There are an estimated 1030 viruses in the ocean and viruses are a major source of mortality for phytoplankton. However, measuring viral-induced mortality in practice is difficult. In our latest paper published in Frontiers in Microbiology, we evaluate the modified dilution method – used by oceanographers to measure rates of viral-induced mortality – using theory and ecological simulations. We also test a viral dilution method, in which only viruses are diluted.
Within the models we test, we find that mortality rate estimates may be improved by shortening incubation times – with the caveat that the incubation period must be longer than the infection latent period. The latent period may not be known for environmental measurements. We also find that phytoplankton niche competition may influence the robustness of dilution based measurements of viral-induced mortality – this complements previous work in which we found niche competition may be conflated with grazing rate measurements.
We also investigated how variability between phytoplankton strains might affect the ability to measure mortality rates using dilution based methods. As different strains may have different growth dynamics and abundances, the ability of the population (non-differentiable strains) to recover from a dilution perturbation may be non-linear. In particular, we find the signal of mortality rates may be dominated by the fastest growing strains which are able to recover faster from the effects of dilution. This signal may not be reflective of the mortality rates in the sampled environment.
Population dynamics of two phytoplankton strains, P1 (green line) and P2 (red line). Before the dilution perturbation (time 0h) P1 has a low population density relative to P2. However, P1 has a faster growth rate than P2 and is able to initially dominate the growth rate signal of the joint population (black line) following dilution.
Overall, whilst we find the viral dilution method may perform better than the modified dilution method in silico, our simulations highlight some of the difficulties in measuring viral-induced mortality using dilution based techniques. Read more in our new paper:
Beckett S.J. and Weitz J.S. 2018. The Effect of Strain Level Diversity on Robust Inference of Virus-Induced Mortality of Phytoplankton. Frontiers in Microbiology 9:1850.
Phage on toast 