Wilhelm SW, MG Weinbauer and CA Suttle. The significance of photoreactivation in marine viral communities, American Society of Microbiology, Miami, FL May 1997.

The paradox of high abundances of infectious viruses in the sea, in the presence of high levels of UV-radiation can be explained by photoreactivation of damaged viral DNA after injection into host cells. In order to quantitatively estimate the amount of DNA repair which occurs in natural systems, we have examined the relationships between viral infectivity, DNA damage and host-mediated photorepair in 4 viral strains: phages H40 and H85 (which infect the offshore bacterial isolate Vibrio sp. strain H40), PWH3a-P1 (which infects the coastal isolate Vibrio natriegens strain PWH3a), and nt-1 (which infects the estuarine isolate Vibrio natriegens ATCC 14048). While all the viruses lost infectivity when irradiated with UV-C (254nm), the ability of the host cells to photoreactivate the viruses varied significantly. Vibrio sp. H40 was more effective in repairing damage to viruses than either of the Vibrio natriegens isolates. To compare the repair efficiencies of the different bacteria, we estimated the number of photon hits (254nm) required to reduce the number of infective units in a viral lysate to 1% of the original titer. We measured the infectivity of UV-damaged viruses when assayed on host cells grown under photoreactivating wavelengths (> 312nm, 200umol photons/m2/s) and in the dark. When titered under dark conditions,the infectivity of viruses H40 and H85 was reduced to 1% after treatment with an estimated 389 and 532 photon hits (254nm) per virus, respectively. However, when titered under photoreactivating conditions, viruses H40 and H85 required 1363 and 1390 more photon hits per virus to reduce infectivity to 1% of the undamaged controls. In contrast, viruses PWH3a-P1 and nt-1 required only 53 and 56 photon hits per virus, respectively, to reduce infectivity to 1% in samples titered in the dark. However, under photoreactivating conditions, 523 and 297 photon hits per virus were required to reduce infectivity to 1% of the undamaged control. Comparing these results with DNA damage in samples from the Gulf of Mexico allows us to infer the photoreactivation which is occurring in the water column. The results demonstrate that host-mediated repair of viral DNA is necessary for the maintenance of high viral titers.

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