AME 15:103-113 (1998)  -  doi:10.3354/ame015103

Size-specific mortality of lake bacterioplankton by natural virus communities

Markus G. Weinbauer*, Manfred G. Höfle

GBF-National Research Center of Biotechnology, AG Microbial Ecology, Mascheroder Weg 1, D-38124 Braunschweig, Germany

ABSTRACT: The potential effect that viral lysis has on the cell size distribution of bacterioplankton was investigated during late summer stratification in Lake Plußsee, Germany. Size-specific bacterial mortality due to viral lysis was estimated from in situ samples by a transmission electron microscopy based examination of visibly infected cells (VIC) and in an experiment with varying concentrations of the natural virus community. In all depth layers the highest percentage of cells was found in a cell length class that was smaller for the entire bacterial community (0.3-0.6 μm) than for VIC (0.6-0.9 μm). For cells <2.4 μm the highest frequency of VIC (FVIC) was detected in the size classes 0.6-0.9 and 0.9-1.2 μm, and the FVIC was high in the size classes 1.2-1.5 (all depth layers) and 1.5-1.8 μm (meta- and hypolimnion). The estimated mortality due to viral lysis in these size classes was significant with maxima of 29 to 55% in the epilimnion, 30 to 59% in the metalimnion and 56 to 107% in the hypolimnion. In all depth layers the FVIC of bacteria <0.3 μm in length was ca 30% of that averaged for the entire bacterial community, and in the experiment the percentage of cells <0.3 μm was highest in enclosures with high viral activity. In the experiment the average cell size was smaller in enclosures with high than in that with low viral activity. The data demonstrate that being small could be a strategy of cells to reduce mortality due to viral lysis probably by reducing the contact rates with viruses. Thus, viral lysis could be one of the mechanisms keeping the cell size small in aquatic ecosystems. In oxic water cells in the largest size class (>2.4 μm) were not infected with viruses, and in enclosures with epilimnetic lake water the percentage of cells >2.4 μm was highest in enclosures with highest viral abundance, suggesting that resistance against infection favored large cells. However, in the meta- and hypolimnion the FVIC was high for cells >2.4 μm and, since the burst size increased with bacterial cell size, lysis of large cells could contribute significantly to viral production. Also, a major portion of biomass was found in cells >2.4 μm. The finding that viral lysis is size-specific and can affect the cell size distribution of bacteria in lake water has important implications for our understanding of the mechanisms which regulate bacterial production and nutrient cycling in pelagic environments.

KEY WORDS: Bacterioplankton size structure · Size-specific viral lysis · Lake Plußsee

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