Although viruses are the most abundant and the most highly diverse “biological entity” within the marine environment, and in fact the world in general it has only been recently that the roles they play within marine systems are beginning to become understood. It wasn’t until Proctor and Fuhrman (1991) that viral lysis of bacteria was first recognized to be an important marine process. Viral lysis creates dissolved organic matter (DOM), which becomes a source of nutrients and the cycling of carbon in marine systems.
This review by Sandaa (2008) examines virus-host interactions. Due to prokaryotes (bacteria and archaea) being the most abundant host type it assumed that the majority of the viruses are bacteriophages. The review goes on to explain that in marine environments double stranded DNA (dsDNA) has a genome size range of 15 to 630 kb and that in order of highest abundance, small genome-viruses (SGV) 20 to 80 kb are top, with middle genome-sized viruses (MGV) 80 to 280 kb being less abundant and large genome-sized viruses (LGV) 280 to 500 kb are the least abundant. And that the genomic size range of marine bacteriophages, which as explained above are thought to be the most abundant marine viruses is 39 to 243 kb. Bacteriophages then fit into the SGV and MGV genomic size range, which makes sense, as they are the most dominant size ranges. Also the bacteriophage thought to be the most numerous group Podoviridae has a genomic size range of 39 to 60 kb and so falls under the category SGV, the most abundant marine genome viral size.
As well as the idea stated above that viruses are important because they release DOM into the environment, it is also suggested that they may play an important role in regulating populations of host bacteria. One mechanism that is given as a potential explanation for this a top-down model labeled “killing the winner”. The role of the virus in this model is to restrict the dominance of a certain bacterial species. The implications are that bacteriophages control the most abundant or competitively superior bacterial populations and therefore allow less abundant or inferior competitively populations to co-exist, rather than be excluded. This allows a higher biodiversity than if a dominant population was left to dominate. Disturbance. Given as an example of evidence that supports this idea comes from a mesocosm study in which the termination of a bloom of the coccolithophorid Emiliania huxleyi was followed by an increase in the specific virus E. huxleyi (EhV). The review does concede however that studies on these ideas have been contradictory in their findings, with some studies suggesting a limited ability of viruses to control hosts’ populations and others suggesting an element of control is visible. It has even been proposed that the most dominant hosts are the least likely to suffer from viral lysis.
The final way that the review exams how viruses may affect their hosts is by horizontal gene transfer. New genetic traits may be introduced from one organism to another from viruses by horizontal gene transfer, e.g. from one bacteria to another. Viruses then are altering the genetics of other organisms, which of course has huge evolutionary implications. This gene transfer can clearly be beneficial to the hosts, given as an example are photosynthetic genes, which code for functional properties and may be beneficial for the host have been uncovered in the viral genome. Gene transfer may lead to metabolic benefits and also as a potential result an increase in fitness of the host.
The author’s conclusion to the question of whether virus-host interactions in the marine environment are a burden or benefit to the host population is favoured on the beneficial side. Although noted that there may be an energy cost from the hosts for carrying the viral genetic material, this is outweighed by the befits of increased fitness from gene transfer as beneficial genes spread through populations, with the virus-host relationship being described as irreplaceable for driving microbial evolution by the author. It is also clear that their is a lot left to discover.
Sandaa (2008) Burden or benefit? Virus-host interactions in the marine environment. Research in Microbiology 159 (2008) 374e381
2 comments:
Hi Joe - this links with earlier posts about "kill the winner" hypothesis and it's good to see some recognition that not al experiMental studies have given the same results. A second point - Does the review make any mention of RNA viruses. these are often overlooked but probably constitute a significant fraction of marine viruses.
Hi Colin. Yes, I really liked this hypothesis and it reminded me of the "Intermediate Disturbance Hypothesis" in ecology in some ways.
The review dies state that recent studies using methods not based on viral cultivation have demonstrated that RNA viruses are important but that in previous studies RNA viruses may not have been detected and are thus often overlooked as you said
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