In an attempt to isolate bacteria solubilizing of silicates in soil, several bacterial isolates were found capable. The isolate TNAU-S was capable of silicate solubilization in vitro. An almost complete 16S rRNA gene sequence of TNAU-S was obtained. The 16S rRNA sequence based phylogenetic analysis of TNAU-S revealed that the isolate were more closely related to Pseudomonas stutzeri and exhibited 100 % homology with type strain of Pseudomonas stutzeri ATCC 17588 NR041715. The 16S rRNA sequences of Pseudomonas stutzeri TNAU-S was deposited in the GenBank database with accession no. KP099588. Evolutionary tree based on 16S rRNA with neighbor-joining method, strain TNAU-S formed a stable clade with Pseudomonas stutzeri ATCC 17588 NR041715, which was supported by strong bootstrap value (100%). The ability of Pseudomonas stutzeri to dissolute silica from different silicate minerals (Talc, Feldspar and Magnesium trisilicate) were studied by in vitro dissolution method. The results showed that maximum dissolution was found in magnesium trisilicate (7.5 mg.l-1 at 16DAI) supplemented with glucose. But in case of Feldspar, maximum dissolution was observed in medium without glucose (6.3 mg.l-1).
Preliminary experiments showed that Pseudomonas stutzeri may have used the silicon compounds as an energy source, enabling them to fix CO2 from the atmosphere. Hence, the Pseudomonas stutzeri culture was grown in silica medium supplemented with and without CO2. The results showed that silica medium supplemented with CO2 had maximum biomass and dissolution of silica (1.5 mg.l-1) when compared to without CO2 (Table 2; Fig. 1). The possibility that bacteria can grow autotrophically under oligotrophic conditions (using energy obtained from hydrogen oxidation) was suggested. Bigger et al. observed that silicon compounds might adsorb ammonia and CO2 from the atmosphere, thereby allowing bacteria to fix CO2, using energy obtained from the oxidation of ammonium. Similar results also obtained by Chakrabarty et al. Although it is generally thought that silicon compounds are biologically unreactive, stated that there is no theoretical reason why the reaction of Si-Si-Si with oxygen or oxygen compounds could not act as an energy-yielding reaction. However, the possibility that fungi and other microorganisms might use silicon-based autotrophy clearly remains speculative. Whatever the mechanism involved, it is clear that silicic acid and other silicon-containing compounds, promote bacterial growth under oligotrophic conditions, a fact which helps explain the ability of bacteria to grow on nutrient-free silica.
SEM-EDAX analysis of the bacterial culture revealed the presence of carbon in the cells grown exclusively on silica (Fig. 2) and this was in agreement to. This is suggestive of bacterial transmutation of silica to carbon. This may be due to the reason that, silicon has four bonding electrons available, like carbon, while Si is a bigger atom and makes longer and weaker bonds. Like that of carbon, silicon also has four open slots in its outer electron shell which may also from a basis for complex biological life.
To further confirm, experiments using minimal broth supplemented with and without carbon under both open and closed conditions was performed. P. stutzeri put forth half of biomass in minimal broth with magnesium trisilicate only as compared to in a medium with glucose. However, minimal broth supplemented with silicate source recorded the highest organic carbon (Table 3). There was no significant changes in pH during the growth of P. stutzeri (data not shown; pH ranged from 6.8-7.1). P. stutzeri grown in a laboratory scale fermenter with 0.25% magnesium trisilicate was producing 0.8 g.l-1 of dry biomass on 4th day. This shows that P. stutzeri is capable of solubilizing silicate in the medium and biologically transmutating it to form carbon.