The effect of divalent metal ions on biomethanation was studied using pre-reduced methanogenic AC21 medium in absence or in presence of leaf or root extracts of Eichornia grassipes (water hyacinth) in vitro. The metal ions like Cu 2+ , Fe 2+ , Ni 2+ , and Co 2+ were found to be effective in increasing methane production in vitro at low concentrations, but toxic inhibitory effects were observed at relatively high concentrations. Zn 2+ ion was inhibitory in character with exceptions. The bioavailability of metal ions in AC21 medium alone or with juices from leaf or root extracts of plant (water hyacinth in the present case) was limited due to complex formation of metal ions with different constituents in the medium, precipitation, etc. However, the results of the microscopic experiments (in vials) with metal ions for methane production by isolated methanogens in the methanogenic medium should not be compared with the conversion of biomass to methane in biodigesters. In such cases, not only a large number of methanogenic bacteria of different strains but also a consortium of bacteria of different specificities is usually involved. Though the biomass may contain sufficient concentrations of metal ions, it is advisable to add metal ions like Cu 2+ , Fe 2+ , Zn 2+ ,Co 2+ , etc. in the biomass, but the concentrations of the metal ions should be limited to 10-20 µM of the biomass to make methane generation successful.

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Twenty-one mixed-ligand complexes of cobalt(II) and cobalt(III) have been screened for their antimicrobial, insect sterilizing and ovicidal activities. Three of these cobalt(III) complexes exhibit broad antimicrobial spectra, including against human bacterial pathogens, dermatophytes and plant pathogenic fungi, while one exhibits feeble activity against a human pathogenic bacterium. These results have been compared with the activity of the corresponding cobalt(II) complexes, which have been found to be inactive, while the free ligands show reduced activity compared with the cobalt(III) complexes. Change in biological activity induced by a particular complex appears to be dependent on the composition of the first co-ordination sphere. Two of these complexes showed 50% inhibition of the conidial germination of Helminthosporium oryzae and Alternaria triticina. These results indicate their potential for use against human and plant pathogenic microbes. Minimum inhibitory concentrations of the cobalt(III) complexes were determined. Three of these cobalt(III) complexes have been tested for insect sterilizing and ovicidal activities on Dysdercus koenigi F. Positive sterilizing and ovicidal actions of [Co(BSOP)(NH3)2]NO3 (where BSOP-H2 is the Schiff base derived from salicylaldehyde and orthophenylene-diamine) were obtained. Possible mechanism(s) of all these activities are discussed qualitatively.

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