The completion of the 5,373,180-bp genome sequence of the marine psychrophilic bacterium Colwellia psychrerythraea 34H, a model for the study of life in permanently cold environments, reveals capabilities important to carbon and nutrient cycling, bioremediation, production of secondary metabolites, and coldadapted enzymes. From a genomic perspective, cold adaptation is suggested in several broad categories involving changes to the cell membrane fluidity, uptake and synthesis of compounds conferring cryotolerance, and strategies to overcome temperature-dependent barriers to carbon uptake. Modeling of three-dimensional protein homology from bacteria representing a range of optimal growth temperatures suggests changes to proteome composition that may enhance enzyme effectiveness at low temperatures. Comparative genome analyses suggest that the psychrophilic lifestyle is most likely conferred not by a unique set of genes but by a collection of synergistic changes in overall genome content and amino acid composition.proteome ͉ psychrophily ͉ bioremediation ͉ astrobiology ͉ threedimensional homology modeling

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The Earth's cryosphere comprises those regions that are cold enough for water to turn into ice. Recent findings show that the icy realms of polar oceans, glaciers and ice sheets are inhabited by microorganisms of all three domains of life, and that temperatures below 0 °C are an integral force in the diversification of microbial life. Cold-adapted microorganisms maintain key ecological functions in icy habitats: where sunlight penetrates the ice, photoautotrophy is the basis for complex food webs, whereas in dark subglacial habitats, chemoautotrophy reigns. This Review summarizes current knowledge of the microbial ecology of frozen waters, including the diversity of niches, the composition of microbial communities at these sites and their biogeochemical activities.

Arctic wintertime sea-ice cores, characterized by a temperature gradient of ؊2 to ؊20°C, were investigated to better understand constraints on bacterial abundance, activity, and diversity at subzero temperatures. With the fluorescent stains 4,6-diamidino-2-phenylindole 2HCl (DAPI) (for DNA) and 5-cyano-2,3-ditoyl tetrazolium chloride (CTC) (for O 2 -based respiration), the abundances of total, particle-associated (>3-m), freeliving, and actively respiring bacteria were determined for ice-core samples melted at their in situ temperatures (؊2 to ؊20°C) and at the corresponding salinities of their brine inclusions (38 to 209 ppt). Fluorescence in situ hybridization was applied to determine the proportions of Bacteria, Cytophaga-Flavobacteria-Bacteroides (CFB), and Archaea. Microtome-prepared ice sections also were examined microscopically under in situ conditions to evaluate bacterial abundance (by DAPI staining) and particle associations within the brine-inclusion network of the ice. For both melted and intact ice sections, more than 50% of cells were found to be associated with particles or surfaces (sediment grains, detritus, and ice-crystal boundaries). CTC-active bacteria (0.5 to 4% of the total) and cells detectable by rRNA probes (18 to 86% of the total) were found in all ice samples, including the coldest (؊20°C), where virtually all active cells were particle associated. The percentage of active bacteria associated with particles increased with decreasing temperature, as did the percentages of CFB (16 to 82% of Bacteria) and Archaea (0.0 to 3.4% of total cells). These results, combined with correlation analyses between bacterial variables and measures of particulate matter in the ice as well as the increase in CFB at lower temperatures, confirm the importance of particle or surface association to bacterial activity at subzero temperatures. Measuring activity down to ؊20°C adds to the concept that liquid inclusions in frozen environments provide an adequate habitat for active microbial populations on Earth and possibly elsewhere.The constraints on and sustainability of life in frozen environments are of considerable importance in a number of contexts, from polar microbial ecology and astrobiology to cryopreservation and other industrial applications (42). For example, a number of subzero environments, such as Antarctic and Arctic lakes (23,25,38), snow (3), glacial ice (46), and permafrost soils (41), have been investigated as Earth analogs for potential extraterrestrial habitats also at subzero temperatures. To date, fundamental questions underlying the behavior of bacteria in any frozen environment have not been adequately addressed: how do bacteria manage to persist and possibly remain active? At the lowest temperatures observed on Earth, what environmental factors enable and control bacterial survival and even sustained activity?This study focused on Arctic wintertime sea ice, the coldest marine habitat on Earth (temperature range of Ϫ2 to Ϫ35°C) (31) and an important component of polar climate and ecos...

