Ecological significance of extracellular vesicles in modulating host-virus interactions during algal blooms

Extracellular vesicles are produced by organisms from all kingdoms and serve a myriad of functions, many of which involve cell-cell signaling, especially during stress conditions and host-pathogen interactions. In the marine environment, communication between microorganisms can shape trophic level interactions and population succession, yet we know very little about the involvement of vesicles in these processes. In a previous study, we showed that vesicles produced during viral infection by the ecologically important model alga Emiliania huxleyi, could act as a pro-viral signal, by expediting infection and enhancing the half-life of the virus in the extracellular milieu. Here, we expand our laboratory findings and show the effect of vesicles on natural populations of E. huxleyi in a mesocosm setting. We profile the small-RNA (sRNA) cargo of vesicles that were produced by E. huxleyi during bloom succession, and show that vesicles applied to natural assemblages expedite viral infection and prolong the half-life of this major mortality agent of E. huxleyi. We subsequently reveal that exposure of the natural assemblage to E. huxleyi-derived vesicles modulates not only host-virus dynamics, but also other components of the microbial food webs, thus emphasizing the importance of extracellular vesicles to microbial interactions in the marine environment.publishedVersio

Grazing on Marine Viruses and Its Biogeochemical Implications

Viruses are the most abundant biological entities in the ocean and show great diversity in terms of size, host specificity, and infection cycle. Lytic viruses induce host cell lysis to release their progeny and thereby redirect nutrients from higher to lower trophic levels. Studies continue to show that marine viruses can be ingested by nonhost organisms. However, not much is known about the role of viral particles as a nutrient source and whether they possess a nutritional value to the grazing organisms. This review seeks to assess the elemental composition and biogeochemical relevance of marine viruses, including roseophages, which are a highly abundant group of bacteriophages in the marine environment. We place a particular emphasis on the phylum Nucleocytoviricota (NCV) (formerly known as nucleocytoplasmic large DNA viruses [NCLDVs]), which comprises some of the largest viral particles in the marine plankton that are well in the size range of prey for marine grazers. Many NCVs contain lipid membranes in their capsid that are rich carbon and energy sources, which further increases their nutritional value. Marine viruses may thus be an important nutritional component of the marine plankton, which can be reintegrated into the classical food web by nonhost organism grazing, a process that we coin the “viral sweep.” Possibilities for future research to resolve this process are highlighted and discussed in light of current technological advancements.publishedVersio

Characterizing the Infectious Pattern of Viruses Infecting the Haptophyte Emiliania huxleyi

Marine viruses play an important role in biodiversity, population abundance and biogeochemical cycling of elements in the environment. They exhibit a broad range of infectious patterns and it is of ecological interest to gain further knowledge about these complex systems. This study investigated infectious patterns by cross-infecting three strains of the ubiquitous coccolithophore Emiliania huxleyi (CCMP374, CCMP371 and B) with three Emiliania huxleyi virus strains (EhV-99B1, EhV-208 and EhV-86). The infectious- and total virus particles were monitored by most probable number (MPN) and flow cytometry (FCM), respectively. Our results presented variations in both infectious and total virus particle production when the three host strains were infected by virus strain EhV-99B1. The two other virus strains, EhV -208 and -86, were only able to propagate on one host strain (CCMP374), but induced a reduction in growth on the other two host strains (CCMP371 and B). EhV -208 and -86 were in this study defined as specialist viruses, however, they did not present any beneficial traits that exceeded the generalist virus EhV-99B1, suggesting the presence of other traits that allow them to persist. On the other hand, the host strains displayed killing the winner dynamics, but further investigations are necessary. Additionally, this study assessed how the three host strains responded to EhVs sampled during a mesocosm experiment, where an E. huxleyi bloom crashed by viral lysis. The same EhV genotype was observed throughout the bloom and was phylogenetically distinct from EhV -208 and -86, but despite this, presented equivalent infectious pattern. At last, we were able to confirm that the burst sizes diminished with increasing MOI, which ultimately led to the same number of virus particles, regardless of the initial MOI. We proposed viral enhanced extracellular vesicles (EVs), which are actively produced by infected cells, as the causative agent for both the reduced growth in resistant host cultures and the diminishing burst sizes with increasing MOIs

Ecological significance of extracellular vesicles in modulating host-virus interactions during algal blooms

Extracellular vesicles are produced by organisms from all kingdoms and serve a myriad of functions, many of which involve cell-cell signaling, especially during stress conditions and host-pathogen interactions. In the marine environment, communication between microorganisms can shape trophic level interactions and population succession, yet we know very little about the involvement of vesicles in these processes. In a previous study, we showed that vesicles produced during viral infection by the ecologically important model alga Emiliania huxleyi, could act as a pro-viral signal, by expediting infection and enhancing the half-life of the virus in the extracellular milieu. Here, we expand our laboratory findings and show the effect of vesicles on natural populations of E. huxleyi in a mesocosm setting. We profile the small-RNA (sRNA) cargo of vesicles that were produced by E. huxleyi during bloom succession, and show that vesicles applied to natural assemblages expedite viral infection and prolong the half-life of this major mortality agent of E. huxleyi. We subsequently reveal that exposure of the natural assemblage to E. huxleyi-derived vesicles modulates not only host-virus dynamics, but also other components of the microbial food webs, thus emphasizing the importance of extracellular vesicles to microbial interactions in the marine environment

Removal of large viruses and their dispersal through fecal pellets of the appendicularian Oikopleura dioica during Emiliania huxleyi bloom conditions

