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We used fluorescent in situ hybridization and epifluorescence microscopy to assess the distribution and diversity of pelagic microorganisms, specifically Bacteria, the Cytophaga-Flavobac- terium (CF) cluster and Archaea, in the cold (-1.5 to 3.5°C) and oligotrophic waters of the Northwest Passage, Canadian Arctic, during September 2000. Total cell abundance ranged from 1.23 to 6.56 × 10 5 cells ml -1 , approximately half of which were hybridizable; Bacteria dominated the region (67 to 99.8% of hybridizable cells). CF were well-represented in the surface-water bacterioplankton, accounting for 9 to 41% of the total cell count (21 to 76% of hybridizable cells), but not in deeper populations: in neph- eloid (particle-rich) layers, they accounted for only 1.6 to 5.4% of total cells (3.2 to 9.5% of hybridizable cells) despite the available substrata for attachment, a behavior common to this group. Over the entire data set, often highly significant (p < 0.001) correlations with environmental variables, including oxy- gen, particulate organic nitrogen (PON) and chlorophyll a (chl a) (positive) and depth, salinity and macronutrients (negative) suggested the importance of CF as aerobic heterotrophic consumers in this environment. In marked contrast, Archaea were present at very low levels (0.1 to 2.6% of total cells; 0.2 to 4.6% of hybridizable cells) in the surface waters, becoming more abundant in nepheloid layers, where they accounted for 2.3 to 13% of total cells (3.9 to 33% of hybridizable cells). Archaea correlated highly significantly (p < 0.001) with concentrations of particles and, in nepheloid layers, with PON. Over the entire data set, Archaea and Bacteria correlated significantly but oppositely to the same en- vironmental variables of depth, salinity, oxygen and macro-nutrients, suggesting separate niches in this setting. In general, our results substantiate and extend the growing evidence for the numerical importance of CF in cold marine surface waters and further document the distribution and oceano- graphic context of the planktonic Archaea to include nepheloid layers.

aquatic microbial ecology

Abundance of Bacteria, the Cytophaga-Flavobacterium cluster and Archaea in cold oligotrophic waters and nepheloid layers of the Northwest Passage, Canadian Archipelago