Seasonal variations of virus- and nanoflagellate-mediated mortality of heterotrophic bacteria in the coastal ecosystem of subtropical western Pacific

Abstract

Since viral lysis and nanoflagellate grazing differ in their impact on the aquatic food web, it is important to assess the relative importance of both bacterial mortality factors. In this study, an adapted version of the modified dilution method was applied to simultaneously estimate the impact of both virus and nanoflagellate grazing on the mortality of heterotrophic bacteria. A series of experiments was conducted monthly from April to December 2011 and April to October 2012. The growth rates of bacteria we measured ranged from 0.078 h⁻¹ (April 2011) to 0.42 h⁻¹ (September 2011), indicating that temperature can be important in controlling the seasonal variations of bacterial growth. Furthermore, it appeared that seasonal changes in nanoflagellate grazing and viral lysis could account for 34% to 68% and 13% to 138% of the daily removal of bacterial production, respectively. We suggest that nanoflagellate grazing might play a key role in controlling bacterial biomass and might exceed the impact of viral lysis during the summer period (July to August) because of the higher abundance of nanoflagellates at that time. Viral lysis, on the other hand, was identified as the main cause of bacterial mortality between September and December. Based on these findings in this study, the seasonal variations in bacterial abundance we observed can be explained by a scenario in which both growth rates and loss rates (grazing + viral lysis) influence the dynamics of the bacteria community

Halobacteriovorax, an underestimated predator on bacteria: potential impact relative to viruses on bacterial mortality

“…A subset of this effort has been devoted to assessing the role of viruses and protists in bacterial mortality. Predation on bacteria in marine, estuarine and freshwater systems has been the subject of many studies in the past two decades (Proctor and Fuhrman 1990;Fuhrman and Noble 1995;Weinbauer and Höfle, 1998;Tsai et al, 2013), prompted largely by the report of Proctor and Fuhrman (1990) that there are 10 6 viruses per ml of seawater in the world's oceans. These studies have shown viruses to be major causative agents of bacterial mortality, sometimes exceeding the importance and activity of protistan grazers in aquatic systems.…”

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing – an overview and synthesis

“…4), because it modifies the export flux and its chemical composition. Hung et al (2013) uses a resuspension model, which is able to provide an estimate of the resuspended fraction collected in sediment traps. This fraction amounts from 27 to 93 % of the total flux.…”

“…Schematic diagram showing the major biogeochemical features in the river-shelf-boundary current systems observed in studies of this special issue. Riverine nutrients-induced phytoplankton bloom in the river plume draws down pCO 2 and produces high particulate organic carbon (POC) flux (Hung et al, 2013), which causes strong community respiration probably driven by active resuspension (Hung et al, 2013). The elevated particle concentrations near the river mouth sustain exceptionally high microbial activities, which include nitrification processes facilitated probably by the particle-borne metal oxides .…”

“…Hung et al (2013) describes the particulate flux of organic carbon from the sea surface to the bottom as measured by sediment traps during summer. One major caveat of these measurements on continental shelves is sediment resuspension (Fig.…”

The effect of strain level diversity on robust inference of virus-induced mortality

“…The modified dilution method (MDiM) (Evans et al, 2003;Kimmance and Brussaard, 2010) is used to quantify the impact of both grazing and viral lysis on microbial populations. Empirical measurements made via the MDiM suggest that both grazing and viral lysis are important mortality drivers of marine microbes; and that both can be the dominant source of mortality (Tsai et al, 2013;Mojica et al, 2015;Pasulka et al, 2015). Yet, the MDiM can yield negative rates of viral lysis (Pasulka et al, 2015) and there are concerns about the ability to detect viral impact e.g.…”