and all cells of the progeny plant contain viral dna. however, viral particles are only produced in the reproductive cells of the algae, while viruses remain latent in vegetative cells. in infected sporophytes, cells undergo meiosis and produce haploid spores. the esv genome is transmitted in a mendelian manner, where half of the progeny contain viral dna. often algae from infected spores are indistinguishable from algae derived from healthy spores, but are partially or fully incapable of reproduction. chlorovirus chloroviruses are the only viruses characterized thus far that infect freshwater algae. the hosts of chloroviruses are zoochlorellae, which are endosymbiotic green algae commonly associated with hosts paramecium bursaria, coelenterate hydra viridis, or the heliozoan acanthocystis turfacea. in the ciliate paramecium bursaria, for example, the algae lives within the cells of the host, providing nutrients via photosynthesis. living inside the cells of the ciliate offers protection for the algae, and a mode of transportation. zoochlorellae are resistant to infection in their symbiotic state. when the relationship between the algae and host is disrupted, for example, through grazing by copepods, infection by chloroviruses is permitted. the life cycle of the chlorovirus infecting paramecium bursaria, known as pbcv-1 has been studied in detail . cryo-electron microscopy and 3d reconstruction of the viral capsid shows that there is a long 'spike' structure which first contacts the cell wall and likely serves to puncture the cell wall of the host. the pbcv-1 virus is specific to its host and recognition is mediated by the interaction of virus surface proteins with algal surface carbohydrates. following attachment of the virus to the host's cell wall, capsid-bound glycolytic enzymes break down the cell wall. the viral membrane likely fuses with the host membrane, allowing the viral dna to enter the cytoplasm, leaving an empty capsid on the outside. as pbcv-1 lacks an rna polymerase gene, the virus must use the host cell's machinery to produce viral rna. thus, the viral dna quickly moves to the nucleus where early transcription is initiated 5–10 minutes post infection. within minutes of infection, host chromosomal degradation occurs, inhibiting host transcription. at 20 minutes post infection, most of the mrnas in the infected cell are viral mrnas. the proteins translated from the early stage of transcription are involved in initiating viral dna replication, occurring 60–90 minutes post infection. the second phase of proteins are translated in the cytoplasm and the assembly of virus capsids begins about 2–3 hours post infection. mature virions are formed with the addition of newly replicated viral dna from the host nucleus, likely facilitated by a virus encoded dna packaging atpase. about 5–6 hours following pbcv-1 infection, the cytoplasm is filled with virions and lysis occurs at 6–8 hours post infection releasing roughly 1000 particles per cell. prymnesiovirus the genus prymnesiovirus currently contains only one species, known as chrysochromulina brevifilum virus pw1 (cbv-pw1). cbv-pw1 infects two species of marine phytoplankton, chrysochromulina brevifilum and c. strobilus, belonging to the genus chrysochromulina. according to the algaebase database, there are currently 63 marine and freshwater species names in the genus, of which 48 are recognized as taxonomically acceptable names. chrysochromulina is a particularly important genus as it can comprise more than 50% of the photosynthetic nanoplanktonic cells in the ocean. little is known about the life cycle of the virus infecting these flagellate-containing planktonic species, chrysochromulina brevifilum and c. strobilus. suttle and chan (1995) were the first to isolate viruses which infect prymnesiophytes or haptophytes. in this study, ultrathin sections of viruses within chyrsochromulina brevifilum were prepared and viewed using transmission electron microscopy. electron micrographs in the early stage of infection suggest that virus replication occurs in the cytoplasm within a viroplasm. a viroplasm is a localized area in the cytoplasm, or around the nucleus of the cell which serves as a 'viral replication factory'. the viroplasm contains components such as virus genetic material, host proteins and ribosomes necessary for replication. virosomes are often surrounded by a membrane; the membrane surrounding the virosome contained in the infected cells in the study was found to consist of a fibrillar matrix. virions are released from infected