Roles of Viruses In Aquatic Ecosystems

Roles of Viruses In Aquatic Ecosystems

Role of Viruses in aquatic ecosystem are important for marine and freshwater microbial communities. In fact, virioplankton are the most numerous members of marine ecosystems. However, quantifying viruses is tricky: The traditional method of plaque formation requires knowledge of both the virus and its host, and the ability to grow the host in the laboratory. Because so few microbes have been cultured, this prevents the measurement of virus diversity by examining actual virus infection. Instead, virus particles may be visualized directly.

How to visualize virus particles?

There are at least two approaches for this. Electron microscopy is the most rigorous, but fluorescence microscopy is a common and convenient method used to visualize viruses in aquatic systems. When seawater is filtered, viruses can be collected on filters with a pore size of 0.02µm. Viral nucleic acids are stained with a fluorescent dye, such as YO-PRO or SYBR Green for fluorescence microscopy.

Microscopy reveals the presence of virions outside the host cell. This does not prove that the virions present can infect a host cell, so viruses enumerated in this way are called viruslike particles (VLPs). Using this approach, the average VLP density in seawater is between 106 to 107 per milliliter (although in some cases, it may be closer to 108 per milliliter); their numbers decline to roughly 106 below about 250m. Marine viruses are so abundant that VLPs are now recognized as the most abundant microbes on Earth.

(a) The biomass of bacteria and archaea far exceeds that of both protists and viruses. (b) When individuals are counted, however, the vast majority of marine microbes are viruses.

Diversity of virus particles

Viral diversity is vast, including single and double-stranded RNA and DNA viruses that infect archaea, bacteria, and protists. Metagenomic analysis of cloned viral genomes has been used to explore viral diversity. In one such study, viral communities were sampled from the Arctic Ocean, North Pacific coastal waters, the Gulf of Mexico, and the Sargasso Sea. Of the roughly 1.8 million nucleotides obtained, 90% had no recognizable match on any database. Thus not only is viral genetic diversity immense, it is largely unexplored.

The abundance of marine viruses indicates that they must be major agents of mortality in the sea. Indeed, viruses are thought to kill on average about 20% of the marine microbial biomass daily. However, measurement of virus induced microbial mortality shows that it is highly variable. Viral abundance frequently corresponds to the microbial host that is most active in the community.

Virus-mediated cell lysis can significantly impact community structure. Models predict that as one microbial species (or strain) becomes numerically dominant, it soon will be targeted by lytic viruses and thus its population will decline. This permits another microbial species (or strain) to flourish, which then becomes subject to intense viral lysis, and so on. This “kill the winner” model has garnered much attention, but more experimental evidence is needed before it is widely accepted.

Perhaps the most compelling data to date follow the fate of blooms of the coccolithophore Emiliania huxleyi. Such blooms are so intense they can be imaged from space, yet their collapse is thought to be (at least in part) the result of viral lysis. Although this phenomenon is interesting, in most cases, viral lysis does not result in the complete collapse of a host population.

This satellite image shows the demise of a 500 km-long bloom (seen as light blue smear) of the coccolithophore Emiliania huxleyi. While the exhaustion of available nutrients also contributes to the death of such an enormous bloom, research has demonstrated that viruses specific to this protist are principle agents in the bloom’s demise. The green land mass is Ireland.

Role of Viruses in the Microbial Loop

This may be due partly to strains of the host species that are resistant to virus infection, as has been shown with members of the abundant cyanobacterial genus Synechococcus. Computer modeling and model experiments indicate that viruses contribute to nutrient cycling by accelerating the rate at which their microbial hosts are converted to POM and DOM, thereby feeding other microorganisms without first making them available for protists and other bacteriovores. This “short-circuits” the microbial loop.

Viral lysis of autotrophic and heterotrophic microbes accelerates the rate at which these microbes are converted to particulate and dissolved organic matter (P-D-OM). This is thought to increase net community respiration and decrease the efficiency of nutrient transfer to higher trophic levels.

As might be predicted, viruses are important vectors for horizontal gene transfer in marine ecosystems. In fact, it has been calculated that in the oceans, phage-mediated gene transfer occurs at an astounding rate of 20 billion times per second. The importance of phage-mediated horizontal gene transfer is demonstrated by cyanophages that infect species of Synechococcus and Prochlorococcus. These phages carry the structural genes for photosynthetic reaction center proteins. By shuttling these genes between strains of cyanobacteria, these phages may play a critical role in the evolution of these important primary producers.

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