Roseobacter fucihabitans sp. nov., isolated from the brown alga Fucus spiralis.

A Gram-negative, aerobic, pink-pigmented, and bacteriochlorophyll a-containing bacterial strain, designated B14T, was isolated from the macroalga Fucus spiralis sampled from the southern North Sea, Germany. Based on 16S rRNA gene sequences, species of the genera Roseobacter and Sulfitobacter were most closely related to strain B14T with sequence identities ranging from 98.15 % (Roseobacter denitrificans Och 114T) to 99.11 % (Roseobacter litoralis Och 149T), whereas Sulfitobacter mediterraneus CH-B427T exhibited 98.52 % sequence identity. Digital DNA-DNA hybridization and average nucleotide identity values between the genome of the novel strain and that of closely related Roseobacter and Sulfitobacter type strains were <20 % and <77 %, respectively. The novel strain contained ubiquinone-10 as the only respiratory quinone and C18 : 1 ω7c, C16 : 0, C18 : 0, C12 : 1 ω7c, C18 : 2 ω7,13c, and C10 : 0 3-OH as the major cellular fatty acids. The predominant polar lipids of strain B14T were phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol. The genome of strain B14T comprises a chromosome with a size of 4.5 Mbp, one chromid, and four plasmids. The genome contains the complete gene cluster for aerobic anoxygenic photosynthesis required for a photoheterotrophic lifestyle. The results of this study indicate that strain B14T (=DSM 116946T=LMG 33352T) represents a novel species of the genus Roseobacter for which the name Roseobacter fucihabitans sp. nov. is proposed.

Antibiotic production by a Roseobacter clade-affiliated species from the German Wadden Sea and its antagonistic effects on indigenous isolates.

A strain affiliated with the Roseobacter clade and producing a new antibiotic named tropodithietic acid (L. Liang, Ph.D. thesis, University of Göttingen, Göttingen, Germany, 2003) was isolated from the German Wadden Sea. The compound showed strong inhibiting properties with respect to marine bacteria of various taxa and marine algae. Antibiotic production was found to occur during the complete growth phase. Strain mutants without antagonistic properties appeared several times spontaneously.

Antagonistic activity of bacteria isolated from organic aggregates of the German Wadden Sea.

Marine aggregates are densely colonized by bacteria, and inter-specific interactions such as inhibition are important for colonization by aggregate-associated bacteria and thus affect the turnover of organic matter in the sea. In order to study antagonistic activities we carried out inhibition tests with 51 isolates obtained exclusively from aggregates of the German Wadden Sea. 16S rRNA gene sequences of all isolates revealed that 35% of the isolates affiliated with the Flavobacteria/Sphingobacteria group, 24% and 16% with alpha- and gamma-Proteobacteria, respectively, 16% with the Bacillus/Clostridium group, and 10% with Actinobacteria. The relatively high percentage of Gram-positive bacteria may be related to specific features of the Wadden Sea environment. After 11 days of incubation using Burkholder agar diffusion assays the percentage of inhibitory isolates was 54.1% and this decreased to 20.7% after 20 days of incubation but it did not decline for members of the Bacillus/Clostridium group. Inhibitory activity was expressed in strain-specific patterns even though some isolates were closely related according to their 16S rRNA gene sequences. Antagonistic activity was lowest for Flavobacteria/Sphingobacteria (35%) and highest for Actinobacteria (80%). We further examined whether growth of isolates was affected when they were placed on lawns of certain other isolates. In parallel with lowest percentage of inhibitory isolates, highest growth occurred on lawns of the Flavobacteria/Sphingobacteria group whereas it was lowest on lawns of Actinobacteria and the Bacillus/Clostridium group. The high inhibitory activity of both groups of Gram-positive bacteria fits well with data from chemical screening using matrix-assisted laser desorption ionization time of flight mass spectrometry. Hence, inhibitory activity greatly influences inter-specific interactions and may impact microbial degradation and remineralization of particulate organic matter in aquatic environments.

Possible quorum sensing in marine snow bacteria: production of acylated homoserine lactones by Roseobacter strains isolated from marine snow.

We report here, for the first time, that bacteria associated with marine snow produce communication signals involved in quorum sensing in gram-negative bacteria. Four of 43 marine microorganisms isolated from marine snow were found to produce acylated homoserine lactones (AHLs) in well diffusion and thin-layer chromatographic assays based on the Agrobacterium tumefaciens reporter system. Three of the AHL-producing strains were identified by 16S ribosomal DNA gene sequence analysis as Roseobacter spp., and this is the first report of AHL production by these alpha-PROTEOBACTERIA: It is likely that AHLs in Roseobacter species and other marine snow bacteria govern phenotypic traits (biofilm formation, exoenzyme production, and antibiotic production) which are required mainly when the population reaches high densities, e.g., in the marine snow community.