METHYLOSPHAERA HANSONII GEN-NOV, SP-NOV, A PSYCHROPHILIC, GROUP-I METHANOTROPH FROM ANTARCTIC MARINE-SALINITY, MEROMICTIC LAKES

Methanotrophic bacteria were enumerated and isolated from the chemocli ne and surface sediments of marine-salinity Antarctic meromictic lakes located in the Vestfold Hills, Antarctica (68 degrees S 78 degrees E) , Most probable number (MPN) analysis indicated that at the chemocline of Ace Lake the methanotroph population made up only a small proporti on of the total microbial population and was sharply stratified, with higher populations detected in the surface sediments collected at the edge of Ace Lake and Burton Lake, Methanotrophs were not detected in P endant Lake. Only a single phenotypic group of methanotrophs was succe ssfully enriched, enumerated and isolated into pure culture from the l ake samples. Strains of this group were non-motile, coccoidal in morph ology, did not form resting cells, reproduced by constriction, and req uired seawater for growth. The strains were also psychrophilic, with o ptimal growth occurring at 10-13 degrees C and maximum growth temperat ures of 16-21 degrees C. The ribulose monophosphate pathway but not th e serine pathway for incorporation of C-1 compounds was detectable in the strains. The guanine plus cytosine (G+C) content of the genomic DN A was 43-46 mol%. Whole-cell fatty acid analysis indicated that 16:1 o mega 8c (37-41 %), 16:1 omega 6c (17-19 %), 16:1 omega 7c (15-19 %) an d 16:0 (14-15 %) were the major fatty acids in the strains. 16S rDNA s equence analysis revealed that the strains form a distinct line of des cent in the family Methylococcaceae (group I methanotrophs), with the closest relative being the Louisiana Slope methanotrophic mytilid endo symbiont (91.8-92.3 % sequence similarity). On the basis of polyphasic taxonomic characteristics the Antarctic lake isolates represent a nov el group I methanotrophic genus with the proposed name Methylosphaera hansonii (type strain ACAM 549).