Cyanophage

The following three families of cyanophages have been recognized by the International Committee on Taxonomy of Viruses (ICTV): Myoviridae, Siphoviridae and Podoviridae; all contain double-stranded DNA. Initially, cyanophages were named after their hosts. However, the ability of cyanophages to infect multiple hosts and lack of a universal naming system can cause difficulties with their taxonomic classification. Many other classification systems used serological, morphological, or physiological properties. Currently, the suggested procedure of naming strains is as follows: Cyanophage Xx-YYZaa, where Xx is the first two letters of the genus and species names of the host that the type specimen phage is found in, YY is the origin of the specimen, Z is the virus family, and aa is the reference number of the virus. ==Morphology==

Like all other tailed bacteriophages cyanophages have a tail and a protein capsid surrounding genetic material. The double-stranded DNA is approximately 45 kilo-base-pairs long and in some cyanophages encodes photosynthetic genes, an integrase, or genes involved with phosphate metabolism (phosphate-inducible). The tail binds the virus to the host cell and transfers viral DNA to the host cell upon infection. Based on morphological characteristics, cyanophages are placed into the families Myoviridae, Podoviridaeand Siphoviridae, and although not formally recognized by the International Committee on Taxonomy of Viruses, historically cyanophages have been further classified into as a Cyanomyovirus, Cyanopodovirus or Cyanostylovirus based on which of the three families in which they are grouped. and was also the first genus recognized. The tails have been observed as either contractile or noncontractile with lengths of 20 to 244 nm, widths of 15 to 23 nm, and a shrinking range of 93 nm. Cyanophages generally have isometric hexagonal heads with diameters ranging from 55 to 90 nm. At the point of attachment between the long tail and the head there is a base plate where short pins are attached, a contractile sheath, and an internal core, similar to other bacteriophages in the Myoviridae. The type specimen of cyanopodovirus is Cyanophage LPP-1, which infects Lyngbya, Plectonema and Phormidium. Their capsids are polyhedrons that appear hexagonal in 2-D. Other genera in this family have tails that range from 200 to 300 nm in length. ==Host==

'' filament The host range of cyanophages is very complex and is thought to play an important role in controlling cyanobacterial populations. Freshwater cyanophages have been reported to infect hosts in more than one genus although this may also reflect problems in the taxonomic classification of their hosts. Nonetheless, they have been classified into three major groups based on the taxonomy of their host organism. This group of cyanophages has the same host same range; however, their serum and other body fluids are not the same. Synechococcus cedrorum, Synechococcus elongatus and Microcystis aeruginosa. There is a new SM-group of virus, known as SM-2, which also lyses Microcystis aeruginosa They play an important role in infecting and causing lysis of members of the genera Nostoc, Anabaena and Plectonema. Lastly, cyanobacterial isolates of Nostoc and Plectonema species are infected by the NP group of viruses. These cyanobacterial isolates closely relate to the taxonomic group of Nostoc. They all have a broad host range and mutations are noticeable in these groups of viruses. ==Replication==

Cyanophage replication has two dominant cycles: the lytic cycle and the lysogenic cycle. Viral nucleic-acid replication and immediate synthesis of virus-encoded protein is considered to be the lytic cycle. Phages are considered lytic if they only have the capacity to enter the lytic cycle; whereas, temperate phage can either enter the lytic cycle or become stably integrated with the host genome and enter the lysogenic cycle. To meet the metabolic demand of replication, viruses recruit a multitude of strategies to sequester nutrients from their host. One such technique is to starve their host cell. This is done by inhibiting the host cells CO2 fixation, which enables the cyanophage to recruit photosynthetically formed redox and ATP from the host cell to meet their nucleotide and metabolic response. Many cyanophages contain genes known as viral-encoded auxiliary metabolic genes (AMGs), which encode critical, rate-limiting steps of the host organism. AMGs also code for proteins, which aid in the repair of the host photosystem, which is susceptible to photodegradation. Field studies also show that the infection and replication of cyanophages is directly or indirectly synchronized with the light-dark cycle. Cyanophages also have several surface proteins with Ig-like domains, which are used for adherence. In one study on cyanomyoviruses infecting marine Synechococcus sp., the lytic phase was shown to last approximately 17 hours with the average number of viruses produced for each cell that was lysed (burst size) ranging from 328 under high light to 151 under low light. There is evidence supporting the premise that there is a correlation between light intensity and burst size. Lysing of the host cell tends to occur after the completion of host DNA replication and immediately prior to cell division. This is likely due to the increased availability of intra-cellular resources for creating viral particles. ==Ecological Significance==

, genus Salacisavirus, pink) infects the ubiquitous Prochlorococcus marinus'' cyanobacteria, it produces a ferredoxin protein that hooks into the bacteria's existing electrical structure and alters its metabolism. Ecosystem Certain cyanophages infect and burst Prochlorococcus, the world's smallest and most abundant primary producers. Cyanophage populations have been found to inhabit microbial mats in the Arctic through metagenomic analysis and hypersaline lagoons. The viruses cannot move independently and must rely on currents, mixing, and host cells to transport them. Viruses cannot actively target their hosts and must wait to encounter them. The higher probability of collision may explain why cyanophages of the Myoviridae family primarily infect one of the most abundant cyanobacteria, Synechoccocus. The dissolved organic matter (DOM) released from viral lysis by cyanophages can be shunted into the microbial loop where it is recycled or rejected by heterotrophic bacteria to form recalcitrant matter that is eventually buried in sediment. This is an important step in atmospheric carbon sequestration, commonly referred to as the biological pump, and maintenance of other biogeochemical cycles. Population, and therefore, rate of oxygen evolution can be regulated by cyanophages. In certain species of cyanobacteria, such as Trichodesmium that perform nitrogen fixation, cyanophages are capable of increasing the supply rate of bioavailable organic nitrogen through lysis. Cyanophages also infect bloom-forming cyanobacteria that can be toxic to health of humans and other animals through the production of microcystin and cause eutrophication, leading to oxygen minimum zones. Cyanophages can infect and kill four common bloom-forming cyanobacteria: Lyngbya birgei, Anabaena circinalis, Anabaena flosaquae, and Microcystis aeruginosa, Spikes in cyanobacteria populations are usually brought on by nutrient increases due to run-off from fertilizers, dust, and sewage. In addition to regulating population size, cyanophages likely influence phylogenetic composition by allowing other phytoplankton normally inhibited by cyanobacteria to grow. Whether the lytic or lysogenic phase dominates in a given area has been hypothesized to depend on eutrophic or oligotrophic conditions, respectively. Increase in number of encounters is directly related to an increase in rate of infection providing more opportunity for selective pressure, making coastal Synechococcus more resistant to viral infection than their off-shore counterparts. ==References==