AME 30:1-9 (2002)  -  doi:10.3354/ame030001

ABSTRACT: Viruses are now known to be significant contributors to bacterial mortality in the marine environment; however, to date, most studies have focused on only the more productive coastal areas. The impact of viruses in oligotrophic environments is still poorly understood, but current literature implies that the percentage of bacteria visibly infected with viruses in these areas is generally low and may often be below the detection limit normally achieved by transmission electron microscopy (i.e. <0.3%). However, from calculations based on typical bacterial abundance, abundance ratios of viruses to bacteria, and plausible turnover times of bacteria and viruses, we hypothesized that the frequency of visibly infected bacteria (FVIB) should be frequently above the detection limit even in oligotrophic environments. We tested this hypothesis in the oligotrophic open waters in the East Sea, Korea, in summer. FVIB ranged from 1.2 to 2.2% and burst size varied from 12 to 15. Viral production ranged from 0.4 to 13.7 x 10⁸ viruses l^-1 d^-1. Estimated viral mortality of bacteria was on average 13.1% of bacterial production, indicating that viruses could be significant contributors to bacterial losses in oligotrophic waters. Comparison of grazing by heterotrophic nanoflagellates and lysis by viruses suggests that, overall, these processes cause comparable losses of bacteria in the oligotrophic East Sea; however, taken together, they accounted for only half of the total mortality.

KEY WORDS: Viral production · Oligotrophy · Bacteriophage · Bacterial mortality · Burst size · East Sea