Interaction of microorganisms with sheet silicates
To study the interaction between microorganisms and sheet silicates, nontronite (NAu-2), and chlinochlore (CCa-2) as a powder form less than 2 µm were incubated with two microorganisms; Streptomyces acidiscabies and Schizophyllum commune strains in liquid culture flasks for two months. CCa-2 is a non swelling mineral, while NAu-2 is a swelling mineral, where hydrated ions incorporate into the mineral interlayer. Consequently, the mineral expands and additional mineral surfaces are more exposed to solution and microbial attack. That is why NAu-2 was more susceptible to microbial dissolution than the CCa-2. X-ray diffraction (XRD) spectra showed that there was no change at all in the structure of CCa-2, while NAu-2 became amorphous to X-rays. S. acidiscabies E13 strain produces some organic acids, exo- polysaccharides (EPS), enzymes, siderophores, and melanin. The production of some of these biomolecules together at the same time might be the main reason that the S. acidiscabies strain was more efficient not only in releasing some elements from both minerals than the S. commune strain, but also in inducing some surface normal retreats of CCa-2 polished pieces. Vertical scanning interferometry (VSI) images showed that the S. acidiscabies strain altered the surface of small polished pieces of CCa-2, and caused a maximum loss of 2.6 µm3 per 1 µm2. S. acidiscabies was also the most effective in comparison to chemical acid treatments regardless organic or non-organic, which indicated that neither protonation nor chelation alone could be the mechanisms for the S. acidiscabies strain to perform such normal surface retreats over the CCa-2. Most probably the actinobacterium strain was able to alter the CCa-2 mineral surface, by the excretion of some organic acids together with siderophores, where both products were concentrated within EPS biofilms. Direct areal contact might be essential also for such dissolution and etch pits formations, as S. commune strain has left the surface of the mineral after colonizing it for a very short time. Therefore, the fungus strain hasn’t caused any effect over the small polished CCa-2 pieces. Microbial pellets formed by the S. acidiscabies strain with CCa-2 had heavily encrusted mineral flakes and smooth hyphae, while in case of NAu-2, no trace for many mineral flakes but the hyphae were almost encrusted. S. commune strain wasn’t able to form microbial pellets with any amount of CCa-2, and formed pellets with only 0.25 g of the NAu-2 minerals. While other fungal strains (Ceratocystis polonica and Alternaria brassicola) which produce melanin have formed microbial pellets with 1 g of NAu-2. FTIR showed that S. acidiscabies strain has produced in the presence of the two minerals some EPS with concentrations higher than what is present in the strains grown alone in the minimal medium. Esters of long chain fatty acids in the form of triglycerides might be present too as they were detected by TEM. Maps showing spatial distribution of some FTIR bands for the S. acidiscbaies strain inoculated with NAu-2 have shown that EPS (polysaccharides and proteins), were localized inside the pellets for mineral attachment. While when the S. acidiscabies strain was incubated with CCa-2 the phosphate ester band at 1245 cm-1 mismatched the spatial distribution of fatty esters, which indicated that phospholipids were not a major component in the sample, however their distribution on the edges of the pellets with the rich Mg CCa-2 mineral showed that they are a potent magnesium chelator. CCa-2 haven’t induced neither S. acidiscabies nor S. commune strains to produce any extracellular proteins, while with NAu-2 both strains have produced some extracellular proteins in the liquid culture medium, which was measured by the Bradford assay.