Manganese Oxidation by Bacterial Isolates from the Indian Ridge System

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Abstract
The abundance and activity of culturable manganese-oxidizing bacteria were assessed from near-bottom water samples of the tectonically active Carlsberg Ridge. Retrievable counts as colony forming units (CFU) on dilute nutrient agar medium (dilNA = 2 gm l−1 nutrient broth+2% agar) and on dilNA supplemented with 1, 2 and 3 mM MnCl2·4H2O were in the order of 106 CFU l−1. Retrievability of heterotrophs ranged from non-detectable levels (ND) to 2.82 × 106 CFU l−1. The retrievable counts on Mn amended dilNA ranged from ND to 3.21× 106, 1.47 × 106 and 1.45 × 106 CFU l−1 on 1, 2 and 3 mM, respectively. About 87% of the Mn tolerant isolates (n = 39) showed taxonomic affinities to Pseudomonas I and II sp. Two representative strains CR35 and CR48 (CR–Carlsberg Ridge) isolated on manganese-supplemented media were tested for their ability to tolerate a range of Mn amendments from 1 nM to 100 mM in terms of growth and respiration. CR35 represents 66% of the total CFU (3.04 × 106 CFU l−1), while CR48 represented only 6% of the total CFU (1.05 × 106 CFU l−1). The colonies of these two isolates were dark brown in color suggesting precipitation of Mn as oxide. Tests for the effect on growth and respiration were conducted in media simulating heterotrophic (amended with 0.01% glucose) and lithotrophic (unamended) conditions. Maximum stimulation in growth and respiration of CR35 occurred at 100 µM Mn both in unamended and amended media. At levels of Mn greater than 100 µM the counts decreased steadily. Total respiring cells of CR48 were stimulated to a maximum at 1 µM Mn in unamended medium and 1 nM in amended medium. Total cells counts for the same decreased beyond 100 µM Mn in unamended and 1 nM in amended medium. The isolates were tested for their ability to oxidize Mn ammendments from 1 µM to 10 mM Mn. At the end of a 76-day incubation period, there was evidence of manganese oxide precipitation at high Mn concentrations (≥1 mM) as a dark brown coloration on the sides of culture tubes. Highest Mn oxidation rates were observed at 10 mM Mn(II) concentration with CR35 oxidizing 27 and 25 µM Mn day−1 in unamended and amended condition, respectively. CR48 oxidized Mn at the rate of 26 µM Mn day−1 in unamended medium and 35 µM Mn day−1 in amended medium. Scanning electron microscope (SEM) observations of both isolates revealed free-living cells in clustered matrices ≈2 µm diameter. Energy dispersive spectrum of the cell matrix of CR35 cultured in 1 mM Mn detected 30% Mn, while the cell aggregates of CR48 harbored 7–10% Mn. The relatively high specific activity of these mixotrophic bacteria under relatively oligotrophic conditions suggests that they may be responsible for scavenging dissolved Mn from the Carlsberg Ridge waters and could potentially participate in oxidation.

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