Discophora

Nelson YM, Lion LW, Ghiorse WC, Shuler ML (1999) Production of biogenic Mn oxides by Leptothrix discophora SS-1 in a chemically defined growth medium and evaluation of their Pb adsorption characteristics. Appl Environ Microbial 65 175-180... 

Huntsman, 1987 Moffett, 1994). At the optimum pH and temperature, the maximum oxidation rate (vmax) was 0.0059 pmol Mn(II)/min-mg cell (at 25°C, pH = 7.5, and a dissolved oxygen concentration of 8.05 mg/1) (Zhang et al., 2002). The halfvelocity coefficient (Ks) for Mn oxidation by L. discophora in the controlled bioreactors was 5.7 pmol Mn(II)/l. This value of Ks is similar to that determined previously under less-controlled conditions using buffers to regulate pH (Adams and Ghiorse, 1987). 

Using the controlled bioreactor experiments described above, we developed a general rate law for biological Mn oxidation by L. discophora that shows the mathematical dependence on cell concentration, Mn concentration, pH, temperature, dissolved oxygen concentration, and copper concentration (Zhang et al., 2002). 

It is interesting to compare the expected rates of Mn oxidation via abiotic mechanisms with the rates expected from the biological kinetic rate law described above. Abiotic Mn oxidation rates at pH 8.03 were measured in seawater by von Langen et al. (1997) who reported a first-order rate constant of l.lxlO-6 (normalized for Po2 = 1 atm and T = 25°C). At this pH and for similar conditions, the cell concentration of L. discophora required to obtain the same rate would be only 0.30 pg/1 (Zhang et al., 2002) (i.e., approximately 3x10s cells/1). It is reasonable to assume that cell populations of Mn-oxidizing bacteria far greater than this would be possible in natural environments. Even smaller population sizes would be required to match abiotic rates (if they could be measured) at lower pH values. 

It should be noted that the kinetic rate law described above is quantitatively applicable only to the strain of L. discophora used in the experiments described. Different Mn-oxidizing bacteria and even different stains of L. discophora would be expected to exhibit different rates of catalysis of Mn oxidation. For example, recent investigations in a wetland in New York State found many different genetic strains of Leptothrix, each exhibiting different rates of catalysis of Mn oxidation (Verity, 2001). However, the general form of the rate law could be expected to be similar for different species of Mn oxidizers. 

The first measurement of trace metal adsorption to laboratory-prepared biogenic Mn oxides was reported by Nelson et al. (1999b). In these experiments both Mn oxidation and trace metal adsorption were carried out under controlled conditions with L. discophora grown in a defined medium. The use of pH controllers in these... 

FIGURE 7.3 Effect of biogenic Mn oxide deposits on Pb adsorption by L. discophora cells at pH 6.0 and 25°C. Cell concentration = 63 mg/1, Mn loading = 0.8 mmol/g cells. (From Nelson, Y.M. et al., Appl. Environ. Microbiol., 65, 175, 1999b. With permission.)... 

FIGURE 7.6 X-ray diffraction pattern of biogenic Mn oxide produced by L. discophora prepared at pH 7.5. Peaks corresponding to polyhydroxybutyrate are labeled PHB, and the peak corresponding to ramsdellite is labeled accordingly... 

Adams, L.F. and Ghiorse, W.C., Characterization of extracellular Mn2+-oxidizing activity and isolation of an Mn2+-oxidizing protein from Leptothrix discophora SS-1, J. Bacteriol., 169, 1279, 1987. 

Brouwers, G.J. et al., Stimulation of Mn2+ oxidation in Leptothrix discophora SS-1 by Cu2+ and sequence analysis of the region flanking the gene encoding putative multicopper oxidase mofA, Geomicrobiol. J., 17, 25, 2000a. 

Zhang, J. et al., Kinetics of Mn oxidation by Leptothrix discophora SSI, Geochim. Cosmochim. Acta, 65, 773, 2002. 

Bacillus sp. SG-1) (Leptothrix discophora SS-1) (Pseudomonas putida GB-1) (plants)... 

Because Mn(II) is stable at pH <8, significant contributions from microorganisms to Mn(II) oxidation has been relatively easy to demonstrate, and a number of Mn(II) oxidizing bacteria have been isolated. Microbial oxidation is considered the primary mechanism of Mn(II) oxidation in circumneutral freshwater (Ghiorse, 1984 Nealson et al., 1988). The sheathed bacterium Leptothrix discophora is perhaps the most-studied species of Mn(II)-oxidizing bacteria, and rate laws have been developed to describe Mn(II) oxidation as a function of pH, temperature, dissolved O2, and Cu concentration (Zhang et al., 2002). Bacillus sp. 

Earlier evidence suggesting the possibility of chemolithotrophic growth of microorganisms on Mn(II) has been reviewed by Mulder (1964) and Johnson and Stokes (1966). One of the most likely candidates for successful chemolithotrophic growth on Mn(II) is Leptothrix discophora. Mulder (1964) presented evidence favouring the production by this organism of hydroxy-... 

Fig. 5.8. Oxidation of Mn(II) by Leptothrix discophora. Treatments as follows 1, suspension of manganese-grown cells 2, as in 1 plus 8 jumol of MnS04 mP 3, as in 2 except cell suspension heated at 93°C for 5 min 4, suspension of cells grown without MnS04 5, as in 4 plus 8 jumol of MnS04 mP 6, control with phosphate buffer plus MnS04 but without bacterium (from Johnson and Stokes, 1966).

Hope, C.K. Bott, T.R. Laboratory modelling of manganese biofiltration using biofilms of Lepto-thrix discophora. Water Res. 2004, 38, 1853-1861. 

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