Characterisation, Sources and Flux of Unmelted Micrometeorites on Earth During the Last ~50,000 Years.

Dust dominates extraterrestrial flux on the earth (30,000 tonnes/yr), however only ~5% of the cosmic dust survives atmospheric entry which is basically in two forms: melted and unmelted. Melted micrometeorites undergo transformational changes due to heating during atmospheric entry which obliterate evidences regarding their precursors. Unmelted micrometeorites (UMM) survive atmospheric entry with minimal alteration, they provide direct evidence for their parent bodies. Recent investigations unravelled a wide range of UMM, there are however no quantitative estimates of sources that contribute to the cosmic dust accreted by the Earth.

Cobalt immobilization by manganese oxidizing bacteria from the Indian ridge system.

Co immobilization by two manganese oxidizing isolates from Carlsberg Ridge waters (CR35 and CR48) was compared with that of Mn at same molar concentrations. At a lower concentration of 10 µM, CR35 and CR48 immobilized 22 and 23 fM Co cell(-1) respectively, which was 1.4 to 2 times higher than that of Mn oxidation, while at 10 mM the immobilization was 15-69 times lower than that of Mn. Scanning electron microscope and energy dispersive X-ray analyses of intact bacterial cells grown in 1 mM Co revealed Co peaks showing extracellular binding of the metal. However, it was evident from transmission electron microscope analyses that most of the sequestered Co was bound intracellularly along the cell membrane in both the isolates. Change in morphology was one of the strategies bacteria adopted to counter metal stress. The cells grew larger and thus maintained a lower than normal surface area-volume ratio on exposure to Co to reduce the number of binding sites. An unbalanced growth with increasing Co additions was observed in the isolates. Cells attained a length of 10-18 µm at 10 mM Co which was 11-15 times the original cell length. Extensive cell rupture indicated that Co was harmful at this concentration. It is apparent that biological and optimal requirement of Mn is more than Co. Thus, these differences in the immobilization of the two metals could be driven by the differences in the requirement, cell physiology and the affinities of the isolates for the concentrations of the metals tested.

Manganese oxidation by bacterial isolates from the Indian ridge system.

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 MnCl(2).4H(2)O were in the order of 10(6) CFU l(-1). Retrievability of heterotrophs ranged from non-detectable levels (ND) to 2.82 x 10(6) CFU l(-1). The retrievable counts on Mn amended dilNA ranged from ND to 3.21 x 10(6), 1.47 x 10(6) and 1.45 x 10(6) 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 x 10(6) CFU l(-1)), while CR48 represented only 6% of the total CFU (1.05 x 10(6) 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 microM Mn both in unamended and amended media. At levels of Mn greater than 100 microM the counts decreased steadily. Total respiring cells of CR48 were stimulated to a maximum at 1 microM Mn in unamended medium and 1 nM in amended medium. Total cells counts for the same decreased beyond 100 microM Mn in unamended and 1 nM in amended medium. The isolates were tested for their ability to oxidize Mn amendments from 1 microM to 10 mM Mn. At the end of a 76-day incubation period, there was evidence of manganese oxide precipitation at high Mn concentrations (>or=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 microM Mn day(-1) in unamended and amended condition, respectively. CR48 oxidized Mn at the rate of 26 microM Mn day(-1) in unamended medium and 35 microM Mn day(-1) in amended medium. Scanning electron microscope (SEM) observations of both isolates revealed free-living cells in clustered matrices approximately 2 microm 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.