The physical properties of Arctic sea ice determine its habitability. Whether ice-dwelling organisms can change those properties has rarely been addressed. Following discovery that sea ice contains an abundance of gelatinous extracellular polymeric substances (EPS), we examined the effects of algal EPS on the microstructure and salt retention of ice grown from saline solutions containing EPS from a culture of the sea-ice diatom, Melosira arctica. We also experimented with xanthan gum and with EPS from a culture of the cold-adapted bacterium Colwellia psychrerythraea strain 34H. Quantitative microscopic analyses of the artificial ice containing Melosira EPS revealed convoluted ice-pore morphologies of high fractal dimension, mimicking features found in EPS-rich coastal sea ice, whereas EPS-free (control) ice featured much simpler pore geometries. A heat-sensitive glycoprotein fraction of Melosira EPS accounted for complex pore morphologies. Although all tested forms of EPS increased bulk ice salinity (by 11-59%) above the controls, ice containing native Melosira EPS retained the most salt. EPS effects on ice and pore microstructure improve sea ice habitability, survivability, and potential for increased primary productivity, even as they may alter the persistence and biogeochemical imprint of sea ice on the surface ocean in a warming climate.ice algae | permeability | polysaccharides | saline ice O ver the past few decades, the extent and thickness of Arctic sea ice have undergone significant climate-driven reductions that show no abatement (1-3). Current polar ecosystems depend on sea ice as a platform for foraging and reproduction by marine organisms and as a porous matrix that supports extensive blooms of ice algae. In the Arctic, these blooms account for a seasonally early and dominant fraction of total spring primary production (4). With continued reductions in areal extent of summer sea ice, however, phytoplankton activity in open waters is expected to dominate total primary production (5), leading to shifts away from ecosystems supported by fluxes of ice-algal material to the seafloor (6) toward pelagic ecosystems characteristic of lower latitudes (7). Predictions of reduced ice-algal production, however, do not consider the possibility that changes to sea ice microstructure and physical properties may stimulate primary productivity in the remaining ice.Here we reframe the question of how large-scale losses of Arctic sea ice will impact ecosystems and ask instead whether organisms-in particular sea-ice algae-have evolved means to alter ice physical properties to their benefit, mitigating impacts of climate change. The mechanism we consider derives from the biological production of extracellular polysaccharide substances (EPS)-organic materials of high surface area and complex behavior in aqueous solution (8)-observed microscopically in the brine inclusions of sea ice, where they are thought to function as cryoprotectants (9) and osmoprotectants (10). Do these substances also alter the microstructure of the...

A comprehensive seafloor biomass and abundance database has been constructed from 24 oceanographic institutions worldwide within the Census of Marine Life (CoML) field projects. The machine-learning algorithm, Random Forests, was employed to model and predict seafloor standing stocks from surface primary production, water-column integrated and export particulate organic matter (POM), seafloor relief, and bottom water properties. The predictive models explain 63% to 88% of stock variance among the major size groups. Individual and composite maps of predicted global seafloor biomass and abundance are generated for bacteria, meiofauna, macrofauna, and megafauna (invertebrates and fishes). Patterns of benthic standing stocks were positive functions of surface primary production and delivery of the particulate organic carbon (POC) flux to the seafloor. At a regional scale, the census maps illustrate that integrated biomass is highest at the poles, on continental margins associated with coastal upwelling and with broad zones associated with equatorial divergence. Lowest values are consistently encountered on the central abyssal plains of major ocean basins The shift of biomass dominance groups with depth is shown to be affected by the decrease in average body size rather than abundance, presumably due to decrease in quantity and quality of food supply. This biomass census and associated maps are vital components of mechanistic deep-sea food web models and global carbon cycling, and as such provide fundamental information that can be incorporated into evidence-based management.

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