INTRODUCTION

It has been shown that microbial communities contribute extensively to the attenuation, mineralization and transport of both organic and inorganic contaminants in the environment. The development of biofilms by microbial communities is often a key factor contributing to the overall efficiency of these processes (Rothemund et al., 1996). For instance, bacterial biofilms are able to accumulate metals through various mechanisms (Marques et al., 1991; Sillitoe et al., 1994). Liehr et al. (1994) showed that biofilms formed by algae could concentrate metals at levels more than four orders of magnitude higher than those in the surrounding water.

The potential of bioremediation as an alternative to physical and chemical remediation strategies has resulted in a significant amount of research effort on degradative biofilms. Although much emphasis has been placed on the degradation of xenobiotic compounds, the knowledge gained through these studies has also contributed to an improved understanding of processes involved in the degradation of naturally occurring molecules as well as nutrient cycling in general. Tank & Webster (1998) suggested that competition for nutrients might regulate heterotrophic microbial processes in natural streams. In their study, they found that nutrient immobilization by leaves partially inhibited other heterotrophic processes, as evidenced by low microbial respiration, fungal biomass and extracellular enzyme activity among wood biofilms in the presence of leaf litter. Lawrence et al. (1998) stressed the applicability of knowledge gained through the study of naturally occurring attenuation mechanisms to remediating contaminated environments. Clearly, the study of degradative biofilms is of both fundamental and applied interest.

Introduction

Traditional microbiological investigations have focused on the culture and analysis of pure cell lines of bacteria, in either batch or chemostat culture. However, it has been clearly established that in nature, disease and industry, the majority of bacteria exist attached to surfaces within biofilms (Costerton et al. 1978, 1987; Lappin-Scott & Costerton 1989; Characklis et al. 1990a). Furthermore, it has also been established that the bacteria which exist in biofilms, termed sessile bacteria, are inherently different from bacteria existing in the planktonic state. In the sessile state, bacteria may express different genes, alter their morphologies, grow at different rates, or produce extracellular polymers in large amounts (Costerton et al. 1978; Wright et al. 1988; Gilbert et al. 1990; Dagostino et al. 1991; McCarter et al. 1992). One significant consequence of sessile growth is that biofilm bacteria are more resistant to medical and industrial control strategies than their planktonic counterparts (Brown et al. 1988; Nichols 1989; Eng et al. 1991; Blenkinsopp et al. 1992).

The development of complex attached and aggregated communities is also important for the survival and reproductive success of microorganisms. These communities have been considered to act as reservoirs for diverse species, sites of specific limited niches, and protective refuges from competition, predation or harsh environmental conditions, allowing otherwise poor competitors to survive. Integration into a biofilm or bioaggregate may be regarded as a survival strategy beyond that of maximizing or increasing the growth rate.