title: Extensive gene acquisition in the extremely psychrophilic bacterial species Psychroflexus torquis and the link to sea-ice ecosystem specialism.
abstract: Sea ice is a highly dynamic and productive environment that includes a diverse array of psychrophilic prokaryotic and eukaryotic taxa distinct from the underlying water column. Because sea ice has only been extensive on Earth since the mid-Eocene, it has been hypothesized that bacteria highly adapted to inhabit sea ice have traits that have been acquired through horizontal gene transfer (HGT). Here we compared the genomes of the psychrophilic bacterium Psychroflexus torquis ATCC 700755(T), associated with both Antarctic and Arctic sea ice, and its closely related nonpsychrophilic sister species, P. gondwanensis ACAM 44(T). Results show that HGT has occurred much more extensively in P. torquis in comparison to P. gondwanensis. Genetic features that can be linked to the psychrophilic and sea ice-specific lifestyle of P. torquis include genes for exopolysaccharide (EPS) and polyunsaturated fatty acid (PUFA) biosynthesis, numerous specific modes of nutrient acquisition, and proteins putatively associated with ice-binding, light-sensing (bacteriophytochromes), and programmed cell death (metacaspases). Proteomic analysis showed that several genes associated with these traits are highly translated, especially those involved with EPS and PUFA production. Because most of the genes relating to the ability of P. torquis to dwell in sea-ice ecosystems occur on genomic islands that are absent in closely related P. gondwanensis, its adaptation to the sea-ice environment appears driven mainly by HGT. The genomic islands are rich in pseudogenes, insertional elements, and addiction modules, suggesting that gene acquisition is being followed by a process of genome reduction potentially indicative of evolving ecosystem specialism.
authors.affiliation: Food Safety Centre, Tasmanian Institute of Agriculture, University of Tasmania, Australia.
authors.forename: Shi
authors.initials: S
authors.lastname: Feng
authors.forename: Shane M
authors.initials: SM
authors.lastname: Powell
authors.forename: Richard
authors.initials: R
authors.lastname: Wilson
authors.forename: John P
authors.initials: JP
authors.lastname: Bowman
mesh_terms: D000222:Adaptation, Physiological; D000864:Antarctic Regions; D001110:Arctic Regions; D003080:Cold Temperature; D017753:Ecosystem; D019143:Evolution, Molecular; D005227:Fatty Acids / Q000096:biosynthesis / Q000235:genetics; D041965:Flavobacteriaceae / Q000145:classification / Q000235:genetics* / Q000378:metabolism; D022761:Gene Transfer, Horizontal; D016680:Genome, Bacterial; D046448:Ice Cover / Q000382:microbiology*; D009792:Oceans and Seas; D010802:Phylogeny; D011135:Polysaccharides, Bacterial / Q000096:biosynthesis / Q000235:genetics; D020543:Proteome / Q000235:genetics / Q000378:metabolism; D011544:Pseudogenes
chemical_list: D005227:Fatty Acids; D011135:Polysaccharides, Bacterial; D020543:Proteome
keywords: ecosystem specialism; horizontal gene transfer; proteomics; psychrophilic bacteria; sea ice