Nutrients biofilms

In fact, significant substrate concentration gradients may exist for cells immobilised in biofilm. Cells located close to the nutrient supply are likely to maintain higher quality and activity compared with cells located relatively further away, leading to differentiation in the quality or activity of the immobilised cell population. This differentiation is more pronounced if there are starvation regions. In practice, zero substrate concentration may exist inside the biofilm, because in these regions the cell physiology may be markedly different from that of the freely suspended cells. 

Keywords Community ecotoxicology, Fluvial biofilm. Metal pollution. Nutrients,... 

Table 2 Fate and effects of metals in a stream receiving a point-source of metals (upper part of the table) or diffuse input via urban runoff (lower part of the table). Summary of the expected influence of four different hydrological situations base-flow in a rainy period a flood after a rainy period low-flow after a long period of low rainfall (water scarcity) and a flood produced after this drought. Metal concentration (M) metal retention efficiency (measured on the basis of the nutrient spiraling concept) exposure (dose and duration) bioaccumulation (in fluvial biofilms) and metal sensitivity (of biofihns)...

Apart from nutrient limitation and diminished growth rates, another reason for this decreased susceptibility is the prevention of access of a biocide to the underlying cells. Thus, in this mechanism, the glycocalyx as well the rate of growth of the biofilm micro-eolony in relation to the diffusion rate of the biocide across the biofilm, can affect susceptibility. A possible third mechanism involves the increased production of degra-dative enzymes by attached cells, but the importance of this has yet to be determined. 

In order to understand current approaches for prevention and control of biofilms, we must first consider the reasons for the failure of conventional antimicrobial protocols. There are thought to be three main reasons as to why biofilm bacteria out-survive their planktonic counterparts during antimicrobial treatments (reviewed by McBain et a/.16).These are i) poor penetration of antimicrobial compounds due to the presence and turn-over of exopolymer slime (glycocalyx) ii) the imposition of extreme nutrient limitation within the depths of the biofilm community and the co-incident expression of metabolically-dormant, recalcitrant phenotypes and (iii) the expression of attachment-specific phenotypes that are radically different and intrinsically less susceptible than unattached ones. 

Although reaction-diffusion limitation and the presence of nutritionally restricted phenotypes are obviously important determinants of biofilm drug resistance, neither, either separately or in combination, provides a complete explanation of the phenomena. Cells on the periphery of the biofilm, subject to nutrient fluxes similar to planktonic organisms would succumb to antibacterial concentrations that are effective against the planktonic cells. Cell-death at the periphery would lead to increased nutrient availability for deeper-lying cells. These would, in turn, grow faster and adopt a more susceptible... 

These characteristics favor the attachment of bacteria to solid surfaces in the form of biofilm so that nutrients flowing in the groundwater can be used. The presence of low nutrient levels in the groundwater also implies that bacteria must regularly use many different compounds as energy sources and, consequently, may select organic contaminants more readily as nutrients. 

In spite of all of this variety of approaches, covering a wide array of metabolism pathways, limitations also exist. Differences in the vulnerability of biofilms have been found to depend on the age, community composition and succession status of the community. In dense biofilms the transfer of contaminants may be limited, resulting in decreased bioavailable concentrations of nutrients or toxicants for the algae. Biofilms show an inverse relationship between metal toxicity and biomass accrual [26], and a similar relationship has been established with nutrients. Therefore, the colonisation time or biofilm thickness are relevant factors to be included in the procedure uses. 

Although the effect of nutrients and classical toxicants (e.g. heavy metals, herbicides) is well known, that of the so-called non-PS in biofilms is still largely unknown. Furthermore, the combination of effects, which operate at the basin scale, requires complex approaches. Therefore, the response of biofilms to these situations should trigger the development of new applications and higher standardisation in the use of biofilms. The standardisation of methods and procedures is a challenge for the future research on the use of natural and laboratory biofilms. 

It is expected that in the very near future, the application of closed water loops will show an intensive growth, strongly supported by the further development of separate treatment technologies such as anaerobic treatment, membrane bioreactors, advanced biofilm processes, membrane separation processes, advanced precipitation processes for recovery of nutrients, selective separation processes for recovery of heavy metals, advanced oxidation processes, selective adsorption processes, advanced processes for demineralisation, and physical/chemical processes which can be applied at elevated temperature. 

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