nagasaki et al. (1993), explored the dynamics between hav and h. akashiwo. algal samples were obtained in the middle or final stages of a red tide in hiroshima bay, japan. using transmission electron microscopy, nagaski et al. identified the hav virus in and around the nuclear area of h. akashiwo cells. further support for the role of the hav virus in bloom termination was provided by a study conducted by nagaski et al. (1994). nagaski et al. (1994) found that proportion of virus-containing cells increased quickly before termination of the red tide; no virus-containing cells were detected three days before termination of the red tide and the sample collected on the last day revealed a high frequency (11.5%) of virus-containing cells. further studies by tarutani et al. (2000) also found an association between a decrease in cell density of h. akashiwo with an increase in the abundance of hav. the researchers found that hav not only plays in important role in controlling biomass, but also influences the clonal composition or characteristics of h. akashiwo cells. the researchers found that most isolates following bloom termination were resistant to hav clonal isolates, while during bloom formation resistant cells were a minor component. the authors suggest that viral infection, during the bloom termination period influences the properties of dominant cells in h. akashiwo populations. selective pressure exerted by the viruses in the later stage of infection may promote genetic diversity, allowing the h. akashiwo population to thrive after bloom termination. as mentioned, h. akashiwo blooms are detrimental to fish populations in temperate and subarctic waters, and continue to pose serious threats for aquaculture. nagasaki et al. (1999) examined the growth characteristics of hav and suggested that hav could be used as a microbial agent against h. akashiwo red tides. the advantages of using hav is that it specifically infects h. akashiwo even when other microorganisms are present. additionally, it has a high growth rate and can be produced at a low cost. using hav as a microbial agent is a promising solution for eliminating red tides to protect fisheries and marine life, but as the authors concluded, the effects of various hav clones on h. akashiwo populations should be explored in greater detail before the virus is used for wide-scale applications. coccolithovirus satellite image of emiliania huxleyi bloom the coccolithovirus (ehv) infects the coccolithophore emiliania huxleyi (e. huxleyi). coccolithophores are marine haptophytes which are surrounded by microscopic plates made of calcium carbonate. they live in the upper layers of the world's oceans and represent the third most abundant group of phytoplankton, containing about 300 species. e. huxleyi is recognized as the most prominent and ecologically important of the coccolithophores. e. huxleyi has a global distribution from the tropics to subarctic waters and occasionally forms dense blooms which can cover 100,000s of square kilometers. these trillions of coccolithophores produced, then die and sink to the bottom of oceans, contributing to sediment formation, and are the biggest producers of calcite in the oceans. thus, coccoliths have significant roles in global carbon fixation and the carbon cycle as well as sulfur cycling. over time, coccolithophores have shaped geological features of our planet. for example, the white cliffs of dover are formed from white chalk, or calcium carbonate produced by coccolithophores over millions of years. coccolithophore blooms are typically not harmful to marine life in the ocean. as these organisms thrive in nutrient-poor conditions, the coccolithophores offer a source of nutrition for small fish and zooplankton. e. huxylei viruses (ehvs) have been shown to be linked to the termination of these blooms. the termination stage of the bloom is indicated by a color change in the water. when large amounts of coccoliths (carbonate shell surrounding e. huxylei) are shed from e. huxylei cells from cell death or lysis, the water turns white or turquoise. in areas of dense bloom termination, the white color is reflective and can be seen in satellite imagery. wilson et al. (2002) used analytical flow cytometry to measure the abundance of viruses at different locations in and around the bloom area. the researchers found that the concentrations of viruses were higher inside the 'high reflectance area', suggesting that virus-induced lysis of e. huxleyi cells resulted in coccolith detachment. other studies by martinez et al. (2007) and bratbak et al. (1993) found higher concentrations of ehv viruses as the e. huxleyi bloom declined, indicating that lytic viral infection was the main cause of bloom termination. ehv viruses therefore have important roles in regulating biomass production in marine environments and ecological succession. this regulation of coccolithophore populations by ehv viruses therefore has significant effects on biogeochemical cycles, particularly the carbon cycle. phaeovirus member of the ectocarpales brown algae: asperococcus bullosus one of the best-studied phaeoviruses, esv-1, infects the small, filamentous brown algae e. siliculosus, which has a cosmopolitan distribution (found in most of the world's oceans). the ectocarpales are closely related to the brown algal group, the laminariales, which are the most economically important group of brown algae, having a wide range of applications in the cosmetics and food industry. muller et al. (1990) were one of the first to explore the causes of gametangium defects in e. siliculosus originating from new zealand. the researchers identified reproductive cells of e. siliculosus filled with hexagonal particles which were then released into culture medium when the cells burst. following release of these particles, sporophytes became infected, shown by pathological symptoms, suggesting that the particles are viruses. such studies allowed for the evaluation of infection potential of e. siliculosus viruses. using pcr amplification of a viral gene fragment, muller et al. (2005) monitored levels of pathogen infection in ectocarpus samples from the gran canaria