It is estimated that the oceans contain 100 million times as many virions (virus individuals) as there are stars in the universe. But unlike the common image of viruses as pathogens, they are also crucial to evolutionary processes and the development of life on Earth.
Different life cycles
Bacteriophages are not necessarily harmful to their host. They have two different life cycles. The most famous, the lytic cycle, is when the virus enters a bacterium and multiplies itself until the host literally bursts with virions, which in turn can infect other cells. In addition, there is also the lysogenic cycle, in which a virus integrates its DNA into that of the host and, as it were, stay dormant, without having any influence on the bacterium. This viral DNA is copied when the bacterium divides and is transferred to the daughter cells. Only under certain circumstances (relatively rarely) does the dormant virus switch to the lytic cycle to actively multiply and search for another host.
The lysogenic life cycle of some bacteriophages and specifically the integration of viral DNA into that of the host has important implications. This is because host DNA is located at both sides of the viral DNA. When the dormant virus wakes up and makes the transition to another host, it usually carries surrounding genes with it. These are then transferred and integrated into the DNA of the next host. A free gene for an overwhelmed new host. Virus-driven genetic transfer between bacteria, called transduction, is an important source of genetic diversity. Important because genetic diversity enhances the adaptability of a species and thus forms the basis for evolution.
Sleeping sea subjects
A recent study published in the scientific journal Nature Microbiology describes the discovery of a lysogenic virus in the most numerous group of marine bacteria. The wide distribution of this group, the SAR11 bacteria, is a result of their adaptability to many different conditions. This study indicates that this adaptability is partly due to a "hidden", lysogenic virus in their DNA. The virus in question appears to be able to selectively scale up the lytic cycle. When a bacterial population has enough nutrients available, the virus is secreted by about 2% of the bacteria via the lytic cycle. However, when a population grows with nutrient deficiency, the virus is shed by about 30% of the bacteria. In short, a deficient bacterial population represents a relatively greater source of transduction. Because the genes transferred via this transduction in some cases influence survival performance during nutrient deficiency, infected bacterial populations can slowly become more resistant to such deficiencies. Partly because of this, the SAR11 bacteria have been able to adapt so well over the years, often to non-optimal living conditions.
Humans go viral
Although the current study is specifically concerned with bacteriophages and the evolutionary development of bacteria, it is believed that similar processes have also taken place in the evolution of eukaryotes, including in humans. Probably, humans would not be humans without the contribution of viruses.