Viral mortality of marine bacteria and cyanobacteria

DESPITE the importance of cyanobacteria in global primary productivity 1 and of heterotrophic bacteria in the consumption of organic matter in the sea 2 , the causes of their mortality, particularly the cyanobacteria, are poorly understood. It is usually assumed that mortality is due to protozoan grazing 3,4 rather than to viral infection, probably because abundances of phage and host in nature are presumed to be low 5 . Previously, either very few marine bacteriophages have been found by plaque assays 6–9 , or viruses have been simply observed 10–12 or counted 13,14 by transmission electron microscopy, with the assumption that 'phage-looking' forms are locally active bacteriophages. Here we report not only high viral abundance in the ocean but also counts of bacteria and cyanobacteria in the final irreversible stage of lytic infection. The latter counts are necessary to evaluate mortality, because the sources, hosts, viability and ages of observed free viruses are unknown; even finding viruses attached to cells does not prove successful infection. Up to 7% of the heterotrophic bacteria and 5% of the cyanobacteria from diverse marine locations contained mature phage; interpretation via culture data indicates that up to 70% of the prokaryotes could be infected. These data demonstrate the existence of a significant new pathway of carbon and nitrogen cycling in marine food webs and have further implications for gene transfer between marine organisms.