The submicron size fraction of seawater containing high numbers of virus particles as bioactive agent in unicellular plankton community successions

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Abstract
The effect of moderately (2.5 times) increasing the concentration of the submicron size fraction (2–200 nm) of seawater on unicellular heterotrophic plankton organisms was tested in a series of experiments during algal bloom conditions. Using electron microscopy, vims-like particles (VLPs) were by far the most visible component in this size fraction. We incubated seawater. which was collected in late winter, in slowly rotating cylindrical tanks that were exposed to spring temperature and light conditions. This led to the onset of a mixed diatom bloom which was delayed by several days in submicron size-enriched treatments. Later, phytoplankton biomass reached values even slightly higher than in unaltered incubations. The abundance of heterotrophic unicellular plankton and VLPs exhibited more pronounced oscillation amplitudes during the early phase of succession, thus being stimulated by additional submicron-sized material (including virions). VLPs are suggested as a possible supplementary diet for heterotrophic flagellates. From 190 h onwards, toward the end of the incubation experiments (550 h). heterotrophic flagellate density was no longer influenced by initial enrichment with the submicron size fraction. In contrast, bacteria were consistently repressed in the last two-thirds of the experimental duration. Our results support the existence of bioactive substances in the 2-200 nm size fraction, most probably virus particles, which are capable of influencing the dynamics and oscillation patterns in plankton community succession according to their concentration during algal bloom events.

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