Size Fractions  
The sites sampled spanned a wide latitudinal range and presented an extensive environmental variability (Supplementary Table S1). The transect South (in the Tasman Sea) was characterised by lower surface seawater temperature (15-17 \({}^{\circ}\)C) and higher nutrient concentrations compared to the EAC drift and the other transects. The EAC sampling sites displayed higher surface seawater temperatures (21-22 \({}^{\circ}\)C) and relatively low nutrient concentrations. The mid-latitude transect was sampled longitudinally from the coast, across the EAC, and then into offshore waters. Along this transect, the coastal site displayed high nutrients and surface seawater temperature between 19-20 \({}^{\circ}\)C, while samples in the EAC and from farther offshore were characterised by high surface seawater temperatures, but low nutrient concentrations. A similarity profile test (SIMPROF) identified seven significant clusters of samples based on the relative abundance of sequence reads mapping to distinct bacteriophage contigs (Figure 3a). Cluster analysis showed a clear separation between samples collected in the euphotic zone compared with those from the aphotic zone. Samples from the euphotic zone grouped in five different clusters. The clusters designated VS_TS includes both surface samples from the Tasman Sea and viriome samples. TS_2 instead grouped just sites from the Tasman Sea. Clusters EAC and EAC_2 and OS grouped site from either the EAC, S.N and Tr_M. The clustering results suggest a distinct partitioning of euphotic zone viral communities between the EAC and Tasman Sea (\(p<0.001\)). The deep samples fell into two clusters, corresponding to different depths: mesopelagic MP (depth 500 m) and bathypelagic BP (depth \(>500\) m). This neat separation of bacteriophage diversity in samples taken from deep vs. photic zone and Tasman Sea vs. EAC was also evident from the non-metric multidimensional scaling (nMDS) analysis (Figure 3b). Figure 2: Depth distribution of novel and known bacteriophage contigs. Similarity to previously sequenced viromes and metagenomes was assigned with a BLASTn analysis against the IMG/VR database (2018) using a similarity cut-off of \(>\)60% genes with \(>\)70% nucleotide identity. Similarity categories: N (novel: shared \(<20\)% genes), LS (low similarity: share \(>20\)% and \(<60\)% genes), S (similar: share \(>\)60% and \(<80\)% genes), K (known phage: share \(>80\)% of the genes). Contig abundance was normalized based on read counts. Figure 3: (**a**) Hierarchical clustering of sites based on the normalized abundance of large viral contigs (\(n=624\)). A SIMPROF analysis based on Bray–Curtis dissimilarity index was used to identify seven significant clusters. The relative abundance of viral contigs was normalized based on reads count and square root transformed. Sites are color-coded bases on significant clusters.