Production of Manganese Oxide Nanoparticles by Shewanella Species.

UNLABELLED: Several species of the bacterial genus Shewanella are well-known dissimilatory reducers of manganese under anaerobic conditions. In fact, Shewanella oneidensis is one of the most well studied of all metal-reducing bacteria. In the current study, a number of Shewanella strains were tested for manganese-oxidizing capacity under aerobic conditions. All were able to oxidize Mn(II) and to produce solid dark brown manganese oxides. Shewanella loihica strain PV-4 was the strongest oxidizer, producing oxides at a rate of 20.3 mg/liter/day and oxidizing Mn(II) concentrations of up to 9 mM. In contrast, S. oneidensis MR-1 was the weakest oxidizer tested, producing oxides at 4.4 mg/liter/day and oxidizing up to 4 mM Mn(II). Analysis of products from the strongest oxidizers, i.e., S loihica PV-4 and Shewanella putrefaciens CN-32, revealed finely grained, nanosize, poorly crystalline oxide particles with identical Mn oxidation states of 3.86. The biogenic manganese oxide products could be subsequently reduced within 2 days by all of the Shewanella strains when culture conditions were made anoxic and an appropriate nutrient (lactate) was added. While Shewanella species were detected previously as part of manganese-oxidizing consortia in natural environments, the current study has clearly shown manganese-reducing Shewanella species bacteria that are able to oxidize manganese in aerobic cultures. IMPORTANCE: Members of the genus Shewanella are well known as dissimilatory manganese-reducing bacteria. This study shows that a number of species from Shewanella are also capable of manganese oxidation under aerobic conditions. Characterization of the products of the two most efficient oxidizers, S. loihica and S. putrefaciens, revealed finely grained, nanosize oxide particles. With a change in culture conditions, the manganese oxide products could be subsequently reduced by the same bacteria. The ability of Shewanella species both to oxidize and to reduce manganese indicates that the genus plays a significant role in the geochemical cycling of manganese. Due to the high affinity of manganese oxides for binding other metals, these bacteria may also contribute to the immobilization and release of other metals in the environment.

Aliidiomarina minuta sp. nov., a haloalkaliphilic bacterium that forms ultra-small cells under non-optimal conditions.

An aerobic haloalkaliphilic bacterium, designated strain MLST1(T), was isolated from filtered (0.22 µm) Mono Lake (USA) waters. The isolate was observed to grow primarily on yeast extract, peptone and tryptone. Optimal growth occurred in media at pH 9.5 containing 5-11 g/l yeast extract, and 70-100 g/l NaCl. When in log phase of growth, cells were found to be mostly curved motile rods (1-3 µm length by 0.4-1 µm diameter). Phylogenetic analysis of the 16S rRNA gene and chemotaxonomic data revealed that the isolate belonged to the family Idiomarinaceae, and is closely related to Aliidiomarina maris (96.67 % sequence similarity). The major fatty acids were identified to be iso-C17:1 ω9c (27.1 %), iso-C17:0 (21.3 %) and iso-C15:0 (12.2 %). Predominant polar lipids included phosphatidylethanolamine, diphosphatidylglycerol, phosphatidylglycerol, and the major respiratory quinone was identified as Q8. The DNA base composition was 46.3 mol% G+C. Survival studies indicated that strain MLST1(T) remains viable after exposure to adverse conditions, particularly in the prolonged absence of a carbon source, at low temperatures and with no NaCl. Under these conditions, the cells shrunk to around 0.2 µm in length by 0.1 µm in diameter and passed through 0.22 µm filters. The ultra-small cells could only be resuscitated in media with low levels of yeast extract, up to 0.6 g/l. Once resuscitated, cells were able to grow to full size. Strain MLST1(T) is clearly a unique bacterium in the waters of Mono Lake and the name Aliidiomarina minuta sp. nov. is proposed. The type strain is MLST1(T) (=JCM 17425(T) = KCTC 23357(T)).