Large viruses in Ross Sea late autumn pack ice habitats

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Large (>110 to 424 nm capsid diameter) extracellular viruses occurred in Ross Sea first year ice samples at 33 stations from 66 to 78° S during the period from 9 May to 11 June 1998. As the viruses were of a size and morphology likely to infect algae and protozoans, infected eukaryotes were present in ice habitats as winter approached. Abundance ranged from below detection limits to 2.5 x 10 6 ml -1 brine (the liquid component of sea ice). It was positively correlated with chlorophyll a (chl a) and phaeophytin and did not differ among vertical location in cores or with ice type. Transmission electron microscopy (TEM) of thin sections of more than 9000 diatoms revealed no viruses; it is therefore unlikely that diatoms were the source of the viruses in the samples. Despite examination of over 2000 other algal cells and heterotrophs, we only observed 5 infected unidentified heterotrophs. They were from bottom communities at 2 stations with chl a ranging from 16 to 73 μg l -1 and where several taxa were present as 10 4 to 10 5 cells ml -1 melted sample. Although viruses of all sizes occurred in fecal pellets from ice habitats and in food vacuoles of phaeodarian radiolarians from water directly below the ice, we did not observe viruses in food vacuoles of ice community microheterotrophs. Viruses were therefore not a major food source in late autumn. Abundance of total (all sizes) viruses (10 8 to 10 9 ml -1 brine, equivalent to 10 6 to 10 8 ml -1 melted core sample) in a subset of samples was comparable to, or higher than, abundance from other polar locations, suggesting the importance of the total viral community in ice community dynamics.