Viral abundance, production, decay rates and life strategies (lysogeny versus lysis) in Lake Bourget (France)

Abstract

We have investigated the ecology of viruses in Lake Bourget (France) from January to August 2008. Data were analysed for viral and bacterial abundance and production, viral decay, frequency of lysogenic cells, the contribution of bacteriophages to prokaryotic mortality and their potential influence on nutrient dynamics. Analyses and experiments were conducted on samples from the epilimnion (2 m) and the hypolimnion (50 m), taken at the reference site of the lake. The abundance of virus-like particles (VLP) varied from 3.4 × 10⁷to 8.2 × 10⁷ VLP ml⁻¹; with the highest numbers and virus-to-bacterium ratio (VBR = 69) recorded in winter. Viral production varied from 3.2 × 10⁴ VLP ml⁻¹ h⁻¹ (July) to 2 × 10⁶ VLP ml⁻¹ h⁻¹ (February and April), and production was lower in the hypolimnion. Viral decay rate reached 0.12-0.15 day⁻¹, and this parameter varied greatly with sampling date and methodology (i.e. KCN versus filtration). Using transmission electron microscopy (TEM) analysis, viral lysis was responsible for 0% (January) to 71% (February) of bacterial mortality, while viral lysis varied between 0% (April) and 53% (January) per day when using a modified dilution approach. Calculated from viral production and burst size, the virus-induced bacterial mortality varied between 0% (January) and 68% (August). A weak relationship was found between the two first methods (TEM versus dilution approach). Interestingly, flow cytometry analysis performed on the dilution experiment samples revealed that the viral impact was mostly on high DNA content bacterial cells whereas grazing, varying between 8.3% (June) and 75.4% (April), was reflected in both HDNA and LDNA cells equally. The lysogenic fraction varied between 0% (spring/summer) and 62% (winter) of total bacterial abundance, and increased slightly with increasing amounts of mitomycin C added. High percentages of lysogenic cells were recorded when bacterial abundance and activity were the lowest. The calculated release of carbon and phosphorus from viral lysis reached up to 56.5 µgC l⁻¹ day⁻¹ (assuming 20 fgC cell⁻¹) and 1.4 µgP l⁻¹ day⁻¹ (assuming 0.5 fgP cell⁻¹), respectively, which may represent a significant fraction of bacterioplankton nutrient demand. This study provides new evidence of the quantitative and functional importance of the virioplankton in the functioning of microbial food webs in peri-alpine lakes. It also highlights methodologically dependent results.

Bacterial Growth and Mortality after Deposition of Saharan Dust and Mixed Aerosols in the Eastern Mediterranean Sea: A Mesocosm Experiment

“…Lysogeny may have been underestimated to some extent, as mitomycin C may not induce nutrient-limited cells (Williamson et al, 2002). Thomas et al (2011) pointed out that bacteria are differentially affected by mitomycin C and this inducing agent may also be toxic. Nevertheless, it is the most commonly used chemical agent for inducing lysis and it allows comparison with other studies.…”

Seasonal determinations of algal virus decay rates reveal overwintering in a temperate freshwater pond

“…Seasonality and variability in rates of decay Although other more general studies of aquatic virus decay have noted the lowest rates in the winter (Thomas et al, 2011), the winter incubation experiment with the freshwater algal viruses ATCV-1, CVM-1 and CpV-BQ1 produced the lowest estimated decay rates observed for any algal viruses in nature. It is likely that these unprecedented low rates of decay were due to the fact that the freshwater pond was frozen over during the winter incubation, which would dramatically reduce exposure to sunlight, especially when ice is covered by snow (Perovich et al, 1993;Bertilsson et al, 2013).…”

Coral Mucus Is a Hot Spot for Viral Infections

“…Alternatively, the centrifugation step of the TEM protocol may also have caused the disruption of virally infected cells. To cope with this last potential methodological bias, we concurrently estimated viral lytic activity with the KCN method, which is frequently used with planktonic and benthic cells (47,55,56). Given the high viscosity and the small volumes of mucus available (Ͻ10 ml), we could not apply the viral reduction approach to estimate viral lytic production rates (57).…”