Estimation of viral-induced phytoplankton mortality using the modified dilution method

Susan A. Kimmance and Corina P. D. Brussaard

Full Citation: Kimmance, S. A., and C. P. D. Brussaard. 2010. Estimation of viralinduced phytoplankton mortality using the modified dilution method, p. 65-73. In S. W. Wilhelm, M. G. Weinbauer, and C. A. Suttle [eds.], Manual of Aquatic Viral Ecology. ASLO. [DOI 10.4319/mave.2010.978-0-9845591-0-7.65]

ABSTRACT: The modified dilution assay aims to partition phytoplankton mortality into virus- versus grazing-induced fractions and has previously been applied to several different environments to determine viral lysis rates of natural phytoplankton. The method involves creating a gradient of both grazing and viral lysis by dilution with different proportions of grazer- and virus-free filtrate, and assessing the subsequent impact on phytoplankton growth rates. We have conducted a critical evaluation of this method, and reviewed published data sets obtained using this approach to examine the utility of the modified dilution assay for estimating viral mortality rates. We provide modifications and improvements that have been incorporated into the method since it was first developed, and suggest recommendations for improving experimental success in less productive oligotrophic environments. Published data show that viral lysis rates vary between different algal groups and in response to environmental conditions. Results also suggest that this method has the potential to be a useful tool for estimating the impact of viruses on phytoplankton populations, but that the measurement of natural, low viral lysis rates (<0.1 d–-1) can challenge the application of this approach. Ultimately, however, the limitation of this method is associated with dilution of specific phytoplankton populations at low abundance.