Polar Biol (2010) 33:543–556 DOI 10.1007/s00300-009-0732-0 123 ORIGINAL PAPER Acquisition of freeze protection in a sea-ice crustacean through horizontal gene transfer? Rainer Kiko Received: 28 December 2008 / Revised: 15 September 2009 / Accepted: 19 September 2009 / Published online: 5 November 2009  Springer-Verlag 2009 Abstract Sea ice is permeated by small brine channels, which are characterised by sub-zero temperatures and vary- ing salinities. Despite sometimes extreme conditions a diverse fauna and Xora thrives within the brine channels. The dominant calanoid copepods of Antarctic sea ice are Stephos longipes and Paralabidocera antarctica. Here, I report for the Wrst time thermal hysteresis (TH) in the hae- molymph of a crustacean, S. longipes, whereas P. antarc- tica has no such activity. TH, the non-colligative prevention of ice growth, seems to enable S. longipes to exploit all available microhabitats within sea ice, especially the surface layer, in which strong temperature Xuctuations can occur. In contrast, P. antarctica only thrives within the lowermost centimetres of sea ice, where temperature Xuctu- ations are moderate. S. longipes possesses two isoforms of a protein with TH activity. A high homology to a group of (putative) antifreeze proteins from diatoms, bacteria and a snow mold and, in contrast, no homologs in any metazoan lineage suggest that this protein was obtained through hori- zontal gene transfer (HGT). Further analysis of available sequence data from sea-ice organisms indicates that these antifreeze proteins were probably transferred horizontally several times. Temperature and salinity Xuctuations within the brine channel system are proposed to provide “natural transformation” conditions enabling HGT and thus making this habitat a potential “hot spot” for HGT. Keywords Sympagic meiofauna · Antifreeze protein · Ice binding protein · In situ hybridization · Stephos longipes · Paralabidocera antarctica · Lateral gene transfer Abbreviations Aa Amino acid AFP Antifreeze protein AF(G)P Antifreeze(glyco)protein eEF-1 Eukaryotic elongation factor 1 alpha FSW Filtered seawater HGT Horizontal gene transfer HSP70 Heat shock protein 70 RI Recrystallization inhibition Rubisco Ribulose-1,5-bisphosphate carboxylase/oxygenase SSH Suppression subtractive hybridization TH Thermal hysteresis Introduction Sea ice is porous and houses a fascinating interstitial Xora and fauna (Thomas and Dieckmann 2002). Three interstitial habitats, characterized by diVerent formation processes, are associated with Antarctic sea ice. From top to bottom these are the surface layer, the brine channel system within and at the bottom of the ice as well as the platelet layer accumulat- ing underneath the ice in the vicinity of ice shelves (Horner et al. 1992). Besides viruses, bacteria and unicellular algae Electronic supplementary material The online version of this article (doi:10.1007/s00300-009-0732-0) contains supplementary material, which is available to authorized users. R. Kiko Institute for Polar Ecology, Wischhofstr. 1-3, Bldg. 12, 24148 Kiel, Germany R. Kiko Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany Present Address: R. Kiko (&) Leibniz Institute of Marine Sciences, IFM-GEOMAR, Hohenbergstr. 2, 24105 Kiel, Germany e-mail: rkiko@ifm-geomar.de; rkiko@ipoe.uni-kiel.de