Significance of bacterivory and viral lysis in bottom waters of Franklin Bay, Canadian Arctic, during winter

Abstract

Little information is currently available about water column microbial processes or mortality during Arctic winter. To address this paucity, we used epifluorescence microscopy and dilution experiments to determine the abundance of flagellates, bacteria and virus-like particles (VLP) and the rates of bacterial growth, bacterivory and virus-induced mortality in subzero-temperature bottom waters (≤ 230 m) of Franklin Bay during February and March 2004, when ice-covered surface waters were highly oligotrophic (maximum chlorophyll a value of 0.09 µg l -1 ). We focused on bottom waters due to the possible importance of sediment resuspension as a source of organic matter. While flagellates were present at low densities (1.5 to 3.1 × 10 2 ml -1 ), bacterial concentrations resembled those from other seasons in the region and increased over the 5 wk sampling period, from 1.4 × 10 5 to 3.0 × 10 5 ml -1 . VLPs were typically an order of magnitude more abundant than bacteria (range of 1.4 to 4.5 × 10 6 VLP ml -1 ) and, like the fraction of particle-associated bacteria (but not total bacteria), correlated with particulate organic carbon concentration (r s = 0.82, p < 0.04, n = 7). Grazing rates, whether measured in dilution experiments or calculated from flagellate abundance, were low or undetectable (maximum of -0.004 h -1 ). Of 3 parallel experiments, 2 yielded substantial virus-induced mortality (-0.006 to -0.015 h -1 ), comparable to or exceeding the intrinsic bacterial growth rate (0.010 h -1 in both experiments) and suggesting viruses were the more important agents of bacterial mortality under these conditions. Using a viral reduction approach, VLP production measured in the water column or ice-moored sediment traps was commonly low (0.3 to 7.7 × 10 4 VLP ml -1 h -1 ) or undetectable, highly variable among replicates and, when measurable, implied viral turnover times between 0.9 and 12 d. In general, our results show that, despite the oligotrophy of Arctic winter, bottom water bacterial communities can remain active and subject to viral predation.KEY WORDS: Virus · VLP · Lysis · Bacterivory · Arctic · Winter · Franklin Bay · Particles Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 43: [209][210][211][212][213][214][215][216][217][218][219][220][221] 2006 ditions with high (> 50%) virus-induced mortality (Murray & Eldridge 1994). Yet such conditions favor lower concentrations of bacteria and viruses (compared to eutrophic circumstances, e.g. Cochlan et al. 1993, Hewson et al. 2001, correspondingly lower encounter rates (Murray & Jackson 1992), reduced virus production and/or virus-induced mortality , Weinbauer & Peduzzi 1995, Guixa-Boixereu et al. 2002 and, perhaps, a greater likelihood of lysogeny (Weinbauer & Suttle 1999, Weinbauer et al. 2003.Viruses may exert a more subtle influence by generating a relatively labile lysate which, though a small component of DOM, may nonetheless be an important source of organic matter to other microorganisms (Middelboe et ...

Experimental evaluation of the warming effect on viral, bacterial and protistan communities in two contrasting Arctic systems

“…This has not been observed in previous studies (Säwström et al 2007b, De Corte et al 2011. Finally, protist grazing in the Arctic community was higher than bacterial mortality due to viruses, as shown in Boras et al (2010), unlike in the Fjord community, where the impact of viruses on bacterial communities was higher than that of bacterivores, as shown by Wells & Deming (2006). These results suggest that both types of bacterial 'predators' could play an important role in controlling bacterial communities.…”

“…Virus and bacterial numbers from the VP incubations were counted by flow cytometry. The number of viruses released by bacterial cells (burst size) was estimated from VP measurements, as in Middelboe & Lyck (2002) and Wells & Deming (2006). The increase in VA over short time intervals (4 h) in VP incubations was divided by the decrease in BA in the same time period.…”

Importance of the Viral Shunt in Nitrogen Cycling in Synechococcus Spp. Growth in Subtropical Western Pacific Coastal Waters

“…Viruses are pervasive in marine environments, causing microbial mortality at a rate equaling that caused by grazing (Steward et al 1996;Weinbauer and Höfle 1998;Wells and Deming 2006). Experimental studies show that although an increase in virus abundance can reduce growth rates in phytoplankton (Suttle et al 1990), viral lysis can ultimately lead to higher phytoplankton productivity (Peduzzi and Weinbauer 1993).…”

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

“…Most studies previously examining the impact of grazing versus viral lysis have focused on bacterial populations (Weinbauer and Peduzzi, 1995;Steward et al, 1996;Weinbauer and Höfle, 1998;Pedros-Alio et al, 2000;Jacquet et al, 2005;Fischer et al, 2006;Wells and Deming, 2006). These studies have shown that both viral lysis and grazing can cause significant mortality, but that the impact of each varies by season, host organism, and environmental conditions.…”

“…When bacterial cells are grazed, energy is made available to higher trophic levels, whereas when they are lysed, organic carbon and nutrients are dissolved and bacterial carbon is recycled as bacterial production in a closed loop (Middelboe et al, 1996;Wilhelm et al, 2002). Viruses have been shown to account for a high percentage of bacterial mortality in some marine environments (Weinbauer and Peduzzi, 1995;Wells and Deming, 2006;Taira et al, 2009), with percentages similar to those due to nanoflagellates when both sources of bacterial loss have been measured simultaneously (Fuhrman and Noble, 1995;Steward et al, 1996). Therefore, it is important to estimate both virus-and grazing-induced bacterial mortality to obtain better insight into the forces underlying nutrient flow within an aquatic food web.…”