Seasonal dynamics in the abundance of Micromonas pusilla (Prasinophyceae) and its viruses in the Gulf of Naples (Mediterranean Sea)

To assess the role of viruses in the bloom dynamics of Micromonas pusilla in the Gulf of Naples (Mediterranean Sea), variations of host and virus abundance were followed over one annual cycle and in late winter-spring of three consecutive years. Micromonas pusilla was recorded from autumn to spring, with peak values up to 6.6 x 10(3) cells ml(-1), but was undetectable in summer. Free M. pusilla viruses were detectable in all seasons, with concentrations from 0.02 viruses ml(-1) to 1.9 X 10(3) viruses ml(-1), exceeding host abundances only in one case. We found a great intraspecific variability in host susceptibility to viruses present in natural samples, with viral titres ranging over one or two orders of magnitude for the same samples incubated on different M. pusilla strains. Over the winter-spring periods, a highly dynamic situation was evident, with wide fluctuations for both host and virus abundances from one week to another. In some cases, peak host concentrations were accompanied by an increase in viral numbers, whereas in other cases the respective fluctuations were uncoupled. Although fluctuations of M. pusilla abundance could be influenced by viral infection, there was no evidence that viruses were able to terminate host blooms. The summer decline of M. pusilla populations did not appear to be related to the impact of viral infection.