Manganese oxidation by bacterial isolates from the Indian Ridge System
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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 sup(1) nutrient broth+2% agar) and on dilNA supplemented with 1, 2 and 3 mM MnCl sub(2) 4H sub(2)O were in the order of 10 sup(6) CFU l sup(1). Retrievability of heterotrophs ranged from non-detectable levels (ND) to 2.82 10 sup(6) CFU l sup(1). The retrievable counts on Mn amended dilNA ranged from ND to 3.21 x 10 sup(6), 1.47 x 10 sup(6) and 1.45 x 10 sup(6) CFU l sup(1) on 1, 2 and 3 mM, respectively. About 87% of the Mn tolerant isolates (n = 39) showed taxonomic affinities to Pseudomonas 1 and 2 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 x 10 sup(6) CFU l sup(1) ), while CR48 represented only 6%of the total CFU (1.05 x 10 sup(6) CFU l sup(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. The isolates were tested for their ability to oxidize Mn ammendments from 1 Mu M to 10 mM Mn. At the end of a 76-day incubation period, there was evidence of manganese oxide precipitation at high Mn concentrations (is greater than or equal to 1 mM) as a dark brown coloration on the sides of culture tubes. Highest Mn oxidation rates were observed at 10 mM Mn(2) concentration with CR35 oxidizing 27 and 25 Mu Mn day sup(-1) in unamended and amended condition, respectively. CR48 oxidized Mn at the rate of 26 Mu Mn day sup(1) in unamended medium and 35 Mu day sup(1) in amended medium. Scanning electron microscope (SEM) observations of both isolates revealed free-living cells in clustered matrices apprrox. 2 Mu 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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