Cyanophage Host-Derived Genes Reflect Changes in Host Ecotype and Depth in the Water Column

Cyanobacteria and cyanophage are abundant in the oligotrophic ocean, but the controls on cyanophage populations are not well known. Cyanophage encode host-derived genes, which are thought to increase viral production. We previously examined cyanophage host genes in the oligotrophic Eastern Tropical North Pacific (ETNP), where a secondary chlorophyll maximum of Low Light IV Prochlorococcus is found in the oxygen minimum zone. The proportion of cyanophage with psbA, a viral host gene for a photosynthesis protein susceptible to light damage, decreased with depth but the proportion with viral host purine synthesis genes increased with depth. However, without similar data in oxic locations, we could not determine if viral host gene changes were due to changes in oxygen, depth, or cyanobacteria ecotype. Here we examine said genes in publicly available cellular metagenomes from 11 depth profiles along GEOTRACES transect GA03, which traversed the North Atlantic Ocean. In the Western Atlantic the deep mixed layer depths caused High Light II Prochlorococcus to dominate throughout the euphotic zone to the exclusion of other cyanobacteria. However, when the mixed layer shoaled Prochlorococcus ecotypes shifted to Low Light I and NC1 below the mixed layer. Cyanomyophage psbA phylogenetically closely related to High Light Prochlorococcus was the dominant cyanophage phylotype in the mixed layer. There was a clear change in both cyanomyophage cyanopodophage psbA phylotypes between the mixed layer and deeper waters. Below the mixed layer, the proportion of both myo and podo cyanophage with psbA, as determined from terL, a phage single copy core, decreased, but not as drastically as in the ETNP where Low Light IV was the dominant ecotype. Consistent with the ETNP, the proportion of cyanomyophage with purine synthesis gene purC increased below the mixed layer. We found that mixed layer depth was a critical variable for cyanobacterial and cyanophage distributions. This data indicates that cyanobacteria ecotypes and the depths where these ecotypes live affect the host gene composition of cyanophage.