Despite their importance in shaping the structure and function of marine microbial food webs, little is known about factors regulating marine virus abundance. Previous work demonstrated clearance of laboratory-cultured Emiliania huxleyi virus by the appendicularian Oikopleura dioica; however, the applicability of this interaction to natural virus assemblages was not investigated. Here, we conducted controlled laboratory experiments using O. dioica and mesocosm water containing natural virus assemblages with high densities of virus, and measured removal of virus by O. dioica using both flow cytometry and molecular methods. Bayesian models based on flow cytometry quantification of virus particles demonstrated efficient removal of viruses (mean 90.3 mL ind−1 d−1), with a clearance efficiency of 42.6% relative to food algae. Molecular detection of virus removal by quantification of viral mcp gene copies revealed a mean clearance rate of 68.1 mL ind−1 d−1. Fecal pellets from these experiments demonstrated that viruses in fecal pellets retain infectivity despite passage through the O. dioica gut. Shotgun metavirome analysis demonstrated O. dioica removal of large virus groups, notably the Phycodnaviridae. The results demonstrate the removal of E. huxleyi virus from natural virus assemblages by O. dioica and the maintenance of viral infectivity when incorporated into fecal pellets, prompting further investigation on the fate of fecal-packaged viruses and their impact on host dynamics. Furthermore, our results indicate the generality of this interaction for other large algal viruses, raising questions about the implications of this mechanism of marine virus redistribution on the broader marine virus community

Adaptive evolution of viruses infecting marine microalgae (haptophytes), from acute infections to stable coexistence

Collectively known as phytoplankton, photosynthetic microbes form the base of the marine food web, and account for up to half of the primary production on Earth. Haptophytes are key components of this phytoplankton community, playing important roles both as primary producers and as mixotrophs that graze on bacteria and protists. Viruses influence the ecology and diversity of phytoplankton in the ocean, with the majority of microalgae–virus interactions described as ‘boom and bust’ dynamics, which are characteristic of acute virus–host systems. Most haptophytes are, however, part of highly diverse communities and occur at low densities, decreasing their chance of being infected by viruses with high host specificity. Viruses infecting these microalgae have been isolated in the laboratory, and there are several characteristics that distinguish them from acute viruses infecting bloom-forming haptophytes. Herein we synthesise what is known of viruses infecting haptophyte hosts in the ocean, discuss the adaptive evolution of haptophyte-infecting viruses -from those that cause acute infections to those that stably coexist with their host - and identify traits of importance for successful survival in the ocean

Removal of large viruses and their dispersal through fecal pellets of the appendicularian Oikopleura dioica during Emiliania huxleyi bloom conditions

Despite their importance in shaping the structure and function of marine microbial food webs, little is known about factors regulating marine virus abundance. Previous work demonstrated clearance of laboratory-cultured Emiliania huxleyi virus by the appendicularian Oikopleura dioica; however, the applicability of this interaction to natural virus assemblages was not investigated. Here, we conducted controlled laboratory experiments using O. dioica and mesocosm water containing natural virus assemblages with high densities of virus, and measured removal of virus by O. dioica using both flow cytometry and molecular methods. Bayesian models based on flow cytometry quantification of virus particles demonstrated efficient removal of viruses (mean 90.3 mL ind−1 d−1), with a clearance efficiency of 42.6% relative to food algae. Molecular detection of virus removal by quantification of viral mcp gene copies revealed a mean clearance rate of 68.1 mL ind−1 d−1. Fecal pellets from these experiments demonstrated that viruses in fecal pellets retain infectivity despite passage through the O. dioica gut. Shotgun metavirome analysis demonstrated O. dioica removal of large virus groups, notably the Phycodnaviridae. The results demonstrate the removal of E. huxleyi virus from natural virus assemblages by O. dioica and the maintenance of viral infectivity when incorporated into fecal pellets, prompting further investigation on the fate of fecal-packaged viruses and their impact on host dynamics. Furthermore, our results indicate the generality of this interaction for other large algal viruses, raising questions about the implications of this mechanism of marine virus redistribution on the broader marine virus community

Removal of large viruses and their dispersal through fecal pellets of the appendicularian Oikopleura dioica during Emiliania huxleyi bloom conditions

Despite their importance in shaping the structure and function of marine microbial food webs, little is known about factors regulating marine virus abundance. Previous work demonstrated clearance of laboratory-cultured Emiliania huxleyi virus by the appendicularian Oikopleura dioica; however, the applicability of this interaction to natural virus assemblages was not investigated. Here, we conducted controlled laboratory experiments using O. dioica and mesocosm water containing natural virus assemblages with high densities of virus, and measured removal of virus by O. dioica using both flow cytometry and molecular methods. Bayesian models based on flow cytometry quantification of virus particles demonstrated efficient removal of viruses (mean 90.3 mL ind−1 d−1), with a clearance efficiency of 42.6% relative to food algae. Molecular detection of virus removal by quantification of viral mcp gene copies revealed a mean clearance rate of 68.1 mL ind−1 d−1. Fecal pellets from these experiments demonstrated that viruses in fecal pellets retain infectivity despite passage through the O. dioica gut. Shotgun metavirome analysis demonstrated O. dioica removal of large virus groups, notably the Phycodnaviridae. The results demonstrate the removal of E. huxleyi virus from natural virus assemblages by O. dioica and the maintenance of viral infectivity when incorporated into fecal pellets, prompting further investigation on the fate of fecal-packaged viruses and their impact on host dynamics. Furthermore, our results indicate the generality of this interaction for other large algal viruses, raising questions about the implications of this mechanism of marine virus redistribution on the broader marine virus community.publishedVersio