PHYTOPLANKTON EXUDATION - EXPLOITATION OF THE MICROBIAL LOOP AS A DEFENSE AGAINST ALGAL VIRUSES

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 phyt oplankton mortality. Diffusion theory shows that for a given biomass, small particles will have a much higher rate of adsorption of solutes than will large ones. Thus colloidal viruses are far more likely to co me into contact with bacteria than with phytoplankton if the same biom asses are present. Bacteria can destroy viruses in a large proportion of contacts. Although flagellate protozoa have lower contact rates wit h viruses, they may consume more of the viruses that they do encounter and they deal with larger viruses particularly effectively. The exude d DOM supports bacterial growth, which in turn may support flagellates . Even fairly low levels of exudation can maintain the biomass of bact eria or small flagellates required to remove >50% of viruses before th ey have a chance to infect their host, at least for larger phytoplankt on. High rates of virus removal may occur at the high exudation rates that are typical of late blooms. It is concluded that healthy phytopla nkton cells exude DOM in order to remain healthy.