Bacteria and viruses in Arctic Sea ice

We studied vertical distribution of bacteria and viruses in different layers of the Arctic sea ice drilled at the North Pole. The sampled multi-year ice was characterized by uneven vertical distribution of bacterial abundance. This characteristic varied within the range of 8±1.2 ×103 to 95±2.6 ×103 cells ml-1. The average bacterial abundance was 28±2.9×103 cells ml-1. The layers with the maximal bacterial abundance were located in the intermediate and lower layers of the ice cores. The morphological composition of bacterial population of the multi-year ice was dominated by the cocci cells (63-92%). Bacterial population of the multi-year ice consisted of rather large cells with the mean cell volume of 0.14 μm3. Bacterial biomass varied from 0.5 to 5 mg C m-3 with the mean value 1.57±0.2 mg C m-3. The vertical distribution of bacterial biomass generally followed the pattern of bacterial abundance distribution. The maximal viral abundance was also located in the upper, intermediate and lower layers of the ice. The ratio of viral to bacterial abundance varied from 0.6 to 28, with the mean value 12.5. The average total number of phages attached to bacteria was 6.2 ×103 viral particles ml-1. The fraction of viruses attached to bacteria varied from 0.5 to 20.7%, with the mean value 7.1% of the total viral abundance. The number of bacterial cells with viral particles attached to them varied from 0.7 to 32×103 cells ml-1, with the mean value 8×103 cells ml-1. The number of viral particles located within bacterial cells varied from 2 to 21 particles per a bacterial cell. The minimum and maximum numbers of viruses located on bacterial cells varied from 1 to 6 viral particles per a bacterial cell. The mean viral capsid size varied from 42 to 83 nm. The frequency of visibly infected bacterial cells (FVIC) calculated for the upper, intermediate and lower layers of the ice was 0.92, 1.23 and 0.8% of the total viral abundance, respectively. The overall frequency of infected cells (FIC) calculated for the same layers was 6.3, 8.4 and 0.8% of the viral numbers, respectively, while the viral-mediated mortality of bacteria (VMB) was 7.1, 9.8 and 6.1 %, respectively. The average number of viral particles located within bacterial cells was 7.3 particles per a cell. Our data show that during the study period the rate of viral infection of bacterial cells and the viral-mediated mortality of bacterial cells in the multy-year ice of the North Pole were relatively low.