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Summary: Metabolic diversity of heterotrophic bacterioplankton was tracked from early winter through spring with Biolog Ecoplates under the seasonally ice covered arctic shelf in the Canadian Arctic (Franklin Bay, Beaufort Sea). Samples were taken every 6 days from December 2003 to May 2004 at the surface, the halocline where a temperature inversion occurs, and at 200 m, close to the bottom. Despite the low nutrient levels and low chlorophyll a, suggesting oligotrophy in the winter surface waters, the number of substrates used (NSU) was greater than in spring, when chlorophyll a concentrations increased. Denaturing gradient gel electrophorisis analysis also indicated that the winter and spring bacterial communities were phylogenetically distinct, with several new bands appearing in spring. In spring, the bacterial community would have access to the freshly produced organic carbon from the early phytoplankton bloom and the growth of rapidly growing specialist phenotypes would be favoured. In contrast, in winter bacterioplankton consumed more complex organic matter originated during the previous year's phytoplankton production. At the other depths we tested the NSU was similar to that for the winter surface, with no seasonal pattern. Instead, bacterioplankton metabolism seemed to be influenced by resuspension, advection, and sedimentation events that contributed organic matter that enhanced bacterial metabolism

Unamended dark incubations of 3 µm-filtered seawater were prepared with surface samples from the Norwegian Sea, the NW Mediterranean and the Indian Ocean. Except for the very oligotrophic Indian Ocean samples, this simple treatment promoted the growth of heterotrophic flagellates, with rates (0.3 to 1.2 d–1) typical of natural assemblages. Marine stramenopile (MAST)-1 and MAST-4 cell counts, obtained by fluorescence in situ hybridization (FISH), increased in most incubations. These yet uncultured protists were first detected in molecular surveys of marine picoplankton and have been recently shown to be globally distributed heterotrophic flagellates. Three incubations from the Norwegian Sea were studied in more detail by denaturing gradient gel electrophoresis (DGGE), 18S rRNA gene clone libraries and FISH counts for 5 MAST groups. Protist diversity changed gradually during the incubation, but the DGGE bands selected were already present at the beginning of the incubation. Clone libraries from the peaks in abundance of heterotrophic flagellates were dominated by MAST sequences. FISH counts revealed MAST-1B to be a very successful organism in the 3 incubations, accounting for 15 to 30% of heterotrophic flagellates after 6 to 8 d; MAST-1A and -1C cells were also abundant. MAST-4 cells peaked before the other groups and MAST-2 was the least represented. We concluded that unamended seawater incubations can select for heterotrophic flagellates abundant in situ but not yet isolated in pure culture. Therefore, they allow investigation of the growth requirements and dynamics of these uncultured protists, and provide promising preliminary stages for their isolation

Bacterivory was determined in surface waters of Franklin Bay, western Arctic, over a seasonal ice-covered period (winter-spring, 2003-2004). The objectives were to obtain information on the functioning of the microbial food web under the ice, during winter (from 21 December 2003 to 21 March 2004) and during spring (from 22 March 2004 to 29 May 2004), and to test whether bacterial losses would increase after the increase in bacterial production following the spring phytoplankton bloom. Chl a concentrations ranged from 0.04 to 0.36 mu g L(-1), increasing in March and reaching a peak in April. Bacterial biomass showed no consistent trend for the whole period, and protist biomass followed a pattern similar to that of Chl a. Bacterial production increased 1 week after Chl a concentrations started to increase, while bacterivory rates increased very slightly. Average bacterivory rates in winter (0.16 +/- 0.07 mu g C L(-1) d(-1)) were not significantly different from those in spring (0.29 +/- 0.24 mu g C L(-1) d(-1)). Average bacterial production, on the other hand, was similar to bacterivory rates in winter (0.19 +/- 0.38 mu g C L(-1) d(-1)), but higher than bacterivory in spring (0.93 +/- 0.28 mu g C L(-1) d(-1)). Therefore, bacterial production was controlled by grazers during winter and by substrate concentration in spring.