## 1 Introduction  
Viruses are the most abundant entity in the marine environment. Their concentration can reach as high as 10\({}^{7}\) mL\({}^{-1}\) and they may outnumber bacteria by more two orders of magnitude [1]. The majority of oceanic viruses infect bacteria and are termed bacteriophages [2]. Bacteriophages (and viruses in general) have a considerable influence on the ecology and the biogeochemical cycles of the ocean [1, 3]. Viral-induced mortality can influence the flux of nutrients in microbial food-webs through the release of \"new\" dissolved organic matter (DOM) [4]. It is estimated that marine viruses promote the release of up to 145 gigatos of carbon per year [5] and therefore represent a major route of C-recycling. The DOM released via viral-induced mortality can either fuel microbial growth through the recycling of nutrients that are limited in the ocean [6, 7], or enhance the efficiency of the carbon pump [8] by enhancing the formation of fast sinking aggregates and colloidal particles [8; 9]. Moreover, viruses can promote genetic variation of their host species via horizontal gene transfer [10] and alter ecosystem productivity through the expression of specific viral-encoded auxiliary metabolic genes (AMGs) [11]. AMGs are widespread in bacteriophage and are believed to encode functions that sustain the host metabolism during infection. In cyanophages for example, the core photosystem II genes _psbA_ and _psbD_[12; 13] play a role in maintaining active photosynthetic electron transport, in order to accumulate energy for phage production. This is particularly important under high light conditions, where the surplus of energy from the upregulation of bacteriophage _psbA_ can support an increased translation rate and is responsible for a shortening of the lytic cycle [14]. Other examples of AMGs are those involved in carbon metabolism, like _cp12_ and _talC_[15; 16; 13] that inhibit the Calvin cycle and redirect carbon flux to the pentose phosphate pathway [17] to promote deoxynucleotide biosynthesis. Despite the importance of bacteriophage in the marine environment there is still a paucity of information on the diversity of viruses in the southern hemisphere. Here, we carried out the first study of the bacteriophage diversity in the oceanic region associated with the Pacific Ocean western boundary current (WBC), the east Australian current (EAC). The EAC is an important model system to investigate the biological impacts of climate change [18; 19; 20]. It supports the poleward transport of heat and nutrient poor waters into temperate latitudes [21; 22] and has played a significant role in shifting the biogeographic boundaries of regional ecological provinces in the Tasman Sea [23]. Previous studies have underlined how the EAC and Tasman Sea maintain distinct microbial assemblages associated with the different oceanographic characteristic of each system [24; 25]. While the balance between microbial dispersal and selection in this dynamic region is complex, we hypothesized that viral-mediated mortality is playing a pivotal role in community turnover, and hence may be a driver of the emergent patterns of microbial diversity across this region. However, there is no information about the identity and diversity of the main viral groups, and especially those infecting primary producers. This gap in knowledge limits our ability to fully understand the factors that govern microbial community structure, which influences present and future productivity in a region undergoing significant climate-related change [18; 19; 20; 21]. Here, we present the first study of the genetic diversity of viral communities in both the euphotic and aphotic zone of the EAC and the Tasman Sea, from samples that cover nine degrees of latitude (28\({}^{\circ}\)-37\({}^{\circ}\) S). Viral genome fragments were assembled from metagenomic contigs, derived from both the microbial (\(>\)0.2 \(\upmu\)m dia.) and the viral size fractions (\(<\)0.2 \(\upmu\)m dia.)