Phytoplankton exudation: exploitation of the microbial loop as a defence against algal viruses

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Abstract
Healthy phytoplankton cells exude dissolved organic matter (DOM). In a model, DOM exudation is demonstrated to be a cost-effective, indirect, means of reducing virus infection, which can be a major cause of phytoplankton mortality. Diffusion theoly shows that, for a given biomass, small particles will have a much higher rate of wlsorption of solutes than will large ones. Thus colloidal viruses are far more likely to come into contact with bacteria than with phytoplankton if the same biomasses are present. B can destroy viruses in a large proportion of contacts. Although flagellate protozoa have lower contact rates with viruses, they may consume more of the viruses that they do encounter and they deal with larger viruses particularly effectively. The exuded DOM supports bacterial growth, which in turn may support flagellates. Even fairly low levels of exudation can maintain the biomass of bacteria or small flagellates required to remove >50% of viruses before they have a chance to infect their host, at least for larger phytoplankton. High rates of virus removal may occur at the high exudation rates that are typical of late blooms, It is concluded that healthy phytoplankton cells exude DOM in order to remain healthy.

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