Effect of Viral Infection on the Functioning and Lysis of Black Sea Microalgae Tetraselmis viridis (Chlorophyta) and Pha
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OPLANKTON, PHYTOBENTHOS, AND PHYTOPERIPHYTON
Effect of Viral Infection on the Functioning and Lysis of Black Sea Microalgae Tetraselmis viridis (Chlorophyta) and Phaeodactylum tricornutum (Bacillariophyta) L. V. Stelmakha, * and O. A. Stepanovab aKovalevskii b
Institute of Biology of the Southern Seas, Russian Academy of Sciences, Sevastopol, Russia Institute of Nature and Technical Systems, Russian Academy of Sciences, Sevastopol, Russia *e-mail: [email protected] Received June 5, 2019; revised July 22, 2019; accepted December 5, 2019
Abstract—Experimental studies have been performed on Tetraselmis viridis (Rouchijajnen) R.E. Norris, Hori & Chihara (Chlorophyta) and Phaeodactylum tricornutum Bohlin (Bacillariophyta) cultures of Black Sea microalgae using TsV-S1 and PtV-S18 algal virus strains isolated from the Black Sea ecosystem. The study assesses the effects of light intensity and the initial abundance of microalgae cells on the onset of their infection by viruses and the decline in their abundance. As early as on the second or third day, the cells changed their shape and increased in volume in the cultures infected by viruses. At this time, a decrease was observed in chlorophyll a red autofluorescence, variable fluorescence per cell, and photochemical efficiency of photosystem 2. In the studied cultures, a reduction in cell abundance due to viral lysis was noted 1 to 2 days later. The threshold cell abundance at which this process was observed was independent of light conditions and amounted to 3 × 105 cells/mL for T. viridis and 18 × 105 cells/mL for Ph. tricornutum. The complete lysis of algae cells was detected by the end of the fourth to sixth day. In the dark, the effect of viruses on microalgae was insignificant. Keywords: marine microalgae, algal viruses, phytoplankton, Black Sea DOI: 10.1134/S1995082920020303
INTRODUCTION Viruses are among the most important components of aquatic ecosystems. There are from 105 to 109 viral particles in every 1 mL of seawater (Stepanova, 2017; Suttle, 2007). Most of them attack bacteria, as well as macro- and microalgae. The main causes of the functionally active cell death in marine phytoplankton include consumption by zooplankton and lysis due to exogenous viral infection (Beckett and Weitz, 2018; Bidle et al., 2007; Pasulka et al., 2015). Viral lysis is considered the primary cause of death of ~3–5% cells in phytoplankton daily (Wommack and Colwell, 2000); this number can peak at 25–30% during particular periods of time (Evans et al., 2003; Mojica et al., 2016). The rather rapid termination of the algal bloom, which is caused by particular microalgae species, may result from large abundance of viruses in the bloom zones and, therefore, the viral lysis of plant cells (Suttle, 2007; Cottrell and Suttle, 1995; Bratbak et al., 1993, 1996; Baudoux et al., 2006, 2008; Vardi et al., 2009; Lehahn et al., 2014). Phytoplankton organic matter, entering the external environment as a result of Abbreviation: F0/cell – the initial level of variable fluorescence; Fv/Fm – photochemic
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