Tailed or not tailed: A paradigm shift in marine viral dominance

The last two decades have revealed a wealth of knowledge to the importance of viruses in the marine environment. Viral lysis is responsible for the structuring and composition of microbial communities and alters the fluctuation of nutrients and organic matter via increased recycling through the microbial loop. They also greatly manipulate the evolution of microorganisms through horizontal gene transfer. However, relatively little is still known about the marine viral world including how viral assemblages vary across oceanic regions. Due to the significance of viruses, it is important that their diversity across marine habitats is understood. This will allow us to better understand their role in ocean processes such as the microbial loop and how they interact with their associated host cells.

Brum et al. (2013) set out to answer the question of global marine virus diversity. They used morphological analyses which are not plagued by metagenomic database issues of previous viral research. The novel use of an air-driven ultracentrifuge with a rotor, designed to quantitatively deposit viruses onto transmission electron microscopy (TEM) grids, resulted in quantitative TEM (qTEM) to analyse samples. The highly contextualised samples were chosen from 14 stations in six global ocean regions collected on the Tara Oceans Expedition. Images of 100 viruses per treatment were analysed to compare morphotype composition and capsid diameter distributions between treatments.

Surprisingly, analyses showed there was little evidence for consistent variation with depth or oceanic region. The proportion of observed morphotypes was highly similar in each oceanographic region. This shows an underlying controlling factor present in all oceans. Salinity, along with temperature and oxygen, are thought to be the main candidates for variation in marine viral assemblages, this is especially present at extremes. This was nevertheless not the most shocking revelation. The majority of cultivated marine bacterial viruses are tailed, however, it would appear from this study that non-tailed viruses dominate numerically in the region of 51-92%. This is a significant paradigm shift of marine viral dominance and shows the ecology of marine viruses is still poorly understood. This study does pave the way for further research and using new approaches such as phageFISH, knowledge of viral diversity, function and importance will surely grow.

This paper shows how morphological analysis is still relevant in a post “omic” world and can be used to great effect, however the limitations are clearly apparent. The analysis of each viral particle separately is time consuming. Although Brum et al. are able to give us a general overview of global virus variation and morphology, they are unable to supply any detailed information on diversity. By the author’s own admission a 5-100 fold increase of viruses per sample is needed to investigate the possible presence of other morphotypes and the variability in subgroups. In addition, I believe a larger number of sample stations are required to better understand what does cause variability in virus morphotypes and to be more confident about findings in order to limit the effect of variability within an ocean ecosystem. This will obviously increase time and costs exponentially, therefore, the combination of morphological analyses and both metagenomics and new emerging analyses is imperative to improve our understanding. 

Brum, J. R., Schenck, R. O., & Sullivan, M. B. (2013). Global morphological analysis of marine viruses shows minimal regional variation and dominance of non-tailed viruses. The ISME journal, 7(9), 1738-1751.
http://www.nature.com/ismej/journal/v7/n9/abs/ismej201367a.html