## 4 Discussion
### Phage Adapt Their Genomic Content Based on Host and Environmental Conditions  
In contrast to deep ecosystems, viral diversity, both at the taxonomic and functional levels in the euphotic zone, presented a marked regional distribution pattern (Figure 6). Due to the reliance of bacteriophage on their hosts for replication, their abundance is usually tightly coupled to the presence and regional distribution of their specific hosts. Moreover, bacteriophage have been strongly correlated with the succession of their specific host ecotypes [57]. Previous work [25] and concurrent genomic (Figure 5) data showed that cyanobacteria are highly abundant in the EAC; not surprisingly phage thought to infect cyanobacteria predominated in the warm water of the samples collected in the EAC (EAC, EAC_2). Here, the cyanophage possess auxiliary metabolic genes to exploit the cyanobacterial photosynthetic apparatus to boost their production. Putative cyanophage contigs from the EAC were indeed enriched in Pfams involved in photosynthesis, including core photosystem components (psbA, psbD), and phycobiliprotein Lyase (Supplementary Table S2). While _psbA_ genes are not essential for phage, they confer a selective advantage in high-light conditions where light damaged cyanobacterial photosystem II machinery can be replaced with functional copies derived from phage genes [13]. T-type phycobiliprotein lyase could provide an advantage for a fast phycobiliprotein assembly during phage infection [58]. Nutrient availability (nitrogen, phosphate, iron) is among the main factors limiting the growth of phytoplankton [59]. Western boundary currents, such as the EAC, are known to be relatively poor in nutrients. Indeed, in our dataset, samples from the EAC were characterised by lower concentrations of nitrogen and phosphorus, compared to those from the Tasman Sea. Genes that encode proteins related to nutrient stress in the cell (2 OG-Fe, MazG, PhoH) and nutrient scavenging (phosphate-binding protein, putative ABC transporter) were highly enriched in the EAC viral contigs. MazG, a nucleoside triphosphate pyrophosphohydrolase, has a role in the hydrolysis of ppGpp (Guanosine pentaphosphate). The cellular concentration of ppGpp usually increases when the cell is in a starvation state. By removing ppGpp, the phage encoded MazG could play a role in \"reactivating\" host metabolism by simulating levels that would normally occur in a nutrient-replete environment, and thus eventually facilitating virus production [60; 61]. Finally, _talC_, a gene that encodes an enzyme responsible for the diversion of carbon from the Calvin cycle to the PP pathway [17], was also significantly abundant in the EAC and offshore samples from the north and middle transect. Transaldolases are commonly encoded in cyanophage genomes [17] and the expression of the phage encoded _talC_ supposedly leads to an increase in energy available for phage production. Altogether, these results highlight how bacteriophage in the EAC may exploit photosynthetic bacterial communities, by modulating their photosynthetic apparatus to cope with higher light intensity, but also expressing carbon cycle genes that divert energy and carbon towards phage production and nutrient scavenging genes to boost the capture of limiting nutrients [17; 42]. While the EAC is usually dominated by prokaryotic primary producers, and hence the phage that infect them, the colder and more mesotrophic Tasman Sea is enriched with eukaryotic primary producers instead of cyanobacteria [24; 25; 62]. Taxonomic classification of the microbial metagenome showed an increase in the Rhodobacteraceae family along the Tasman Sea; _Roseobacter_ belongs to this family, and this is consistent with our observed decrease in cyanophage and relative increase in Cellulophaga and Roseobacter phages, but also Puneceispirillum phages.