| Summary: | Planktonic protists have traditionally been categorized based on their modes of energy and carbon acquisition as either phototrophic (algal) or heterotrophic. However, mixotrophy, the combination of phototrophy and heterotrophy within the same individual, is widespread in oceanic systems, but neither the presence nor the ecological impact of these organisms has been identified in an Arctic marine environment. We quantified nano- and picoplankton during early autumn in the Beaufort Sea and Canada Basin and determined relative rates of bacterivory by heterotrophs and mixotrophs. Results confirmed previous reports of low microbial biomass for Arctic communities in autumn. The impact of bacterivory was relatively low, ranging from 0.6 x 103 to 42.8 x 103 bacteria mL-1 day-1, but it was often dominated by pico- or nano-mixotrophs. From 1-7% of the photosynthetic picoeukaryotes were bacterivorous, while mixotrophic nanoplankton abundance comprised 1-22% of the heterotrophic and 2-32% of the phototrophic nanoplankton abundance, respectively. The estimated daily grazing impact was usually < 5% of the bacterial standing stock, but impacts as high as 25% occurred. Analysis of denaturing gradient gel electrophoresis band patterns indicated that communities from different depths at the same site were appreciably different, and that there was a shift in community diversity at the midpoint of the cruise. Sequence information from recovered DGGE bands reflected microbe types related to ones observed in other Arctic studies, particularly from the Beaufort Sea. Cruise: ancillary participant on HLY0806
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