Mortality of marine bacteria in response to enrichments of the virus size fraction from seawater

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Abstract
The potential for viral lysis of marine bacteria in seawater enriched with the virus size fraction from seawater was investigated in seawater samples from Long Island Sound, USA, the eastern Pacific Ocean and the Caribbean Sea. Ultrafiltration was used to concentrate material from seawater in the >0.05 mum to <0.22 mum size fraction. Electron microscopy counts of virus-like particles in the high molecular weight concentrate (HMWC) correlated closely to epifluorescence microscopy counts of <0.22 mum DAPI-positive, DNase-resistant particles of the HMWC. Acridine orange direct counts of bacterial abundances significantly declined (often to 1/2 or less of controls) when seawater was incubated with 4- to 16-fold enrichments of HMWC. Microwave treatment of the HMWC before addition to seawater virtually eliminated the declines in bacterial abundance. The combined evidence of the size range of particles, the heat lability of the HMWC and the presence of <0.22 mum DAPI-fluorescent, DNase-resistant particles by epifluorescence microscopy and abundant virus particles by electron microscopy suggests that a dominant bacterial mortality agent in the seawater concentrate was bacteriophage, although we could not rule out an effect of high molecular weight proteins.