Abundant dissolved genetic material in Arctic sea ice Part II: Viral dynamics during autumn freeze-up

Viruses play a significant role in nutrient cycling within the world’s oceans and are important agents of horizontal gene transfer, but little is know about their entrainment into sea ice or their temporal dynamics once entrained. Nilas, grease ice, pancake ice, first-year sea ice floes up to 78 cm in thickness, and under-ice seawater were sampled widely across Amundsen Gulf (ca. 71^° N, 125^° W71^\circ \hbox{N}, 125^\circ \hbox{W}) for concentrations of viruses and bacteria. Here, we report exceptionally high virus-to-bacteria ratios in seawater (45–340) and sea ice (93–2,820) during the autumn freeze-up. Virus concentrations ranged from 4.8 to 27 × 10⁶  ml⁻¹ in seawater and, scaled to brine volume, 5.5 to 170 × 10⁷ ml⁻¹ in sea ice. Large enrichment indices indicated processes of active entrainment from source seawater, or viral production within the ice, which was observed in 2 of 3 bottle incubations of sea ice brine at a temperature (-7^°C-7^\circ\hbox{C}) and salinity ( 110 \permille110 \permille) approximating that in situ. Median predicted virus-to-bacteria contact rates (relative to underlying seawater) were greatest in the top of thick sea ice (66–78 cm: 130×) and lowest in the bottom of medium-thickness ice (33–37 cm: 23×). The great abundance of viruses and more frequent interactions between bacteria and viruses predicted in sea ice relative to underlying seawater suggest that sea ice may be a hot spot for virally mediated horizontal gene transfer in the polar marine environment.