%0 Journal Article %J Environmental Microbiology %D 2021 %T Seasonal dynamics of natural Ostreococcus viral infection at the single cell level using VirusFISH %A Castillo, Yaiza M. %A Forn, Irene %A Yau, Sheree %A Morán, Xosé Anxelu G. %A Alonso-Sáez, Laura %A Arandia-Gorostidi, Néstor %A Vaqué, Dolors %A Sebastián, Marta %K RCC2590 %K RCC4221 %K RCC809 %X Ostreococcus is a cosmopolitan marine genus of phytoplankton found in mesotrophic and oligotrophic waters, and the smallest free-living eukaryotes known to date, with a cell diameter close to 1 μm. Ostreococcus has been extensively studied as a model system to investigate viral–host dynamics in culture, yet the impact of viruses in naturally occurring populations is largely unknown. Here, we used Virus Fluorescence in situ Hybridization (VirusFISH) to visualize and quantify viral-host dynamics in natural populations of Ostreococcus during a seasonal cycle in the central Cantabrian Sea (Southern Bay of Biscay). Ostreococcus were predominantly found during summer and autumn at surface and 50 m depth, in coastal, mid-shelf and shelf waters, representing up to 21% of the picoeukaryotic communities. Viral infection was only detected in surface waters, and its impact was variable but highest from May to July and November to December, when up to half of the population was infected. Metatranscriptomic data available from the mid-shelf station unveiled that the Ostreococcus population was dominated by the species O. lucimarinus. This work represents a proof of concept that the VirusFISH technique can be used to quantify the impact of viruses on targeted populations of key microbes from complex natural communities. This article is protected by copyright. All rights reserved. %B Environmental Microbiology %V n/a %G eng %U http://sfamjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1462-2920.15504 %R 10.1111/1462-2920.15504 %0 Journal Article %J Frontiers in Microbiology %D 2020 %T Visualization of viral infection dynamics in a unicellular eukaryote and quantification of viral production using virus fluorescence in situ hybridization %A Castillo, Yaiza M. %A Sebastián, Marta %A Forn, Irene %A Grimsley, Nigel %A Yau, Sheree %A Moraru, Cristina %A Vaqué, Dolors %K culture system %K marine picoeukaryote %K Ostreococcus tauri %K Ostreococcus tauri virus 5 %K RCC4221 %K virus fluorescence in situ hybridization %K virus-host interactions %X One of the major challenges in viral ecology is to assess the impact of viruses in controlling the abundance of specific hosts in the environment. To this end, techniques that enable the detection and quantification of virus-host interactions at the single-cell level are essential. With this goal in mind, we implemented virus fluorescence in situ hybridization (VirusFISH) using as a model the marine picoeukaryote Ostreococcus tauri and its virus Ostreococcus tauri virus 5 (OtV5). VirusFISH allowed the visualization and quantification of the proportion of infected cells during an infection cycle in experimental conditions. We were also able to quantify the abundance of free viruses released during cell lysis, discriminating OtV5 from other mid-level fluorescence phages in our non-axenic infected culture that were not easily distinguishable with flow cytometry. Our results showed that although the major lysis of the culture occurred between 24 and 48 h after OtV5 inoculation, some new viruses were already produced between 8 and 24 h. With this work, we demonstrate that VirusFISH is a promising technique to study specific virus-host interactions in non-axenic cultures and establish a framework for its application in complex natural communities. %B Frontiers in Microbiology %V 11 %P 1559 %8 jul %G eng %U https://www.frontiersin.org/article/10.3389/fmicb.2020.01559/full %R 10.3389/fmicb.2020.01559