Sphingomonas alaskensis strain AFO1, an abundant oligotrophic ultramicrobacterium from the North Pacific

Numerous studies have established the importance of picoplankton (microorga nisms of less than or equal to2 mum in length) in energy flow and nutrient cycling in marine oligotrophic environments, and significant effort has bee n directed at identifying and isolating heterotrophic picoplankton from the world's oceans. Using a method of diluting natural seawater to extinction followed by monthly subculturing for 12 months, a bacterium was isolated th at was able to form colonies on solid medium. The strain was isolated from a 10(5) dilution of seawater where the standing bacterial count was 3.1 X 1 0(5) cells ml(-1). This indicated that the isolate was representative of th e most abundant bacteria at the sampling site, 1.5 km from Cape Muroto, Jap an. The bacterium was characterized and found to be ultramicrosized (less t han 0.1 mum(3)), and the size varied to only a small degree when the cells were starved or grown in rich media. A detailed molecular (16S rRNA sequenc e, DNA-DNA hybridization, G+C mol%, genome size), chemotaxonomic (lipid ana lysis, morphology), and physiological (resistance to hydrogen peroxide, hea t, and ethanol) characterization of the bacterium revealed that it was a st rain of Sphingomonas alaskensis. The type strain, RB2256, was previously is olated from Resurrection Bay, Alaska, and similar isolates have been obtain ed from the North Sea. The isolation of this species over an extended perio d, its high abundance at the time of sampling, and its geographical distrib ution indicate that it has the capacity to proliferate in ocean waters and is therefore likely to be an important contributor in terms of biomass and nutrient cycling in marine environments.