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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-Flavobacterium (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 nepheloid (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 oxygen, 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 environmental 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 oceanographic context of the planktonic Archaea to include nepheloid layers.KEY WORDS: Cytophaga-Flavobacterium · CF · Archaea · Nepheloid layer · Arctic · Particleassociated Bacteria · Amundsen Gulf Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 31: [19][20][21][22][23][24][25][26][27][28][29][30][31] 2003 clivity for attachment, constituents of the CytophagaFlavobacterium-Bacteroides phylum, including the CF cluster, represented 18 to 55% of nonplastid-like ribosomal sequences cloned from bacteria associated with marine snow in Californian coastal waters (DeLong et al. 1993) and in the Adriatic Sea (Rath et al. 1998). They were also members of the communities found attached to suspended particles in the Columbia River estuary (Crump et al. 1999) and to deposited sediments in the eastern Arctic (Ravenschlag et al. 2001). CF association with pelagic particles has not been addressed explicitly in high latitude waters; however, they appear again as abundant members ...

Little information is currently available about water column microbial processes or mortality during Arctic winter. To address this paucity, we used epifluorescence microscopy and dilution experiments to determine the abundance of flagellates, bacteria and virus-like particles (VLP) and the rates of bacterial growth, bacterivory and virus-induced mortality in subzero-temperature bottom waters (≤ 230 m) of Franklin Bay during February and March 2004, when ice-covered surface waters were highly oligotrophic (maximum chlorophyll a value of 0.09 µg l -1 ). We focused on bottom waters due to the possible importance of sediment resuspension as a source of organic matter. While flagellates were present at low densities (1.5 to 3.1 × 10 2 ml -1 ), bacterial concentrations resembled those from other seasons in the region and increased over the 5 wk sampling period, from 1.4 × 10 5 to 3.0 × 10 5 ml -1 . VLPs were typically an order of magnitude more abundant than bacteria (range of 1.4 to 4.5 × 10 6 VLP ml -1 ) and, like the fraction of particle-associated bacteria (but not total bacteria), correlated with particulate organic carbon concentration (r s = 0.82, p < 0.04, n = 7). Grazing rates, whether measured in dilution experiments or calculated from flagellate abundance, were low or undetectable (maximum of -0.004 h -1 ). Of 3 parallel experiments, 2 yielded substantial virus-induced mortality (-0.006 to -0.015 h -1 ), comparable to or exceeding the intrinsic bacterial growth rate (0.010 h -1 in both experiments) and suggesting viruses were the more important agents of bacterial mortality under these conditions. Using a viral reduction approach, VLP production measured in the water column or ice-moored sediment traps was commonly low (0.3 to 7.7 × 10 4 VLP ml -1 h -1 ) or undetectable, highly variable among replicates and, when measurable, implied viral turnover times between 0.9 and 12 d. In general, our results show that, despite the oligotrophy of Arctic winter, bottom water bacterial communities can remain active and subject to viral predation.KEY WORDS: Virus · VLP · Lysis · Bacterivory · Arctic · Winter · Franklin Bay · Particles Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 43: [209][210][211][212][213][214][215][216][217][218][219][220][221] 2006 ditions with high (> 50%) virus-induced mortality (Murray & Eldridge 1994). Yet such conditions favor lower concentrations of bacteria and viruses (compared to eutrophic circumstances, e.g. Cochlan et al. 1993, Hewson et al. 2001, correspondingly lower encounter rates (Murray & Jackson 1992), reduced virus production and/or virus-induced mortality , Weinbauer & Peduzzi 1995, Guixa-Boixereu et al. 2002 and, perhaps, a greater likelihood of lysogeny (Weinbauer & Suttle 1999, Weinbauer et al. 2003.Viruses may exert a more subtle influence by generating a relatively labile lysate which, though a small component of DOM, may nonetheless be an important source of organic matter to other microorganisms (Middelboe et ...

A cold-active bacteriophage designated 9A was isolated against Colwellia psychrerythraea Strain 34H at near in situ temperature (-1°C) by enrichment of seawater from an Arctic nepheloid layer, using a newly developed isothermal overlay technique. Phage 9A is classified as a Siphoviridae with a genome size of 80 to 90 kb. In addition to 34H, 9A infects C. demingiae ACAM 459 T ; no other hosts (of 22 tested) were identified. In replete media, 9A formed plaques on 34H from -6 to 4°C and on C. demingiae from -6 to 8°C; the temperature range of plaque formation on 34H could be extended to 8°C by prior host starvation. An indirect plating method and microscopic evaluation also determined phage production at temperatures between -13 and -10°C. At -1°C, 34H had a broader salinity range of plaque formation than C. demingiae: 20 to 50 (but not 65) psu vs. 27 to 34 (but not 50) psu. As monitored by epifluorescence microscopy, phage production by 34H was observed at 1, 10, 100 and 200 atm (all at -1°C), but not at 400 or 600 atm. The 9A-34H system commonly had a low efficiency of plating (EOP; typically ~1%) which varied with culture age. Despite repeated attempts, no meaningful adsorption rate could be determined at -1 or 8°C. This result, the low EOP, and the effect of starvation on plaque formation suggest that fluctuating host phenotypes may play an important role in the dynamics of this system. One-step growth curves (using 34H as host) revealed a longer latent period (4 to 5 vs. 2.5 to 3 h) and greater burst size (55 vs. 5) at -1 than 8°C; at temperatures between -10 and -12°C, the estimated latent period was 5-10 d and the burst size 5. At both -1 and 8°C, rise times were comparable to latent periods. Although the cycle from infection to burst at -1°C required only 10 to 20% of the generation time of 34H at this temperature, the amount of viral DNA synthesized was comparable to the size of the host genome, suggesting very efficient and cold-active virus-encoded enzymes.KEY WORDS: Virus · Phage 9A · Colwellia psychrerythraea 34H · Colwellia demingiae · Arctic · Adsorption · One-step growth curve · Pressure · Salinity Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 45: [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] 2006 'psychrophilic ' (e.g. Stamatin 1963, Olsen 1967, Ackermann & DuBow 1987 or 'psychrotrophic' (Greer 1982, 1983, Patel & Jackman 1986, the terms are ambiguous or misleading, since they imply growth characteristics of the host, not features necessarily specific to the virus (e.g. the temperature range of infection by a given virus may change with different hosts). For the sake of clarity, we define as 'cold-active' those viruses capable of infection and production at ≤ 4°C. This upper limit accords with the sparse literature about phage infection at low temperature and encompasses the temperatures of the perennially cold regions of the ocean. By this definition, several phage or phage-host systems (PHS) termed 'psychrophilic' or 'p...

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