Bacterial viruses in coastal seawater: lytic rather than lysogenic production

It has recently been suggested that the majority of virus production in seawater is through lysogen induction. Here, we examine the mechanism of virus production in coastal surface seawater. Bacteria and viruses were removed from seawater by ultrafiltration (0.2 or 0.02 pm) and introduced back to the water in 0.6 pm filtered inocula. Viral and bacterial abundance was followed during sample incubations using transmission electron microscopy and acridine orange direct counts. Bacteria increased in abundance in aU cases. When initial abundances were less than 1.3 X 106 viruses rnl-', (with extremely low initial bacterial densities), viruses disappeared at a rate of 20 to 50% d-l for the 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 lead to detectable virus production, indicating that natural UV light did not induce lysogens. In samples with initial abundances above 1.3 X 106 viruses ml-l, significant increases as well as fluctuations in 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 experiments. Since lytic infection is dependent on encounter frequency, which is controlled by densities of both viruses and bactena, the product of these 2 concentrations determines if infection may take place. We observed lytic infection only when the product of virus and bacteria numbers equalled 10'' or more. Reported abundances of viruses and bacteria in natural seawater san~ples frequently fall above this level in surface waters and below it in mid and deep waters, suggesting that lytic infection may occur in all but deeper waters. However, due to the specificity of hosthirus interactions, infection is also dependent on the quantitative bacterial species composition and diversity, which are presently unknown.