cells following disruption of the organelles and lysis of the host cell membrane. suttle and chan (1995) counted more than 320 viruses in an ultrathin section of an infection cell. estimates for burst sizes range from 320 to 600 viruses per cell. prasinovirus members of the genus prasinovirus infect small unicellular green algae in the order mamiellales, commonly found in coastal marine waters. a species of the genus prasinovirus is micromonas pusilla virus sp1 (mpv-sp1), which was isolated from a water sample collected off of san diego. the prasinovirus mpv-sp1 infects micromonas pusilla which is a dominant photosynthetic marine picoeukaryote. and which infects micromonas pusilla (utex 991, plymouth 27). common hosts of prasinoviruses include members from the genera ostreococcus and micromonas. three potential species of ostreococcus have been identified and differ based on their light requirements. one of the most widely studied prasinoviruses, strain otv5 whose genome is fully sequenced infects ostreococcus tauri, the smallest free-living eukaryotes currently known. prasinoviruses employ a nucleo-cytoplasmic replication strategy where virions adhere to the host-cell surface, followed by injection of dna into the host cytoplasm. researchers found that 'empty' otv5 viruses, or viruses with only the capsid attached to the host membrane, were rarely seen at any stage of the infection, suggesting that virions detach from the host membrane after injection of their dna. the authors also found that a high proportion of viruses did not attach to cells after inoculation and suggest that viral attachment may be a limiting step in the infection. the viral dna is then replicated inside the nucleus by the host cell's machinery. virus particles are assembled in the cytoplasm, usually occupying a space near the inner face of the nucleus. due to the extremely small size of the algae cells, the average burst size was found to be 25 virus particles per cell. viral production without cell lysis has recently been observed in o. tauri cells. thomas et al. (2011) found that in resistant host cells, the viral genome was replicated and viruses were released via a budding mechanism. this low rate of viral release through budding allows for prolonged survivability of the host and virus progeny, resulting in a stable co-existence. encoded proteins ectocarpus siliculosus virus (esv-1), belonging to the genus phaeovirus, and paramecium bursaria chlorella virus (pbcv-1), belonging to the genus chlorovirus, are two well-studied viruses, whose genomes have been found to encode many proteins. these proteins function in virus stability, dna synthesis, transcription, and other important interactions with the host. enzymes for glycosylation pbcv-1 has a 54-kda glycosylated major capsid protein, which comprises about 40% of total viral protein. unlike most of the viral structural proteins which are glycosylated in the endoplasmic reticulum (er) and golgi apparatus by host-encoded glycosyltransferases, pbcv-1 glycosylates its major capsid protein independently by encoding most of the enzymes necessary for constructing the complex oligosaccharides, which then attach to the major capsid protein of pbcv-1 to form the glycoprotein. therefore, the glycosylation of the major capsid protein of pbcv-1 happens independently of the er and golgi apparatus in host cells. ion channel proteins the first known viral protein that functions as a potassium-selective ion channel was found in pbcv-1. the protein (called kcv) consists of 94 amino acids and is encoded from a small open reading frame (orf) (orf a250r) in pbcv-1, which can produce potassium-selective and voltage-sensitive conductance in xenopus oocytes. the supposed pbcv-1 protein has a short cytoplasmic n-terminus (12 amino acids) containing one consensus protein kinase c site and it has 2 transmembrane domains. the different amino acid sequences and lack of cooh-terminal cytoplasmic tail make the kcv protein different from other potassium channels. esv-1 encodes a 124 codon orf that has significant amino acid similarity to pbcv-1 kcv (41% amino acid identity). however, the esv-1 protein has a longer n-terminus (35 amino acids) containing two consensus protein kinase c sites and it has three transmembrane domains. it is unknown whether the esv-1 protein can form a functional channel in heterologous cells. the esv-1 genome also encodes several proteins with hydrophobic amino acid rich regions that resemble helical transmembrane domains. among these proteins, the input domain of