Microorganisms mobility/transport

All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

Post depositional mobility by diffusion or by microorganism transport in pore water. 

Activation occurs at aqueous-solid phase interfaces where microorganisms are active and the products thus created may have a short residence time or persist for long periods (Fig. 9). The harmful product may be an intermediate in mineralization, yet it may persist long enough to create a pollution problem. Moreover, the mobility of the activation product is sometimes different from that of its precursor, so that the product may be transported to distant sites to a greater or to a smaller extent than the contaminant molecule from which it was formed [43,73,194,195]. 

Electric fields use in soil restoration has been focused on contaminant extraction by their transport under electroosmosis and ionic migration. Contaminant extraction by electric fields is a successful technique for removal of ionic or mobile contaminants in the subsurface. However, this technique might not be effective in treatment of soils contaminated with immobile and/or trapped organics, such as dense non aqueous phase liquids (DNAPLs). For such organics, it is possible to use electric fields to stimulate in situ biodegradation under either aerobic or anaerobic conditions. It is necessary to evaluate the impact of dc electric fields on the biogeochemical interactions prior to application of the technique. It is not clear yet how dc electric fields will impact microbial adhesion and transport in the subsurface. Further, the effect of dc fields on the activity of microorganisms in a soil matrix is not yet well understood. 

The microbial cycling of phosphorus does not alter its oxidation state. Most phosphorus transformations mediated by microorganisms can be viewed as inorganic to organic phosphate transfers or as transfers of phosphate from insoluble, immobilized forms to soluble or mobile compounds. Various microorganisms have evolved transport systems for the regulated acquisition of phosphate from the environment. 

Heavy metal mobilization is often followed by microorganism and plant uptake, and intracellular accumulation. Filamentous fungi transport heavy metals and radionuclides along their hyphae. This may be a mechanism of mobilization from mycor-rhizal fungi to higher plants. An alternative pathway involves direct root uptake of heavy metals mobilized by microbial acid production or chelation. 

Some metals may need to be mobilized from the environment to make them bioavailable. Iron in particular must be rendered more soluble to be accessible for uptake. Microorganisms and some plants have evolved with secreted ligands known as siderophores (or phytosiderophores). These ligands bind Fe + with extraordinary affinity. For example, a complex of the siderophore enterobactin with ferric iron has a formal stability constant of 10 (19). Once siderophores compete with other environmental ligands for iron, the ferric iron-siderophore complex then binds to specific transport proteins at the microbial... 

M 38). For microorganisms such as enteric bacteria, which need at least a total concentration of iron of 5 10 7 M for optimal growth, this concentration is many orders of magnitude too low. Only powerful chelating agents such as the siderophores (see the chapter on thermodynamics for details) can mobilize iron from the environment and facilitate transport of iron into the microbial cell. 

The aerial transport of pollen and microorganisms has received some attention (Gregory, 1973, 1978 R. Campbell, 1977). Bacteria (size < 1 p.m) are difficult to discern directly, and their study requires cultural growth techniques. In contrast to fungi spores, they usually occur attached to other aerosol particles because they are mobilized together with dust. Concentrations number several hundred per cubic meter in rural areas and several thousand in the cities. Air is not their natural habitat, so that multiplication does not take place. On the contrary, the atmospheric aerosol appears to have definite germicidal qualities (Riiden and Thofern, 1976 Riiden et al., 1978). 

Harms H, Wick LY. (2004). Mobilization of organic compounds and iron by microorganisms. In Physicochemical Kinetics and Transport at Chemical-Biological Interphases (eds. HP van Leuven, W Koester). Chichester WUey, pp. 401-444. 

---