No abstract

A set of eight large (20 m 3 ) mesocosms were moored in Johnson's Dock (62Њ39.576ЈS, 60Њ22.408ЈW, Livingston Island, Antarctica) to experimentally generate a gradient of phytoplankton biomass and production in order to test the extent of coupling between bacteria (heterotrophic Bacteria and Archaea) and phytoplankton, as well as the role of bacterial losses to protist grazers. This was achieved by imposing four light levels (100%, 50%, 25%, and 10%) in the presence or absence of nutrient additions (0.1 mol NH 4 Cl, 0.1 mol F 6 Na 2 Si, and 0.01 mol KH 2 PO 4 per day per mesocosm). The experimental treatments resulted in a broad range of chlorophyll a (Chl a) (0.31-93.5 g Chl a L Ϫ1) and average primary production rates, while bacteria responded in a much narrower range of biomass (3-447 g C L Ϫ1) and production (0.21-15.71 g C L Ϫ1 d Ϫ1

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 , 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 mg L 21 , 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 6 0.07 mg C L 21 d 21 ) were not significantly different from those in spring (0.29 6 0.24 mg C L 21 d 21 ). Average bacterial production, on the other hand, was similar to bacterivory rates in winter (0.19 6 0.38 mg C L 21 d 21 ), but higher than bacterivory in spring (0.93 6 0.28 mg C L 21 d 21 ). Therefore, bacterial production was controlled by grazers during winter and by substrate concentration in spring.

The phylogenetic and functional diversity of the bacterioplankton assemblage associated with blooms of toxic Alexandrium spp. was studied in three harbours of the NW Mediterranean. Denaturing gradient gel electrophoresis and DNA sequence analysis revealed the presence of a bacterium within the Roseobacter clade related to the presence of Alexandrium cells. Phylogenetic diversity was affected by the presence of Alexandrium spp., geographic situation and seasonality. In contrast, functional diversity, assessed with Biolog plates, was clearly affected by seasonality, but not by the presence of Alexandrium, indicating that the presence of the bacterium associated with the blooms was not enough to modify the metabolic pattern of the bacterioplankton assemblage.