the supposed hybrid his-kinase 186 and the orf 188 resemble ion channel proteins. dna replication-associated proteins both esv-1 and pbcv-1 encode dna polymerase which belong to the dna polymerase-δ family, and they all contain a proof-reading 3'-5' exonuclease domain. additionally, both pbcv-1 and esv-1 encode a sliding clamp processivity factor protein (pcna), which interacts with proteins involved in dna replication as well as proteins involved in dna repair and postreplicative processing (e.g. dna methylases and dna transposases). heteropentameric replication factor c (rfc) is a complex which is responsible for the atp-dependent loading of pcna onto dna; esv-1 encodes five proteins which can form a rfc complex. pbcv-1 encodes a single protein which resembles the found in the archae rfc complex. pbcv-1 also encodes other proteins involved in dna replication including an atp-dependent dna ligase, a type ii dna topoisomerase, and rnase h. although both esv-1 and pbcv-1 possess genes for essential elements of the eukaryotic replication system, neither have complete replicative genes, since they all lack genes for primase. transcription-associated proteins neither esv-1 nor pbcv-1 encode a complete rna polymerase, but they produce several transcription factor-like proteins to assist the host transcription system. esv-1 encodes two small polypeptides (orf 193 and orf 196) for transcriptional regulation; the proteins resemble the α/β/α domain of tfiid-18 subunit. the tfiid complex is necessary for transcription of eukaryotes, as it binds to the tata box in the core promoter of the gene to initiate the assembly of rna polymerase. besides, polypeptides resemble to the set, btb/poz (i.e. broad complex, tramtrack, and bric-a-brac/poxvirus and zinc finger) (orf 40), and baf60b (orf 129) domains are also encoded by esv-1 to regulate chromatin remodeling and transcription repression. four transcription factor-like proteins have been found in pbsv-1, including tfiib (a107l), tfiid (a552r), tfiis (a125l), and a vltf-2 type transcription factor (a482r). in addition, pbcv-1 also encodes two enzymes involved in forming a mrna cap structure, an rna triphosphatase and a mrna guanylyltransferase. the pbcv-1 enzymes are more closely related to yeast enzymes than to poxvirus multifunctional rna capping enzymes according to its size, amino-acid sequence, and biochemical properties. pbcv-1 also encodes rnase iii, which is involved in virus mrnas processing. nucleotide metabolism-associated proteins to supply deoxynucleotides for viral production in the low proliferating host cells, large dna viruses possess genes to encode deoxynucleotide synthesis enzymes themselves. thirteen nucleotide metabolic enzymes have been found in pbcv-1, two of which include dutp pyrophosphatase and dcmp deaminase, which can produce dump (i.e. the substrate for thymidylate synthetase). in comparison, esv-1 only encodes an atpase (orf 26) as well as both subunits of ribonucleotide reductase (orf 128 and 180), which is a key enzyme in deoxynucleotide synthesis. other enzymes other enzymes such as methyltransferases, dna restriction endonucleases, and integrase were also found in pbcv-1. pbcv-1 also encodes a 187-amino-acid protein that resembles the cu-zn sod with all of the conserved amino acid residues for binding copper and zinc, which can decompose the rapid accumulated superoxide in host cells during infection, thereby benefiting virus replication. ecological implications raphidovirus sketch of heterosigma akashiwo cell: internal anatomy heterosigma akashiwo forms dense, harmful blooms in temperate and subarctic waters, occurring at densities up to 5 ×106 cells/ml. these algal blooms can be extremely harmful to aquatic life, causing mortality in wild and cultured fish, such as salmon, yellowtail and sea bream. the severity and duration of these blooms varies from year, and damage to aquaculture by h.akashiwo has been increasing. in 1989, a noxious algal bloom off the coast of new zealand resulted in the loss of seventeen million new zealand dollars worth of chinook salmon. in 1995 and 1997 in japanese coastal waters in kagoshimo bay, 1,090 million and 327 million yen worth of fish were killed, respectively. the hav virus, infecting h. akashiwo has been shown to be a factor in bloom termination. suttle et al. (1990) suggested that viral infection of algae could have a role in regulating population densities of phytoplankton communities, thus having significant roles in their dynamics in the oceans. earlier studies, such as the study by