Index >> Biodegradation of Pesticides and Pollutants >> Biovailability of Pesticides Pollutants

The manner in which organic substrates from NAPLs are mineralized has been a subject of speculation. One obvious explanation is that microorganisms use the substrates as and when they are solubilized and enter the water phase as exemplified by the dissolution and utilization of phenanthrene by Flavobacterium sp. and Beijerinckia sp. A second hypothesis is that bacteria attach to substrates by fimbriae and pili and directly dissolve the substrate from NAPL as has been shown in the case of Acinetobacter calcoaceticus in a NAPL composed of pure alkane.

An interesting pseudosolubilization hypothesis involves the bacterial excretion of emulsifiers composed mainly of polysaccharides, polysaccharide protein complexes or glycolipids that serve not only to increase the surface of the interphase between the NAPL (pure alkanes or oils) and water but also act as surfactants to reduce tensions between the different phases. This dual role of emulsifying agents results in the conversion of the NAPL to droplets or particles of 0.1 to 1.0 µm dimensions. These molecules have hydrophobic (water repellant) and hydrophilic (water loving) ends that aggregate to form micelles. In a micelle, the hydrophilic ends tend to face outwards to the waterphase while the hydrophobic ends get clustered in the centre. An analogy may make the layout of the micelle easy to understand. If several roundshaped small rubber balloons, each tied with a thread are arranged in a circle by facing the balloons (the' hydrophilic ends) to the circumference and the ends of threads (hydrophobic ends) coalesing to the centre of the circle, we can then imagine how micelle appears in water. It is presumed that the hydrophobic substrate contained in the NAPL would be entrapped in the inner region of the micelle in a semi soluble form and hence the usage of the word pseudosolubilization in this context. One can surmise that a continuity may exist from hydrophobic to hydrophilic portions of the surfactant molecules thereby rendering the substrate optimally available for microbial action. There are other hypotheses put forward to explain how immiscible substrates get mineralized by microorganisms. One possibility can be the occurrence of an emulsifier in the form of a thin layer on the bacterial surface. Another proposition has been the existence of a lipophilic layer, on the bacterial surface that facilitates the binding of the substrate to the cell.

The terms aged residue and bound residue have been used in discussions on bioavailability of pesticides for microbially mediated biodegradation. When pollutants get incorporated into humic compounds via oxidative coupling reactions by changing or altering the original chemical/biological activity and remain non extractable even with vigorous solvents, they are referred to as bound residues. On the otherhand, if the residue are amenable to extraction by solvents, they are referred to as aged residues. This can be illustrated in the case of the insecticide aldrin which is converted to dieldrin by soil microorganisms. When monitored at yearly intervals, it was observed that the residues in the form of both the compounds were traceable even after 5 years that get integrated in organic matter within compact deeper layers of soils. Such soils become deep polluted soils with limited access to degrading bacteria for interaction. These aged residues were released however by solvent extraction.