2.1 Introduction

According to many definitions [1], biodegradation of plastics is usually primarily induced by the action of various microorganisms, although often non-biotic effects such as irradiation, thermal degradation or chemical hydrolysis contribute to the degradation process. The activity of microorganisms is closely connected to the presence of water. The supply of nutrients to the microorganisms and the transportation of excreted enzymes and metabolic products takes place by diffusion in an aqueous environment surrounding the cells. (Thus, it can be said that an aqueous environment is actually the natural one for a microbe). However, in environments regarded as non-liquid such as soil, compost or surfaces of solids, microbes can also be active as far as a certain aqueous micro-environment allows transportation processes necessary for the biological activity. For example in soil, microbial life takes place in the thin water-films located between the particles or in water-filled cavities in the soil components. A soil-humidity of around 50-60% is optimal for aerobic biological processes, where the humidity is given as percentage of the maximum water holding capacity, which takes into account also structural elements of the soil (actually it reflects the filling of the cavities in the material).

Although water is a basic component of the microbial world, many organisms need or prefer the contact to a solid matrix. For example many fungi exhibit a better growth on surfaces than in agitated liquids, which can, besides other effects, be attributed to the sensitivity of the fungal mycelium to mechanical forces.