ABSTRACT: The influence of viruses on bacterial net growth and respiration was investigated in batch cultures with natural assemblages of marine bacterioplankton, which were manipulated with respect to abundance of natural virioplankton. In 1 set of cultures (-virus), a virus-free water sample (0.02 µm filtered) was inoculated with a bacterioplankton concentrate, and in a parallel set of cultures (control) a virus-containing water sample (0.2 µm filtered) was inoculated with the bacterioplankton concentrate. The 0.02 µm filtration procedure reduced viral abundance by 62 to 92% in the -virus cultures relative to the parallel control cultures with the natural density of viruses (i.e. the fraction of natural viruses <0.2 µm). This approach allowed us to examine the effects of reduced viral densities on the production of natural assemblages of bacteria and viruses and on the distribution of added ³H-thymidine into size fractions (the bacterial size fraction, viral size fraction, dissolved size fraction and respired fraction). The results showed significantly higher bacterial net growth and growth efficiency in cultures with a reduced abundance of viruses relative to control cultures with natural viral abundance, and indicated viral regulation of bacterial abundance in the control cultures. We suggest that viral lysis significantly affected the bacterial carbon cycling in the cultures by liberating a fraction of the organic matter already taken up by the bacteria, thus stimulating recycling of bacterial carbon and reducing the net bacterial production. The implications of such regeneration of dissolved organic matter by viral lysis for pelagic carbon cycling and for measurements of bacterial production are discussed.

KEY WORDS: Viruses · Bacterioplankton · Recycling · Dissolved organic matter

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Published in AME Vol. 27, No. 2 (2002) on March 15
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 2002