AME 30:207-219 (2003)  -  doi:10.3354/ame030207

Direct estimates of the contribution of viral lysis and microzooplankton grazing to the decline of a Micromonas spp. population

Claire Evans1,2,3, Stephen D. Archer1,*, Stéphan Jacquet4,5, William H. Wilson1,2,6

1Plymouth Marine Laboratory, Prospect Place, Plymouth PL1 3DH, United Kingdom
2Marine Biological Association, Citadel Hill, Plymouth PL1 2PB, United Kingdom
3School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom
4Department of Microbiology, University of Bergen, Jahnebakken 5, 5020 Bergen, Norway
5Station INRA d¹Hydrobiologie Lacustre, 75 Avenue de Corzent, BP 511, 74203 Thonon-les-Bains, France
6Department of Biological Sciences, University of Stirling, Stirling FK9 4LA, United Kingdom
*Corresponding author. Email:

ABSTRACT: During a mesocosm study in Raunefjorden, Norway, a Micromonas spp. population, initially showing exponential net growth, dramatically declined after Day 4 of the experiment. Using a modification of the dilution approach originally developed to quantify grazing by microzooplankton on phytoplankton, it was possible to partition the mortality of Micromonas spp. between grazing and viral lysis on Days 5, 6 and 7 during the population decline. Parallel dilution experiments were carried out in which 0.2 μm- and 10 kDa-filtered water was used as the diluents. In this way, gradients of grazing pressure (0.2 μm series) and grazing pressure + viral concentration (10 kDa series) were produced. Model 1 linear regression of the fraction of whole water versus the apparent growth rate of chlorophyll a and Micromonas spp. provided an estimate of mortality in the 0.2 μm and 10 kDa dilution series. On Days 5, 6 and 7, the slopes of the linear regressions of 0.2 μm and 10 kDa dilution series were significantly different at p = 0.083, 0.001 and 0.093 respectively. From the differences in slope between the series, estimates of viral mortality amounted to a turnover rate of the Micromonas spp. standing stocks of 10, 25 and 9% d-1. This compares to a turnover rate by the microzooplankton of 48, 26 and 23% d-1. On all 3 d the combined viral lysis and grazing mortality exceeded estimates of the potential production of Micromonas spp., in part accounting for the population decline. This study demonstrates that the dilution approach can be adapted to directly determine virus-induced mortality rates of specific phytoplankton. However, further work is required to determine how the specificity of viral infection and variety of viral infection cycles affect the results of this modified dilution approach when applied to other phytoplankton taxa and communities.

KEY WORDS: Viral lysis · Microzooplankton grazing · Micromonas spp. · Dilution technique

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