Viruses exist in all the marine environments of the world, but their abundance can be very difficult to measure. To circumvent many of the difficulties associated with trying to grow viruses, scientists have begun isolating large communities of viruses and sequencing all the DNA found in such a community. This provides a “virome”, representing all of the DNA in a given population. Using this, scientists have made some stunning discoveries concerning the roles viruses are playing in the world’s water.

One of the most important impacts viruses can have on organisms occurs through horizontal gene transfer. Horizontal gene transfer occurs when DNA is passed between organisms which are not parent and offspring. This can occur through a number of methods, one of which is viral infection. Viruses infect their hosts by injecting DNA into them. Often the DNA will then cause the organism to start making many copies of the virus and die, but it is also possible that the organism can undergo a phase in which the DNA will just be incorporated into the organism without immediately resulting in death. These genes can then be passed from one generation to the next.

A startling example for how important this process might be for all life on earth concerns cyanobacteria and their viruses, cyanophages. Cyanobacteria are algae that can perform photosynthesis. Two types of cyanobacteria are responsible for 25% of global photosynthesis. The cyanophages that infect these organisms carry genes essential to photosynthesis. It has recently been estimated that 60% of cyanobacteria express a gene required for photosynthesis that has originated from the cyanophages. That would make viruses responsible for a little over 10% of the photosynthesis that occurs on earth.

The authors of this review propose a role for marine viruses as gene banks, floating around with a wealth of genetic information that may be useful when the virus encounters the right host in the right environment.

This possibility is very appealing for explaining some of the more unexpected aspects of life in the ocean. For example deep-sea thermal vents are a strenuous environment, with many hazards that could threaten a living organism. Yet, life flourishes at these underwater hot spots.  Scientists have found extremely large viromes located at these deep-sea thermal vents. The DNA sampled from these populations of viruses shows that only 25% of the DNA contains sequences already represented in GenBank, an on-line database of all known genes from every organism that humans have studied. This means that these viromes present a huge variety of novel DNA which could be useful in allowing marine organisms to adapt to life at high temperatures, high pressures or high concentrations of inorganic material.

Another possibility for horizontal gene transfer to affect dramatic changes arises from the fact that viral-like particles have been found which can even affect organisms across the separate domains of life. This means that genetic information could theoretically be passed between organisms as different as bacteria and animals, plants and prokaryotes. While gene transfer across such wide divides is rare it can happen when viruses get involved, causing surprising results.

An example of far-flung genetic transfer is the solar powered sea slug. These sea slugs will eat algae and in the process steal chloroplasts whole from their dinner, in a process that has been named kleptoplasty. By hijacking the chloroplasts they become capable of photosynthesis. This kleptoplasty provides 10-20% of the genes needed for photosynthesis. The rest are actually produced in the nucleus of the sea slug and research suggests that the sea slug gained these by horizontal gene transfer from a eukaryotic virus.