Water-dissolved substrates, uptake

The type of carrier that is involved in substrate mobilisation largely depends on the chemical and physical state of the substrate. It is generally agreed that most microorganisms rely on the presence of water-dissolved substrate molecules. This is because microbial cell walls exclude the uptake of molecules larger than about 600 g moL1 [3]. Furthermore, the activity and growth of most... 

Micelles forming above the c.m.c. incorporate hydrophobic molecules in addition to those dissolved in the aqueous phase, which results in apparently increased aqueous concentrations. It has to be noted, however, that a micelle-solubilised chemical is not truly water-dissolved, and, as a consequence, is differently bioavailable than a water-dissolved chemical. The bioavailability of hydrophobic organic compounds was, for instance, reduced by the addition of surfactant micelles when no excess separate phase compound was present and water-dissolved molecules became solubilised by the micelles [69], In these experiments, bacterial uptake rates were a function of the truly water-dissolved substrate concentration. It seems therefore that micellar solubilisation increases bioavailability only when it transfers additional separate phase substrate into the aqueous phase, e.g. by increasing the rates of desorption or dissolution, and when micelle-solubilised substrate is efficiently transferred to the microorganisms. Theoretically, this transfer can occur exclusively via the water phase, involving release of substrate molecules from micelles, molecular diffusion through the aqueous phase and microbial uptake of water-dissolved molecules. This was obviously the case, when bacterial uptake rates of naphthalene and phenanthrene responded directly to micelle-mediated lowered truly water-dissolved concentrations of these chemicals [69]. These authors concluded from their experiments that micellar naphthalene and phenanthrene had to leave the micellar phase and diffuse through the water phase to become... 

We have discussed the positive effects that bacterial displacement and adhesion to substrates exerts on the diffusive mass transfer. Theoretically, direct contact with the substrate could also allow microorganisms to employ other modes of uptake in addition to absorption of water-dissolved molecules. So, which are the physical states for which chemicals can be ingested by microorganisms ... 

Environmental chemicals occur as pure liquid or solid compounds, dissolved in water or in nonaqueous liquids, volatilised in gases, dissolved in solids (absorbed) or bound to interfaces (adsorbed). Figure 5 gives a schematic view of the different physical states at which substrates are taken up by microbial cells. There is a consensus that water-dissolved chemicals are available to microbes. This is obvious for readily soluble chemicals, but there is also clear evidence for microbial uptake of the small dissolved fractions of poorly water soluble compounds. Rogoff already had shown in 1962 that bacteria take up phenanthrene from aqueous solution [55], In the intervening time many other researchers have made the same observation with various combinations of microorganisms and poorly soluble compounds [14,56,57]. 

Figure 5. Schematic representation of the different physical states at which environmental chemicals occur, with an indication of how these substrates are taken up by bacteria. Note that there is unanimous agreement only on the uptake of water-dissolved low-molecular-weight molecules...

It can be concluded that the uptake of molecules by microorganisms in other than the water-dissolved state seems to be the exception rather than the rule. Neither the observation of direct contact of microorganisms with nondissolved substrates, nor the fact that in some cases this contact is needed for microbial growth or substrate degradation, provide evidence for the direct uptake of the separate phase substrates. 

A simple biodegradation model is one in which microorganisms are in contact with water containing a dissolved organic chemical that serves as the energy substrate. Because chemical uptake into a cell is followed by enzymatic transformation, biodegradation and uptake rates are equivalent in this model. It is... 

Since the product of flow rate, time and concentration equal the input mass, a constant input concentration permits the calculation of mass from either time or retention volume. Empty columns provide an essentially constant ratio of input mass to time at constant flow rate with the concentration of water vapor fixed by the temperature of the carrier gas saturated with water vapor (100Z RH or Aw of 1 see Figure 6). This state can be achieved with substrates that do not dissolve in water when saturated (for example, starches and many proteins), or when the relative humidity is constant but insufficient to allow uptake to produce a highly multilayered or clustered water state in the substrate equivalent to a continuous water phase or solution. This condition requires a source of humidified gas as in (7.). The sorption isotherm equation is then given by... 

A number of different mechanisms have therefore clearly emerged and it seems premature to draw general conclusions especially with respect to the application of these natural surfactants to bioremediation that is discussed in greater detail in Chapter 8, Section 8.2.1. It is important to note that production of biosurfactants may not be the only mechanism for facilitating the uptake of substrates with low water solubility. For a strain of Rhodococcus sp. that did not produce surfactants, the rates of degradation of pyrene dissolved in water in the presence of insoluble, nondegradable 2,2,4,4,6,8,8-heptameth-ylnonane exceeded those predicted for physicochemical transfer from the solvent to the aqueous phase, but could be accounted for on the basis of uptake both from the interface and from the aqueous solution (Bouchez et al. 1997). 

---