Grazing and virus-induced mortality of microbial populations before and during the onset of annual hypoxia in Lake Erie

Abstract

Lake Erie is the most productive of the North American Great Lakes and experiences annual periods of hypolimnetic hypoxia with unknown consequences for the microbial food web. We established the abundances and mortality rates of microbes in Lake Erie during thermal stratification and determined how they varied with changes in bottom-water dissolved oxygen concentrations. The microbial plankton community (heterotrophic bacteria, Cyanobacteria, eukaryotic phytoplankton, nanozooplankton, microzooplankton) was quantified in surface and bottom waters along with measurements of herbivory and bacterivory rates on eukaryotic and prokaryotic picoplankton and rates of viral lysis of bacteria. High rates of grazing mortality of prokaryotic picoplankton (1.4 ± 0.6 d ) and significant correlations between microzooplankton abundances and all picoplankton populations quantified demonstrated the strong impacts of grazing on Lake Erie picoplanktonic communities. Microbial herbivory accounted for half of total phytoplankton mortality per day. Bacterivory and viral lysis turned over 85% of the heterotrophic bacterial community each day. During the onset of hypolimnetic hypoxia, abundances of ciliates and rotifers decreased significantly and herbivory was undetectable. Concurrently, bacterivory persisted at rates equal to those found in shallower oxygenated waters, and abundances of heterotrophic nanoflagellates did not change significantly. These results suggest that, during hypoxia events in Lake Erie, herbivory by microzooplankton is disrupted, but bacterivory by heterotrophic nanoflagellates persists. Finally, rates of viral lysis of heterotrophic bacteria were higher in the hypolimnion than in surface waters, suggesting that increased viral lysis may enhance regeneration of organic matter in bottom waters during hypoxic events.

Seasonal variation in viral-induced mortality of bacterioplankton in the water column of a large mesotrophic lake (Lake Biwa, Japan)

“…*p < 0.05, **p < 0.01, ***p < 0.001. FVIC = frequency of visibly infected cells, HNF = heterotrophic nanoflagellate, ns = not significant by inadequacies in knowledge regarding factors that control variation in viral parameters (abundance, infectivity, and viral-induced mortality of bacteria), especially in freshwater lakes (Gobler et al 2008, Pradeep Ram et al 2009). Data are also scarce regarding the relative importance of viral lysis and HNF grazing as mortality forces of bacteria.…”

High Lytic Infection Rates but Low Abundances of Prokaryote Viruses in a Humic Lake (Vassivière, Massif Central, France)

“…Recent studies have suggested that viral lytic infection contributes significantly to the bulk of prokaryotic mortality in aquatic ecosystems, and the VIC (expressed as the percentage of total prokaryotic cells) is a measure of the magnitude of this process (16,38). We used the transmission electron microscopy (TEM) method (i.e., whole-cell approach) for the determination of the VIC, which provided direct evidence of phage infection.…”

“…As lakes are characterized by steep changes in environmental gradients over depth, most of the studies on viral ecology have focused on the vertical spatial variability (11,16,37) rather than trophic gradients (13). The apparent positive relation reported between viral lytic infection and the trophic state of aquatic systems is based more upon extrapolation than on direct measurements (12,14).…”

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

“…The importance of viruses as agents of freshwater phytoplankton mortality has also been established through a number of modified dilution experiments (Gobler et al, 2008;Tijdens et al, 2008;Staniewski et al, 2012), and algal virus diversity surveys of various lakes and rivers have been conducted (e.g., Short and Short, 2008;Clasen and Suttle, 2009;Gimenes et al, 2012;Zhong and Jacquet, 2014).…”