Organic mobilisation

Tyre-derived rubber materials should not be used in areas with high pH (e.g., pH 8 or above) or very low pH (e.g., pH 5 or below) as there is a greater potential for metal/organic mobilisation. 

The subject of this chapter will be the most important mechanisms by which microorganisms mobilise substrates. We will thereby focus on the mobilisation of nonliving food molecules, and will not deal with living food organisms. Besides organic compounds, we will also treat the biological mechanisms to acquire iron as the least bioavailable inorganic nutrient in many environments. 

MOBILISATION OF HYDROPHOBIC ORGANIC SUBSTRATES USING BIOSURFACTANTS... 

Microbes were frequently found to synthesise surface-active molecules in order to mobilise hydrophobic organic substrates. These biosurfactants, which are either excreted by the producing organisms or remain bound to their cell surfaces, are composed of a hydrophilic part making them soluble in water and a lipophilic part making them accumulate at interfaces. With respect to their physical effects, one can distinguish two types of biosurfactants firstly, molecules that drastically reduce the surface and interfacial tensions of gas-liquid, liquid-liquid and liquid-solid systems, and, secondly, compounds that stabilise emulsions of nonaqueous phase liquids in water, often also referred to as bioemulsifiers. The former molecules are typically low-molar-mass... 

At concentrations above their aqueous solubility, the so-called c.m.c., low-molar-mass biosurfactants form micelles in the aqueous phase. Micelles are spherical or lamellar aggregates with a hydrophobic core and a hydrophilic outer surface. They are capable of solubilising nonpolar chemicals in their hydrophobic interior, and can thereby mobilise separate phase (liquid, solid or sorbed) hydrophobic organic compounds. The characteristics for the efficiency of (bio)surfactants are the extent of the reduction of the surface or interfacial tension, the c.m.c. as a measure of the concentration needed to bring about this reduction, and the molar solubilisation ratio MSR, which is the number of moles of a chemical solubilised per mole of surfactant in the form of micelles [96]. 

The acquisition and assimilation of bioelements are the most fundamental processes in an organism s struggle for life. It is therefore obvious that in complex natural systems the competition between thousands of species for limited quantities of a small number of elements is a major evolutionary factor. However, the individual contributions of the physical and biochemical aspects of nutrition to the fitness of an organism are widely unknown. The frequent observation that biodegradation processes, e.g. in soil remediation, are limited by physical obstacles to substrate acquisition, rather than by biochemical incapacities, points at the importance of substrate mobilisation strategies. 

Speciation in solution is considered a major factor in the mobilisation and leaching of metal cations (DeKoninck, 1980 Bloomfield, 1981 Stevenson and Fitch, 1986). Complexation increases the total soluble concentration of a metal and hence increases its potential to be leached. Organic ligands (e.g. humate, ful-vate, citrate, polyphenols) are the major complexers involved in this mechanism, but they are effective only if the soluble organic complex does not become saturated and precipitate (DeKoninck, 1980). 

In assessing environmental risks, however, the lack of specificity can be not so crucial. It might be more interesting to get information on the possibility of mobilisation or on bioavailability of PTMs than to identify the exact chemical species of metals in soil. It could be not so important to know whether the metals come from sulphides or organic matter, but could be more useful to know that they can be released under reducing or oxidising conditions . 

Are they involved in the activation and mobilisation of CO2 Van den Brenk et al. suggested this from their studies of patellamide D and ascidiacyclamide, which formed carbonate bridged 2Cu complexes. They suggested that these complexes fix CO2 for use in the formation of CaCOs (used in the internal skeleton of the tunicate).. One argument against this is that marine photosynthetic organisms fix HCOs which is plentiful in seawater (0.002 M). The proposed process, CO2 -> HCOa is therefore the reverse of that expected. 

The type Ilia /3-lactamase [141] can be mobilised on the R factor RPl [142] into strains of Ps. aeruginosa. The crypticity of the type Ilia /3-lactamase can then be determined against a number of substrates. That an efficient penetration barrier exists in these organisms between the substrate located outside the cell and the /8-lactamase located in the periplasmic space [141] can be seen from the following crypticity values for Ps. aeruginosa (1822 RPl) penicillin G, 80 ampicillin, 60 carbenicil-lin, 60 cephaloridine, 50. 

---