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Category: Applied and Industrial Microbiology; Environmental Microbiology
Life in Ice Formations at Very Cold Temperatures, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815813/9781555814229_Chap10-1.gif /docserver/preview/fulltext/10.1128/9781555815813/9781555814229_Chap10-2.gifAbstract:
The most vibrant and extensive of within-ice microbial ecosystems are those that develop in seasonal ice formed from seawater. Permanently frozen soil, or permafrost, represents an endmember to glacial ice in being primarily lithogenic by definition, with only a limited volume of either frozen or liquid water held within the soil matrix. The fraction of liquid inclusions is relatively large as a result of the non-linear process of ice-formation from seawater, which promotes the retention of pockets of seawater as the ice grows. With shrinking pore space, the encased liquid becomes increasingly salty. Temperature thus determines the salinity of the brine, though not in linear fashion, in part because the subzero precipitation points of individual sea salts differ. Enzymes may have been present in the initial seawater prior to freezing, and thus derived from an unknown myriad of possible organisms, only to partition and eventually concentrate in the brine inclusions during the freezing process and onset of winter. Tracers to measure the synthesis must enter the cell at lower temperatures permissive of solute diffusion, but incorporation into protein appears possible via conformational changes of enzymes that do not require diffusion. The wonders of life in the extreme cold continue to beckon.
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Comparative (generic) temperature profiles over depth in three types of ice formations: glacial ice at South Pole, Antarctica (dotted line), with basement temperature varying depending on method of assessment (hatched box; adapted from Price et al., 2002 ); Arctic permafrost, with the seasonal swing in the active surface layer (bold dashed lines; adapted from Hinkel et al., 2003 ; Osterkamp, 2003 ; Oelke and Zhang, 2004 ; Smith et al., 2005 ); and Arctic sea ice, with a seasonal swing that results in its melting, first at the surface as atmospheric temperatures warm in summer (bold lines; adapted from Krembs et al., 2002 ; Eicken, 2003 ). Upper right image (A) depicts pore microstructure (liquid phase in black) in a 20-mm wide section of sea ice at –30°C; lower right image (B) same, but at –2.4°C with greater pore connectivity (see Eicken et al., 1998 ; Deming and Huston, 2000 ).
Microscopic images of thin sections of Arctic winter sea ice, taken by transmission light at –20°C, showing the size and interconnectivity of brine pores (BP) and some particulate matter within them; note scale bar of 50 μm (courtesy of H. Eicken; see Stierle and Eicken, 2002 , for details).
Concentration of EPS (◆) or DOC (·) scaled to the brine volume of ice sections cored from Arctic winter sea ice, where the sampled temperature gradient spanned –4 to –18°C (data from Krembs et al., 2002 ).
Fraction of actively respiring cells determined by CTC stain and evaluated as originally attached to surfaces in the brine pores of Arctic winter sea ice (◆), where the sampled temperature gradient spanned –2 to –20°C, and fraction of the total bacterial population that fluoresced in response to in situ hybridization probes for Cytophaga- Flavobacterium- Bacteroides (▲), a group generally known for EPS production and an attached lifestyle. Error bars are standard error of the mean for triplicate samples; at –20°C, the symbol for CTC-stained bacteria obscures the error bars (data from Junge et al., 2004b ).
Frequency of detection of cold-adapted extracellular protease activity (T opt ≤ 15°C; A. L. Huston and J. W. D., unpublished), as measured by the protocols of Huston et al. (2000) ; inset depicts an individual experiment based on a sample of sinking particulate matter originally at –1°C (data from Huston et al., 2000 ).
Temperature-dependent extracellular enzyme activity (EEA) measured by fluorescent substrate assay (for proteases, as in Huston et al., 2000 ) in first-year Arctic winter sea-ice brines from Franklin Bay, N.W.T. (from Deming, 2004 ). Ice sections originally at –20°C in situ were melted into 0.2-μm filtered, artificial brine solutions to achieve final meltwater salinities (‰) of 220 (squares), 210 (circles), 180 (triangles), or 22 (inverted triangles). The dotted line connects the solid symbols, which represent EEA at salinities nearest to those of the microbial habitat at each temperature.
Viral and bacterial dynamics in a section of Arctic winter sea ice melted into an artificial (0.2-μm filtered) brine solution to achieve a final sample salinity of 160‰, mimicking the in situ brine salinity of the ice at –12°C (data from Wells and Deming, 2006 ). In many cases, symbols (▲ for bacteria, ■ for viruses) for the mean of the microscopic counts obscure the error bars (standard error of the mean).
Schematic diagram of the potential roles of EPS in winter sea-ice brine pores (including freeze depressant, attachment facilitator, salt segregator, and physical buffer against osmotic damage) and as enzyme localizer and stabilizer (including virus-borne polysaccharide depolymerases; adapted from Hughes et al., 1998 ).
Comparative features of three types of ice formations subject to very cold temperature