Sphingomonas alaskensis Strain AFO1, an Abundant Oligotrophic Ultramicrobacterium from the North Pacific

Abstract

Numerous studies have established the importance of picoplankton (microorganisms of <2 m in length) in energy flow and nutrient cycling in marine oligotrophic environments, and significant effort has been 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 that 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 ؋ 10 5 cells ml ؊1 . This indicated that the isolate was representative of the most abundant bacteria at the sampling site, 1.5 km from Cape Muroto, Japan. The bacterium was characterized and found to be ultramicrosized (less than 0.1 m 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 sequence, DNA-DNA hybridization, G؉C mol%, genome size), chemotaxonomic (lipid analysis, morphology), and physiological (resistance to hydrogen peroxide, heat, and ethanol) characterization of the bacterium revealed that it was a strain of Sphingomonas alaskensis. The type strain, RB2256, was previously isolated from Resurrection Bay, Alaska, and similar isolates have been obtained from the North Sea. The isolation of this species over an extended period, its high abundance at the time of sampling, and its geographical distribution 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.

Cultivation and Growth Characteristics of a Diverse Group of Oligotrophic Marine Gammaproteobacteria

“…T and AFO1 formed colonies on nutrient-rich media and became facultative oligotrophs (17,47,48). The oligotrophic characteristics of the HTCC isolates in the OMG group were comparable to those of S. alaskensis.…”

“…HTCC isolates in the OMG group, therefore, can be considered model oligotrophic bacteria, similar to well-studied oligotrophic species, such as Sphingomonas alaskensis (17,55) and "Candidatus Pelagibacter ubique" (42) in the SAR11 clade. Since S. alaskensis strain RB2256…”

“…It was able to grow in very low nutrient media (Ͻ1 mg of C liter Ϫ1 ) but not in 5 mg of DOC liter Ϫ1 , and it has an ultramicrosize of Ͻ0.1 m 3 , a relatively low growth rate ( ϭ Ͻ0.2 h Ϫ1 ), and high-affinity substrate uptake systems (7,16,17,20,21,40,47,48). After prolonged storage at 4 to 5°C and laboratory adaptation, strains RB2256…”

Detection and Characterization of Conjugative Degradative Plasmids in Xenobiotic-Degrading Sphingomonas Strains

“…Sphingomonas strains appear to be widely distributed in various aquatic and terrestrial environments. They have been isolated from anthropogenic polluted river water and sediments (17,45,67) and medical material (69), and they constitute an important part of the marine bacterial plankton (11). Recently, sphingomonads have also been detected in rather high cell densities on the surfaces of various plants (31) and in biofilms found in drinking water supplies (32).…”

Biogeography, Evolution, and Diversity of Epibionts in Phototrophic Consortia

“…While studies of marine and freshwater microorganisms indicate that most of the species are ubiquitous (3,13,15,16,17,21,22,46,51), some soil bacteria appear to have a more limited distribution (10,47). Limited migration between North America and Central America has been shown for Rhizobium leguminosarum biovar phaseoli (47).…”