Marine viruses Marine viruses are defined by their habitat as viruses that are found in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Viruses are small infectious agents that replicates only inside the living cells of a host organism, because they require the replication machinery of the host to replicate.[4] They can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.[5] When not inside a cell or in the process of infecting a cell, viruses Structure of a typical virus, in this case a phage.[1] exist in the form of independent The appearance of these viruses has been likened to a [2] particles called virions. A virion miniature lunar lander. They are essential to the [3] contains a genome (long molecules regulation of marine ecosystems. that carry genetic information in the form of either DNA or RNA) surrounded by a capsid (a protein coat protecting the genetic material). The shapes of these virus particles range from simple helical and icosahedral forms for some virus species to more complex structures for others. Most virus species have virions that are too small to be seen with an optical microscope. The average virion is about one one-hundredth the linear size of the average bacterium. A teaspoon of seawater typically contains about ten million marine viruses. Most of these viruses are bacteriophages which infect and destroy marine bacteria and control the growth of phytoplankton at the base of the marine food web. Bacteriophages are harmless to plants and animals, but are essential to the regulation of marine ecosystems. They supply key mechanisms for recycling ocean carbon and nutrients. In a process known as the viral shunt, organic molecules released from dead bacterial cells stimulate fresh bacterial and algal growth. In particular the breaking down of bacteria by viruses (lysis) has been shown to enhance nitrogen cycling and stimulate phytoplankton growth. Viral activity also affects the biological pump, the process which sequesters carbon in the deep ocean. By increasing the amount of respiration in the oceans, viruses are indirectly responsible for reducing the amount of carbon dioxide in the atmosphere by approximately 3 gigatonnes of carbon per year Marine microorganisms make up about 70% of the total marine biomass. It is estimated marine viruses kill 20% of this biomass every day. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms which often kill other marine life. The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms. Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution. It is thought viruses played a central role in early evolution before the diversification of bacteria, archaea and eukaryotes, at the time of the last universal common ancestor of life on Earth. Viruses are still one of the largest areas of unexplored genetic diversity on Earth. Contents Background Bacteriophages Archaeal viruses Fungal viruses Eukaryote viruses Marine protists Marine invertebrates Marine vertebrates Giant marine viruses Virophages Role of marine viruses Viral shunt Limiting algal blooms Gene transfer Marine habitats Along the coast At the ocean surface In the water column In sediments In hydrothermal vents Polar regions Distribution References Background Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the three domains.[6] They are found wherever there is life and have probably existed since living cells first evolved.[7] The origins of viruses in the evolutionary history of life are unclear because they do not form fossils. Molecular techniques are used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose.[8] Some viruses may have evolved from plasmids— pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.[9] Opinions differ on whether viruses are Bacteriophages (phages) a form of life or organic structures that interact with living organisms.[10] They are considered by some to be a life form, because they carry genetic material, reproduce by creating multiple copies of themselves through self-assembly, and evolve through natural selection. However they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because Multiple phages Diagram of a typical they possess some but not all such attached to a bacterial tailed phage qualities, viruses have been described cell wall at 200,000x [11] as replicators and as "organisms at magnification the edge of life".[12] The existence of viruses in the ocean was discovered through electron microscopy and epifluorescence microscopy of ecological water samples, and later through metagenomic sampling of uncultured viral samples.[13][14] Marine viruses, although microscopic and essentially unnoticed by scientists until recently, are the most abundant and diverse biological entities in the ocean. Viruses have an estimated abundance of 1030 in the ocean, or between 1 and 100,000x106 per millilitre.[4] Quantification of marine viruses was Phage injecting its genome into bacteria originally performed using transmission electron microscopy but has been replaced by epifluorescence or flow cytometry.[15] Bacteriophages Bacteriophages, often just called phages, are viruses that parasite bacteria. Marine phages parasite marine bacteria such as cyanobacteria.[16] They are a diverse group of viruses which are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. There are up to ten times more phages in the oceans than there are bacteria,[17] reaching levels of 250 million bacteriophages per millilitre of seawater.[18] These viruses infect specific bacteria by binding to surface receptor molecules and then entering the cell. Within a short amount of time, in some cases just minutes, bacterial polymerase starts translating viral mRNA into protein. These proteins go on to become either new virions within the cell, helper proteins, which help assembly of new virions, or proteins involved in cell lysis. Viral enzymes aid in the breakdown of the cell membrane, and there are phages that can replicate three hundred phages twenty minutes after injection.[19] Bacteria defend themselves from bacteriophages is by producing enzymes that destroy foreign DNA. These enzymes, called restriction endonucleases, cut up the viral DNA that bacteriophages inject into bacterial cells.[20] Bacteria also contain a system that uses CRISPR sequences to retain fragments of the genomes of viruses that the bacteria have come into contact with in the past, which allows them to block the virus's replication through a form of RNA interference.[21][22] This genetic These are cyanophages, viruses that infect system provides bacteria with acquired cyanobacteria (scale bars indicate 100 nm) immunity to infection.[23] Adsorption of cyanophages onto a marine Prochlorococcus (a) Slice (~20 nm) through a reconstructed tomogram of P-SSP7 phage incubated with MED4, imaged at ~86 min post-infection. FC and EC show full-DNA capsid phage and empty capsid phage, respectively. (b) same image visualised by highlighting the cell wall in orange, the plasma membrane in light yellow, the thylakoid membrane in green, carboxysomes in cyan, the polyphosphate body in blue, adsorbed phages on the sides or top of the cell in red, and cytoplasmic granules (probably mostly ribosomes) in light purple.[24] scale bar: 200 nm Process of a phage "landing" on a bacterium The phage first adheres to the cell surface with its tail parallel to or leaning at an angle to the cell surface in the pre-infection stage. The tail then firmly stands on the cell surface and extends its fibers horizontally, rendering the phage infection-competent, after which viral DNA is released into the cell through an extensible tube.[24] based on observations of the model cyanophage P-SSP7 interacting with the marine Prochlorococcus MED4 bacterium Microbes drive the nutrient transformations that sustain Earth’s ecosystems,[26] and the viruses that infect these microbes modulate both microbial population size and diversity.[27][24] The cyanobacterium Prochlorococcus, the most abundant oxygenic phototroph on Earth, contributes a substantial The lytic cycle, the reproductive cycle of the bacteriaphage, has fraction of global primary six stages: carbon production, and • Attachment – the phage attaches itself to the surface of the often reaches densities of host cell over 100,000 cells per • Penetration – the phage injects its DNA through the cell milliliter in oligotrophic membrane and temperate oceans.[28] • Transcription – the host cell's DNA is degraded and the cell's Hence, viral (cyanophage) metabolism infection and lysis of is directed to initiate phage biosynthesis Prochlorococcus represent • Biosynthesis – the phage DNA replicates inside the cell an important component of • Maturation – the replicated material assembles into fully formed viral phages the global carbon cycle. In • Lysis – the newly formed phages are released from the infected addition to their ecological cell role in inducing host (which is