Bacterial and viral abundance in Ross Sea summer pack ice communities

Abundance of single, non-attached bacteria and viruses < 110 nm (likely to have infected bacteria) were determined for surface, interior and bottom ice microhabitats between 66 and 75degreesS in the Ross Sea during the austral summer of 1999. Emphasis was on sites of ice algal blooms, and bacterial abundance was examined with respect to physical characteristics, chlorophyll a, phaeophytin and, in particular, microbial communities. Bacterial abundance ranged from 1.5 x 10(5) to 6.7 x 10(6) ml(-1) melted sample and viral abundance from 6.3 x 10(6) to 1.19 x 10(8) ml(-1) melted sample over all the microhabitats. Neither bacterial nor viral abundance differed among microhabitats, and bacterial abundance was not related to physical characteristics of the habitats. Although bacterial abundance was positively correlated with chlorophyll a and phaeophytin concentrations, only chlorophyll a was significant in-explaining a small (28%) degree of the variability in bacterial abundance. Abundance of diatoms, heterotrophic dinoflagellates and other flagellates, however, explained 85% of the variability in bacterial abundance; these groups were positively correlated with bacterial abundance. Neither viral lysis nor grazing by bacteriovores appeared to be strong controls of bacterial abundance. Community analysis showed that samples were > 90% similar with respect to abundance of bacteria, viruses, and microeukaryote groups. Distinct clusters could be attributed to different algal bloom stages, with, relationships to Phaeocystis spp. blooms being particularly apparent, indicating the strength of algal blooms as processes structuring microbial communities. Microbial communities in Ross Sea summer ice microhabitats could furthermore be viewed within the same general successional sequence characteristic of algal blooms in polar and temperate marine waters.