Jennifer R. BRUM, Ryan O. SCHENCK and Matthew B. SULLIVAN
The ISME Journal, 2 May 2013.
Viruses influence oceanic ecosystems by causing mortality of microorganisms, altering nutrient and
organic matter flux via lysis and auxiliary metabolic gene expression and changing the trajectory of
microbial evolution through horizontal gene transfer. Limited host range and differing genetic
potential of individual virus types mean that investigations into the types of viruses that exist in the
ocean and their spatial distribution throughout the world’s oceans are critical to understanding the
global impacts of marine viruses. Here we evaluate viral morphological characteristics (morphotype,
capsid diameter and tail length) using a quantitative transmission electron microscopy (qTEM)
method across six of the world’s oceans and seas sampled through the Tara Oceans Expedition.
Extensive experimental validation of the qTEM method shows that neither sample preservation nor
preparation significantly alters natural viral morphological characteristics. The global sampling
analysis demonstrated that morphological characteristics did not vary consistently with depth
(surface versus deep chlorophyll maximum waters) or oceanic region. Instead, temperature, salinity
and oxygen concentration, but not chlorophyll a concentration, were more explanatory in evaluating
differences in viral assemblage morphological characteristics. Surprisingly, given that the majority
of cultivated bacterial viruses are tailed, non-tailed viruses appear to numerically dominate the
upper oceans as they comprised 51–92% of the viral particles observed. Together, these results
document global marine viral morphological characteristics, show that their minimal variability is
more explained by environmental conditions than geography and suggest that non-tailed viruses
might represent the most ecologically important targets for future research.