Loss of Estuarine Bacteria by Viral Infection and Predation in Microcosm Conditions

The bacterioplankton density in Ria de Aveiro, a shallow estuarine ecosystem, varied in the broad range of 1.9-10.6 x 109 cells L-1. The range of values was about 2 times higher in brackish water than in marine water. At high tide bacterial abundance was 2-3 times lower than at low tide. The overall variation in virioplankton was in the range of 2.4-25.0 x 1010 particles L-1. Brackish water was about 2 times richer in viral particles than the marine water. Near low tide the virioplankton was 2-3 times higher that at high tide. Viral density followed the pattern of bacterial abundance (it explained 40% of virioplankton variation). The viruses to bacterium ratio varied, throughout tidal cycles, by a factor of about 10 establishing the range 4.7-55.6 (average 17.6). This ratio was rather similar in the two estuarine zones. We compared the effects of infection and predation on the control of bacterioplankton size in the two zones of the estuary. The approach to this question was conducted in experimental microcosms, set up in six combinations of plankton variables affecting the presence/absence of predators, virus-to-bacterium ratio (10-fold increase), virus-to-bacterium distance (2.2-fold increase), and bacterial growth rate. The results showed that predation was similar, in a percent basis, in marine (69%) and brackish water (73%). Viral infection was, however, higher in brackish water (59%) than in the marine water (36%). We conclude that the bacterioplankton along the salinity gradient evolves under biological pressures that are in different balance in the marine and brackish water zones. The effect of viral lysis on bacterial communities with enhanced growth (after yeast extract addition) was masked even when the initial ratio was 10-fold greater than in the natural samples. The high density of the virioplankton did not preclude the large and rapid increase in bacterial density. We suggest that the dynamics of the equilibrium between bacteria and viruses in the environment is driven to higher numerical levels during periods of intensive bacterial growth. On the contrary, at low bacterial growth rates the temporarily increased virus-to-bacterium ratio may drive the equilibrium to its lowest levels.