Biouptake

If areas identified as likely to receive significant atmospheric contaminant concentrations include areas supporting edible biota, the biouptake of contaminants must be considered as a possible environmental fate pathway. Direct biouptake from the atmosphere is a potential fate mechanism for lipophilic contaminants. Biouptake from soil or water following transfer of contaminants to these media must also be considered as part of the screening assessments of these media. 

Important intermedia transfer mechanisms affecting soil contaminants include volatilization or resuspension to the atmosphere and biouptake by plants and soil organisms. These, in turn, introduce contaminants into the food chain. 

FIGURE 5.1 Pathways of bioaccessibility, biouptake, and bioavaUabUity leading to exposure. Source Modified from Escher and Hermens 2002.)... 

Jansen, S., Gonzalez-Gil, G., and Leeuwen, H.P.V., The impact of Co and Ni speciation on methanogenesis in sulfidic media—biouptake versus metal dissolution, Enzyme Microb Technol, 40 (4), 823-830, 2007. 

Oliver, B.G. (1987a) Biouptake of chlorinated hydrocarbons from laboratory-spiked and field sediments by oligochaete worms. Environ. Sci. Technol. 21, 785-790. 

The membrane is the regulating barrier for exchange of chemical species between the environmental medium and cell interior. It may be practically impermeable to one type of species and highly permeable to another. In the chain of transport steps from the bulk of the medium to the cell interior, the membrane transfer step may thus vary from fully rate-limiting to apparently fast with respect to transport in the medium. The overall rate of this biouptake process is determined by mass transport either in the medium or through the membrane the actual rate-limiting step will depend on a large variety of factors. Membrane... 

As noted above, biouptake involves a series of elementary processes that take place in the external medium, in the interphasial region, and within the cell itself. One of the most important characteristics of the medium is the chemical speciation of the bioactive element or compound under consideration. Speci-ation not only includes complexation of metal ions by various types of ligands, but also the distribution over different oxidation states, e.g. Fe(II) and Fe(III), and protonation/deprotonation of organic and inorganic acids of intermediate strength. The relationship between speciation and the direct or indirect bioavailability1 of certain species has received a lot of recent attention. 

Figure 4. Schematic representation of the various processes involved in the transfer of metal ions from a complex medium to an organism. The free metal ion and the lipophilic complexes ML are effectively bioactive. Bioinactive complexes MY, present in the medium, can only contribute to biouptake processes via dissociation into M...

Pinheiro, J. P., Galceran, J. and van Leeuwen, H. P. (2003). Metal speciation dynamics and bioavailability. Bulk depletion influence in biouptake, Environ. Sci. Technol., submitted. 

Dynamics of Biouptake Processes the Role of Transport, Adsorption and Internalisation... 

Biouptake When Mass Transfer is Coupled With... 

Modelling biouptake processes helps in the understanding of the key factors involved and their interconnection [1]. In this chapter, uptake is considered in a general sense, without distinction between nutrition or toxicity, in which several elementary processes come together, and among which we highlight diffusion, adsorption and internalisation [2-4], We show how the combination of the equations corresponding with a few elementary physical laws leads to a complex behaviour which can be physically relevant. Some reviews on the subject, from different perspectives, are available in the literature [2,5-7]. 

Steady-State If one defines the limiting biouptake flux for each site (which would appear for much larger than each KMj) as [26] ... 

Equation (17) shows quite elegantly that the biouptake flux is governed by the two fundamental parameters a and b. A set of limiting values of J is easily derived by using that (1 — x)1 2 approaches (l jx) for x transport flux (b [Pg.156]

We have seen that purely diffusion-controlled biouptake fluxes may require time spans of O(103) s to decay to their eventual steady-state values (see Section 2.3.6). In reality the situation of pure diffusion as the mode of mass transfer in... 

BIOUPTAKE WHEN MASS TRANSFER IS COUPLED WITH CHEMICAL REACTION (COMPLEX MEDIA)... 

The use of the excess ligand condition, equation (57), spares the need to consider the continuity equation (52) for the ligand. Then, two limiting cases of kinetic behaviour are particularly simple the inert case and the fully labile case. As we will see, these cases can be treated with the expressions (for transient and steady-state biouptake) developed in Section 2, and they provide clear boundaries for the general kinetic case, which will be considered in Section 3.4. 

If the rate constants for interconversion between M and ML are infinitesimally small (on the effective timescale of the experimental conditions), the complex does not contribute significantly to the supply of metal to the biosurface. The equilibrium equation (50) behaves as if frozen. In a biouptake process, the complex ML then does not contribute to the supply of metal towards the biosurface, and all the expressions given in Section 2 apply, with the only noteworthy point that the value of c"M to be used differs from the total metal concentration. In this case, the complexed metal is not bioavailable on the timescale considered, as metal in the complex species is absent from any process affecting the uptake. 

KEY FACTORS AND CHALLENGES FOR FUTURE RESEARCH IN BIOUPTAKE MODELLING... 

The preceding sections have demonstrated the considerable quantitative understanding of biouptake that can be attained by models with a sound theoretical basis. We have shown solutions for a range of conditions, ranging from relatively simple limiting cases to more involved situations involving kinetically limited metal complex dissociation fluxes. In this section, we highlight key points that should be considered in future refinements of biouptake models. 

In principle, any kind of limitation of the medium (e.g. due to some kind of clustering in a zone) tends to diminish the individual uptake rate [31]. From the point of view of modelling, the breaking of the symmetry rapidly complicates the problem (see Chapter 3 in this volume). As an exception to the general rule of decreased uptake due to inter-cell competition, it has been shown [49] that biouptake through siderophore excretion is only viable for nonisolated cells. 

Thus, the community effect can have a positive impact on the biouptake of certain trace nutrients. 

The finite kinetics of the adsorption/desorption steps at the interface have been extensively studied by Hudson and Morel [13,15]. A wealth of literature is available on dealing with such interfacial processes [94-96] and its inclusion in the biouptake model should be implemented when experimental evidence of its necessity arises. 

Another factor to take into account in biouptake studies is the possibility that the organism develops strategies of eliminating toxic species by means of efflux [38,52,101]. As a first approach, the efflux rate can be set proportional to the amount of species taken up that has been internalised, thus converting the boundary condition of flux balances for two sites, equation (4), into ... 

Modelling biouptake requires the judicious consideration and selection of the underlying physical phenomena responsible for the experimental observations. We have seen that three fundamental phenomena may play a key role in biouptake mass transfer, adsorption, and internalisation. The inclusion of additional phenomena or refinements (such as nonexcess ligand complexation, non-first-order kinetics, nonlinear isotherms, etc.) may be essential to describe certain cases, but they have handicaps, such as ... 

Galceran, J. and van Leeuwen, H. P. (2004). Dynamics of biouptake processes. The role of transport, adsorption and internalisation. In Physiochemical Kinetics and Transport at Biointerfaces, eds. van Leeuwen, H. P. and Koster, W., Vol. 9, IUPAC Series on Analytical and Physical Chemistry of Environmental Systems. Series eds. Buffle, J. and van Leeuwen, H. P., John Wiley Sons Ltd, Chichester, UK, pp. 147-203. 

Nature of the Rate-Limiting Step for Metal Biouptake... 

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