A single drop of sea-water contains one million bacteria and ten million viruses. The viruses in our oceans outnumber all other organisms on Earth ten to one. These colossal numbers are almost beyond our imagination. Viruses are not generally considered to be living organisms. They lack a metabolism, are unable to move independently and do not eat. They also need other organisms, known as hosts, to reproduce. The virusus ‘hijack’ a host cell and use it to make new viruses.
The engine of evolution
Throughout the world’s oceans around one hundred trillion (1023) virus infections occur each second. The organisms being infected are mainly bacteria. These unicellular hosts usually die within a matter of hours or days. In this way it is estimated that marine viruses kill 40% of all the bacteria in the ocean every day. This is very important, because viruses accelerate the recycling of nutrients from dying microbes by this means. They are also the engine of evolution forcing underwater organisms to adapt to the constant threat of viral infections.
5,500 new virus populations
However it was unclear exactly how many different species of viruses are floating around in the oceans. Thus for many years researchers at the University of Arizona, in the USA, took water samples and analysed them in their lab. They developed a method for identifying the viruses based on DNA. By comparing a random virus gene with their database, they were able to determine whether it was one they had found previously. At length they discovered no more new genes and concluded that they had fished up all the viral DNA. Using this statistical trick they discovered nearly 5,500 new virus populations, whereas only 39 had previously been described. Further research should reveal whether these are indeed different virus species. Hence the word “populations”.
The researchers also indicated that this number will only continue to increase. This was because they only sampled the upper ocean layers and their filters excluded the so-called ‘megaviruses’. Also they only studied DNA while many virusesinstead use RNA as a carrier for their genetic information. So perhaps there are thousands more marine RNA-virus species.