Introduction

Hydrocarbon degradation by microorganisms is a natural manifestation of the carbon cycle. Only when hydrocarbon pollutants occur in excessively high concentrations, or prove highly toxic or are recalcitrant to existing bacterial degradative processes do they become the focus of remediation efforts. Intrinsic (or natural) bioremediation therefore defines the fate of most environmental chemicals, whether natural or anthropogenic. Indeed, through chemical modifications such as halogenation, a great deal of effort has gone into creating classes of hydrocarbons, such as the polychlorinated biphenyls (PCBs), that are more environmentally stable than their unmodified analogs. The recalcitrance of these compounds has led us to attempt to determine why the organisms normally responsible for carbon cycling fail when challenged by certain pollutants. Through the process of genetic engineering, strain development, enzyme redesign and pathway construction we can then undertake the molecular manipulation of biodegradative systems to improve their utility in bioremediation.

Biodegradation depends on the microbial production of enzymes capable of catalyzing chemical reactions that will transform (or, ideally, mineralize) pollutants. Bioremediation technologies can be enhanced in several ways, but the existing biochemical capabilities of the organisms should be reviewed before any anticipated genetic alterations are considered.

The most frequent means of stimulating bioremediation include:

1. Chemical modifications of the environment (Nelson et al., 1986; Mueller et al., 1991a, b, 1992; Pritchard & Costa, 1991; McCarty & Semprini, 1994) can include the addition of chemicals which act as electron acceptors or supplemental electron donors, or which enhance bioavailability (Rosenberg et al., 1979; Georgiou, Lin & Sharma, 1992).

2. Physical modifications of polluted sites may relate to ex situ or in situ treatments.