island, north atlantic and southern chile. the researchers found high levels of pathogen prevalence; 40–100% of ectocarpus specimens contained viral dna. similar estimates have been given by sengco et al. (1996) who estimated that at least 50% of ectocarpus plants in the world contain viral dna. this high frequency of viral infection among globally distributed ectocarpus plants has ecological implications. viral infection by esv-1 in e. siliculosus plants, as mentioned, limits reproductive success of infected plants. thus, the esv-1 virus plays a key role in regulating populations of e. siliculosus, having further effects on local ecosystem dynamics. chlorovirus zoochlorellae (green) living inside the ciliate stichotricha secunda members of the genus chlorovirus are found in freshwater sources around the world and infect the green algae symbionts zoochlorellae. there is a lack of information about the role chloroviruses play in freshwater ecology. despite this, chloroviruses are found in native waters at 1–100 plaque-forming units (pfu)/ml and measurements as high as 100,000 pfu/ml of native water have been obtained. a plaque-forming unit is the number of particles capable of forming visible structures within a cell culture, known as plaques. abundances of chloroviruses vary with season, with the highest abundances occurring in the spring. chloroviruses, such as pbcv-1, play a role in regulating host populations of zoochlorella. as mentioned previously, infection of zoochlorella occurs only when the symbiotic relationship with its host is disrupted. infection of the algae during this stage of host/algae independence will prevent the host and algae relationship from being restored, thus decreasing the survivability of the endosymbiotic hosts of the zoochlorellae, such as paramecium bursaria. thus, chloroviruses play in important role in freshwater ecosystems by not only regulating populations of their host, zoochlorellae, but also regulating, to an extent, populations of zoochlorellae hosts as well. chloroviruses and viruses in general cause death and lysis of their hosts, releasing dissolved organic carbon, nitrogen and phosphorus into the water. these nutrients can then be taken up by bacteria, thus contributing to the microbial loop. liberation of dissolved organic materials allows for bacterial growth, and bacteria are an important source of food for organisms in higher trophic levels. consequently, chloroviruses have significant effects on carbon and nutrient flows, influencing freshwater ecosystem dynamics. prymnesiovirus prymnesiovirus, cbv-pw1, as mentioned infects the algal genus chyrsochromulina. chyrsochromulina, found in global fresh and marine waters, occasionally forms dense blooms which can produce harmful toxins, having negative effects on fisheries. a particularly toxic species called c. polylepis has caused enormous damage to commercial fisheries in scandinavia. in 1988, this bloom caused a loss of 500 tons of caged fish, worth 5 million us. given that chyrsochromulina is a widespread species, and is of significant ecological importance, viral infection and lysis of genus members is likely to have significant impacts on biogeochemical cycles, such as nutrient recycling in aquatic environments. suttle and chan suggest that the presence of viruses should have a strong regulatory effect on chyrsochromulina populations, thus preventing bloom formation or enabling bloom termination, explaining why persistent blooms are an unusual phenomenon in nature. prasinovirus a commonly studied prasinovirus, otv5, as mentioned, infects the smallest currently known eukaryote, ostreococcus tauri. o. tauri is about 0.8 micrometers in diameter and is within the picosize fraction (0.2–2 micrometers). picoeukaryotes, such as ostreococcus tauri are widely distributed and contribute significantly to microbial biomass and total primary productivity. in oligotrophic environments, marine picophytoplankton account for up to 90% of the autotrophic biomass and thus are an important food source for nanoplanktonic and phagotrophic protists. as picoeukaryotes serve as the base for marine microbial food webs, they are intrinsic to the survival of higher trophic levels. ostreococcus tauri has a rapid growth rate and dense blooms have been observed off the coasts of long island and california. samples collected from long island bay were found to contain many virus-like particles, a likely cause for the decline of the bloom. despite the large abundances of picoeukaryotes, these unicellular organisms are outnumbered by viruses by about ten to one. viruses such as otv5, play important roles in regulating phytoplankton populations, and through lysis of cells contribute to the recycling of nutrients back towards other microorganisms, otherwise known as the viral shunt. as mentioned, the prasinovirus mpv-sp1 infects micromonas pusilla which is a major component of the picophytoplankton of the world's oceans. m. pusilla lives from tropical to polar marine ecosystems. cottrell & suttle (1995) found that 2–10% of the m. pusilla population in an inshore environment was lysed per day, with an average of 4.4%. higher estimates have been given by evans et al. (2003), who suggest that m. pusilla viruses can lyse up to 25% of the micromonas population per day. this suggests that viruses are responsible for a moderate amount of mortality in m. pusilla populations. on a larger scale, viral infection of m. pusilla is responsible for nutrient and energy recycling in aquatic food webs, which is yet to be quantified. pathology until recently phycodnaviruses were believed to infect algal species exclusively. recently, dna homologous to chlorovirus acanthocystis turfacea virus 1 (atcv-1) were isolated from human nasopharyngeal mucosal surfaces. the presence of atcv-1 in the human microbiome was associated with diminished performance on cognitive assessments. inoculation of atcv-1 in experimental animals was associated with decreased performance in memory and sensory-motor gating, as well as altered expression of genes in the hippocampus related to synaptic plasticity, learning, memory formation, and the viral immune response.