Virusesare abundant in every environment on earth from high up in the atmosphere tothe deepest depths of the ocean.
Approximately 71% of the earth is covered bywater, of which 96.5% is in the ocean where the true power of viruses can beseen. Viruses have the highest abundance of any other life form in the ocean,occurring at approximately ten billion per litre of water with a vast range ofgenetic diversity (Fuhrman, 1999). Microorganisms make up 90% of the biomass inthe ocean and are essential to nutrient and energy cycles. Yet, approximately20% of this living biomass is lost to viruses every day (Suttle, 2007). Viruseshowever to do not just affect microorganisms, they can cause mortality in almostevery lifeform in the ocean such as bacteria, archaea and eukaryotic organisms (Rohwerand Thurber, 2009).
The development of methods to estimate the abundance and diversity of virusesin the ocean has proved to be quite difficult for researchers. It was oncethought that the quantity of viruses in the ocean was linked to prokaryoticabundance and activity but researchers have now found dissimilarity occurringbetween them in different marine environments. Researchers have also found it difficult to quantify the exact effectsviruses have on heterotrophic and autotrophic communities in the ocean (Suttle,2007).However in recent years our knowledge of the vast range of viruses in themarine environment has grown by using metagenomics processes. The relationshipbetween viruses and the organisms that they effect appears to limit and controlthe genetic diversity of viruses in that area (Suttle, 2007). Recent studieshave revealed that viruses are capable of manipulating entire life historiesand future evolution of the organism that have infected, which shows that theyare of major importance in the marine food web. Since viruses can impactbacteria, archaea and eukaryotic communities, they have the ability to changethe composition of almost any organism in our oceans (Rohwer and Thurber,2009). A virus is a small particle with RNA or DNAgenetic material which can be single or double stranded surrounded by a proteincoat and is generally about 20-200nm in length.
The infectious virus particleis composed of a nucleic acid surrounded by a protein shell (Lodish et al., 2000). Viruses need a host toreproduce and multiply as they cannot do it by themselves. With almost everycellular organism being vulnerable to infection by a vast array of viruses,this makes viruses one of the most diverse and dangerous entities on earth. Virusesuse the hosts exposed cellular structures as a method to attach and enter theorganism, while using passive transport to come in contact with a host (Fuhrman,1999).The three major types ofreproduction in viruses are: Lytic infection, chronic infection and lysogeny.Lytic infection is when the host cell is penetrated by a lytic phage. A nucleicacid is injected into the cell which encourages the host cell to producemultiple progeny viruses.
The cell will then burst causing the death of thecell, which allows the process to start once again as the virus particles thatare released can infect other cells of the host (Clyde and Glaunsinger, 2010).Chronic infection does not result in the fatality of the cell. Instead, theprogeny virus can be released out of the cell through budding or extrusion overnumerous generations (Fuhrman, 1999). Lysongeny (also referred to as thelysogenic cycle) is when the host cell can carry the cell of the virus in arelatively stable state.
The nucleic acid of the virus is reproduced with thegenome of the host cell and is called a prophage. However, a stressful event tothe host cell can trigger a change from the lysogenic cycle to lytic infection(Guttman, 2001). The bacterial virus “phage” was discovered by FrederickW. Thwort in 1915 and also discovered by Félix d’Hérelle in 1917 in France, independently of eachother.
Some say that d’Hérelle’s claims that he had no knowledge ofThworts earlier discovery when he published his work in 1917 are false (Duckworth, 1976). In 1979 Francisco Torellaand Richard Morita discovered that there are many morphological similaritiesbetween phage and marine viral particles and that they are extremely abundant inthe ocean with approximately 100,000 occurring in every millilitre of water. Inthe 1990’s, a lot of research was put into diversity and abundance of marinephage and the ecological effect these viruses have on marine planktoncommunities. Research showed that viruses and protists gave major contributionsto the global biogeochemical cycle.
Soon the very first marine viral genomeswere sequenced and developed what we know today as metagenomics to characterizethe different types of DNA and RNA viruses in the water (Rohwer and Thurber,2009). The timeline shown in the figure below shows the major events that have occurredin the research of marine viruses.