SEASONAL-VARIATIONS OF VIRUS ABUNDANCE AND VIRAL CONTROL OF THE BACTERIAL PRODUCTION IN A BACKWATER SYSTEM OF THE DANUBE RIVER

The abundance of virus-like particles in a backwater system of the Danube River covered a range of 1.2 x 10(7) to 6.1 x 10(7) mi(-1) from 1992 to 1993. Measurements of head diameters for these particles, all of which were presumed to be viruses, led to four defined size classes, ranging from < 60 nm to > 150 nm. The 60- to < 90-nm size class contained the largest fraction of total particles (41%), followed by the 90- to < 150-nm size class (33%). The frequency of size classes was not significantly different between the two years. The frequency of bacteria with mature phages ranged from 1 to 4% over the seasons, with mean burst sizes ranging from 17 to 36 phage per host cell. Among the bacterial morphotypes, rods and vibrios were the major host systems for phages, while coccoid and filamentous cells were considered negligible. Counts from transmission electron microscopy and acridine orange direct counts confirmed that rods and vibrios accounted for 85 to 95% of the bacterial population over the seasons, Virus decay experiments showed lower decay rates for temperatures between 5 and 15 degrees C (52 to 70% of the virus population remained) relative to 18 and 25 degrees C (31 to 51% of the virus remained). Bacterial production measurements, performed at the same time and under the same conditions as decay experiments, allowed us to estimate virus-induced death rates, which ranged from 15.8 to 30.1% over the year, with an average of 20% viral control of the bacterial production. Considering that mature phage particles are visible only in the last phase of the latent period and using a mean conversion factor of 5.4 from the literature, based on descriptions of various phage host systems to relate the percentage of visibly infected cells to the total percentage of the bacterial community that is phage infected, we estimate that some 5.4 to 21.6% of the bacterial population is infected with viruses. This would imply that virus-induced death rates of bacteria range from 10.8 to 43.2%. The data on virus-induced bacterial mortality obtained by both the viral decay method and the determination of the frequency of infected cells are compared and discussed.