Bioavailability organic substrates

The process of hydrolysis, as defined here, converts dead particulate organic matter into bioavailable dissolved substrate. It is usually described by a simple first-order transformation. The process involves extracellular enzyme reactions breaking down high-molecular-weight particulate material that cannot be directly metabolized by microorganisms. Table 16.12 shows a simple representation of this process. It is quantified by an empirical specific hydrolysis rate constant and a temperature-dependence coefficient. 

Resource limitations of productivity in wetlands and aquatic ecosystems have focused on inorganic nutrient availability and loading rates from drainage basins. Recently, research has shown that much of the nutrient bioavailability in the soil and water column is associated with nutrients bound in or with naturally dissolved organic compounds. Many of the nutrient and organic substrate limitations are focused on hydrolytic activity of enzymes. The role of enzymes in regulating elemental cycles is addressed in various chapters of this book. 

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. 

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... 

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