BACTERIAL-VIRUSES IN COASTAL SEAWATER - LYTIC RATHER THAN LYSOGENIC PRODUCTION

It has recently been suggested that the majority of virus production i n seawater is through lysogen induction, Here, we examine the mechanis m of virus production in coastal surface seawater. Bacteria and viruse s were removed from seawater by ultrafiltration (0.2 or 0.02 mu m) and introduced back to the water in 0.6 mu m filtered inocula. Viral and bacterial abundance was followed during sample incubations using trans mission electron microscopy and acridine orange direct counts. Bacteri a increased in abundance in all cases. When initial abundances were le ss than 1.3 x 10(6) viruses ml(-1), (with extremely low initial bacter ial densities), viruses disappeared at a rate of 20 to 50% d(-1) for t he first 2 d and did not increase in abundance over the 7 d incubation period. Continuous sunlight exposure or a pulse of sunlight did not l ead to detectable virus production, indicating that natural UV Light d id not induce lysogens. In samples with initial abundances above 1.3 x 10(6) viruses ml(-1), significant increases as well as fluctuations i n virus numbers were observed after the second day. This suggests that lytic infection, rather than the induction of lysogenic bacteria, is responsible for the majority of bacteriophage production in these expe riments. Since lytic infection is dependent on encounter frequency, wh ich is controlled by densities of both viruses and bacteria, the produ ct of these 2 concentrations determines if infection may take place. W e observed lytic infection only when the product of virus and bacteria numbers equalled 10(12) or more. Reported abundances of viruses and b acteria in natural seawater samples frequently fall above this level i n surface waters and below it in mid and deep waters, suggesting that lytic infection map occur in all but deeper waters. However, due to th e specificity of host/virus interactions, infection is also dependent on the quantitative bacterial species composition and diversity, which are presently unknown.