North Pacific metagenomes from Monterey Bay to Open Ocean (CalCOFI Line 67) October 2007
Phosphate (PO4) is an important limiting nutrient in
marine environments. Marine cyanobacteria scavenge
PO4 using the high-affinity periplasmic phosphate
binding protein PstS. The pstS gene has
recently been identified in genomes of cyanobacterial
viruses as well. Here, we analyse genes encoding
transporters in genomes from viruses that infect
eukaryotic phytoplankton. We identified inorganic
PO4 transporter-encoding genes from the PHO4
superfamily in several virus genomes, along with
other transporter-encoding genes. Homologues of
the viral pho4 genes were also identified in genome
sequences from the genera that these viruses infect.
Genome sequences were available from host genera
of all the phytoplankton viruses analysed except the
host genus Bathycoccus. Pho4 was recovered from
Bathycoccus by sequencing a targeted metagenome
from an uncultured Atlantic Ocean population. Phylogenetic
reconstruction showed that pho4 genes from
pelagophytes, haptophytes and infecting viruses
were more closely related to homologues in prasinophytes
than to those in what, at the species level, are
considered to be closer relatives (e.g. diatoms).
We also identified PHO4 superfamily members in
ocean metagenomes, including new metagenomes
from the Pacific Ocean. The environmental sequences
grouped with pelagophytes, haptophytes,
prasinophytes and viruses as well as bacteria. The
analyses suggest that multiple independent pho4
gene transfer events have occurred between marine
viruses and both eukaryotic and bacterial hosts.
Additionally, pho4 genes were identified in available
genomes from viruses that infect marine eukaryotes
but not those that infect terrestrial hosts. Commonalities
in marine host-virus gene exchanges indicate
that manipulation of host-PO4 uptake is an important
adaptation for viral proliferation in marine systems.
Our findings suggest that PO4-availability may not
serve as a simple bottom-up control of marine phytoplankton.