Marine viruses - major players in the global ecosystem?

This paper opens by reminding us that oceans account for more than 70% of the Earth’s surface, control the climate, provide significant amounts of globally consumed protein and produce approximately half of the planet’s oxygen. Microorganisms are a major force behind the nutrient and energy cycles in the world’s oceans and constitute more than 90% of the living biomass in the sea. It is estimated that viruses kill approximately 20% of this biomass per day. The virosphere is conceived as encompassing all environments on the planet. It is increasingly evident that viruses are a fundamental force that exert strong effects on the structure of marine microbial assemblages and, subsequently are a governing force in patterns of nutrient and energy cycling.

In this paper by Curtis A. Suttle (2007) our current knowledge of marine viruses is reviewed. Attention is paid to areas of rapid advance, and to potential for paradigm-shifting discoveries.

The advances in techniques common to microbiology have allowed further and more accurate knowledge in marine virology to flourish over the past decade. These are discussed in some detail within the paper; of particular interest are  advances in epifluorescence microscopy and flow cytometry, which have led to confident, and replicable estimates of viral abundance within the water column. However, it is still a challenge to enumerate abundance within marine sediments. 

Viral abundance is in excess of that of archaea and bacteria by “15-fold”, though their size constrains them to represent only 5% of ‘prokaryotic’ biomass. However viral abundance varies with productivity and 'prokaryote’ abundance, “(as reflected by the concentration of chlorophyll a)”. It has thus been found that viral numbers diminish with an increase distance from the shore and with depth. These are large-scale patterns, however, viral production occurs at microbial hot spots.

It has been observed that members of the Phycodnaviridae family infect eukaryotic phytoplankton and have been coupled with chlorophyll a concentrations. If viruses tend infect cells that grow more rapidly this may affect nutrient cycling and efficiency of carbon fixation in surface waters and its transport to the benthos.

Many models on the impact of viral infection assume that every member of the prokaryotic community is equally affected by viral infection, such as large lytic events, which can lead to a cessation of phytoplankton blooms, though more often infection is subtler. Still an increase in the growth rate of host cells can increase viral reproduction rates and act as a strong feedback mechanism, preventing dominance by fastest growing taxa. 

Concluding remarks

This is a review paper that links many topics. It discusses the connections between viruses and their effect on marine microbial communities, macro-ecology and biogeography. The examples given above are representative of research that the author has carefully knitted together in this intriguing and well-written paper. The links between virology and global ecosystems are clarified. Suttle highlights that our knowledge of marine virology is far from complete, but new methods employed over the past decade have put us on the right track for the future.

Curtis A. Suttle. (2007). Marine viruses — major players in the global ecosystem. Nature Reviews Microbiology. 5, 801-812.

Available at: http://www.nature.com/nrmicro/journal/v5/n10/abs/nrmicro1750.html#top