The potential for horizontal gene transfer in sea ice R. Eric Collins1,2 andJody W. Deming1, 1School of Oceanography and Astrobiology Program, University of Washington, USAand 2Origins Institute, McMaster University, Canada (rec3141@mcmaster.ca)

Although ‘follow the water’ has long been amantra in astrobiology, the dominant state of waterin our solar system is not as a liquid but as a solid.Water ice is present in abundance on the satellitesof Saturn and Jupiter, including Titan and Europa,on Mars, on the Moon, and it may have played aprominent role in the evolution of life on Earth dur-ing putative ‘snowball Earth’ episodes.

One form of solid water present on Earth todayis sea ice, an extreme environment characterized bylow temperature (−2 to −35◦C) and high salinity(35 to 270 parts per thousand) in its brine inclu-sions, where microbes have been observed to reside.The physical concentration of microorganisms andthe subsequent imposition of physical, chemical, andbiological stresses in autumn and winter sea ice brinemay favor the occurrence of horizontal gene trans-fer there, which in turn is expected to aid in theadaptation of microorganisms to this seasonally andepochally dynamic environment.

As part of the International Polar Year (IPY)Circumpolar Flaw Lead System Study (CFL) wefound high concentrations of bacteria, viruses, andextracellular free DNA in sea ice brine relative tounderlying seawater (Fig. 1), each a proxy for theraw materials required for horizontal gene transferby conjugation, transduction, and transformation,respectively. Transduction is suggested as a promi-nent form of horizontal gene transfer in sea ice due toextremely high virus-to-bacteria contact rates pre-dicted in sea ice brine (Fig. 2) and the presenceof stressors, like rapid changes in temperature andsalinity, that have been shown to promote the induc-tion of prophage into the lytic cycle.

Horizontally transferred genes with particularutility in sea ice may include those encoding forthe production, degradation, or alteration of ex-tracellular polymeric substances or compatible so-lutes, compounds thought to have utility as cryopro-tectants and osmoprotectants. Complete genomecomparisons using a sea ice affiliated psychrophilicgammaproteobacterium, Colwellia psychrerythraea,revealed evidence for the horizontal acquisition ofgenes necessary for the complete catabolism ofglycine betaine (Fig. 3), a common compatible soluteused for protection against salt stress by membersof all three Domains of life. Growth experiments us-ing a defined medium indicated that sarcosine (an

intermediate in the catabolism of glycine betaine)was utilized by Colwellia psychrerythraea and sev-eral other isolates of Colwellia as a sole source of or-ganic carbon and nitrogen, suggesting that the hor-izontally transferred operon was expressed in vitro.

Figures

Figure 1: Concentrations of DNA in bacteria,viruses and extracellular DNA in seawater and seaice from Amundsen Gulf and Beaufort Sea, Cana-dian Arctic sector.

Figure 2: Virus-bacteria contact rates relative toseawater at −1.7◦C in sea ice from Amundsen Gulfand Beaufort Sea, Canadian Arctic sector.

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Figure 3: Phylogeny of 16S rRNA gene sequences from selected genomes in the Joint Genome InstituteIntegrated Microbial Genomes database. Alignment and neighbor-joining tree were acquired from SILVAversion 98. Large blue leaf font indicates the genome included at least one copy of the genes encodingheterotetrameric sarcosine oxidase.

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