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Appl Environ Microbiol, February 1998, p. 431-438, Vol. 64, No. 2
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Significance of Viral Lysis and Flagellate Grazing as Factors
Controlling Bacterioplankton Production in a Eutrophic
Lake
Markus G.
Weinbauer* and
Manfred G.
Höfle
GBF
National Research Center of
Biotechnology, AG Molecular Microbial Ecology, D-38124
Braunschweig, Germany
Received 19 August 1997/Accepted 14 November 1997
The effects of viral lysis and heterotrophic nanoflagellate (HNF)
grazing on bacterial mortality were estimated in a eutrophic lake (Lake Plußsee in northern Germany) which was separated by a steep
temperature and oxygen gradient into a warm and oxic epilimnion and a
cold and anoxic hypolimnion. Two transmission electron
microscopy-based methods (whole-cell examination and thin sections)
were used to determine the frequency of visibly infected cells,
and a model was used to estimate bacterial mortality due to viral
lysis. Examination of thin sections also showed that between 20.2 and
29.2% (average, 26.1%) of the bacterial cells were empty (ghosts) and
thus could not contribute to viral production. The most important
finding was that the mechanism for regulating bacterial production
shifted with depth from grazing control in the epilimnion to control
due to viral lysis in the hypolimnion. We estimated that in the
epilimnion viral lysis accounted on average for 8.4 to 41.8% of the
summed mortality (calculated by determining the sum of the mortalities due to lysis and grazing), compared to 51.3 to 91.0% of the summed mortality in the metalimninon and 88.5 to 94.2% of the summed mortality in the hypolimnion. Estimates of summed mortality values indicated that bacterial production was controlled completely or almost
completely in the epilimnion (summed mortality, 66.6 to 128.5%) and
the hypolimnion (summed mortality, 43.4 to 103.3%), whereas in the
metalimnion viral lysis and HNF grazing were not sufficient to control
bacterial production (summed mortality, 22.4 to 56.7%). The
estimated contribution of organic matter released by viral lysis
of cells into the pool of dissolved organic matter (DOM) was low;
however, since cell lysis products are very likely labile compared to
the bulk DOM, they might stimulate bacterial production. The high
mortality of bacterioplankton due to viral lysis in anoxic water
indicates that a significant portion of bacterial production in the
metalimnion and hypolimnion is cycled in the bacterium-virus-DOM loop.
This finding has major implications for the fate and cycling of organic
nutrients in lakes.
*
Corresponding author. Mailing address: GBF
National
Research Center of Biotechnology, AG Molecular Microbial Ecology,
Mascheroder Weg 1, D-38124 Braunschweig, Germany. Phone:
49-531-6181-440. Fax: 49-531-6181-411. E-mail: mgw{at}gbf.de.
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