Accumulation of damaged bacteria and viruses in lake water exposed to solar radiation

Ambient ultraviolet radiation (UVR) is known to have direct negative effects on aquatic bacteria at the molecular level, damaging DNA, and at the level of community metabolism, but little is known about the effects of UVR at the cellular level. In order to study the direct and indirect effects of UVR on bacterioplankton, we developed a staining protocol based on the exclusion nucleic acid stain TOPRO-1 that allows the detection of cells with damaged membranes. We used this protocol to assess the effect of UVR in water samples collected from Lac Cromwell, a small dystrophic Canadian Shield lake. Samples were incubated 5 d in situ in a series of dialysis bag experiments under natural solar radiation. Samples with and without grazers were suspended in the lake's epilimnion and received graded amounts of ambient UVR. Abundances of bacteria, viruses and heterotrophic nanoflagellate grazers, and the number of TOPRO+ cells were monitored. In samples exposed to full solar radiation, a marked increase in the number of bacteria that took up the exclusion nucleic acid stain TOPRO, an indicator of cellular damage, was observed. The rate of accumulation of damaged bacteria reached a maximum of 13.5 % d(-1) and was a direct function of the UVR intensity in solar radiation. There was also an increase in the mean total viral abundance in bags exposed at the surface. We hypothesize that this increase in viral number was due to the induction by UVR of the lytic cycle in lysogenic bacteria. In the presence of grazers, however, the virus-to-bacteria ratio was lower and the bacterial mortality attributable to viral lysis was reduced by half, from 10.7 to 4.6 % d(-1). These observations strongly suggest that grazers and viruses are competing for the same bacteria and that grazers may play an important role in regulating the proportion of bacterial mortality caused by viruses. Our results show that high intensities of ambient UVR, particularly in the surface waters of lakes with shallow epilimnia or those that undergo periods of microstratification, may be an important factor influencing bacterial mortality, either directly or indirectly via the induction of lysogenic bacteria.