Bacterial sources and sinks of isoprene, a reactive atmospheric hydrocarbon.

Physiological and biochemical processes in bacteria can have important impacts on the atmosphere. For example, methane is produced by anaerobic methanogens and then released from soils, sediments and ruminant animals to contribute to the atmospheric `greenhouse' effect (Crutzen, 1991; Conrad, 1996). Interestingly, it is estimated that the fraction of methane that reaches the atmosphere is only about 10% of that formed in anaerobic environments ± aerobic methanotrophs in soils and sediments and associated with plant roots are very adept at scavenging methane before it can be released to the atmosphere (King, 1992; Hanson and Hanson, 1996). Methanotrophs can also take up methane from the atmosphere, although the magnitude of this sink is unclear (Potter et al., 1996). Thus, the concentration of one of the most important trace gases in the atmosphere is controlled in part by the balance between bacterial methane producers and consumers. As reviewed by Conrad (1996), similar patterns of bacterial producers and consumers control emissions of other important atmospheric trace gases, such as CO, NO and N2O. From recently reported work, it appears that yet another bacterial source±sink phenomenon occurs with the volatile hydrocarbon, isoprene (2-methyl-1,3-butadiene). Isoprene emission has been detected from many different bacteria, especially bacilli (Kuzma et al., 1995; Wagner et al., 1999a), soils have been shown to contain signi®cant levels of isoprene-consuming bacteria (Cleveland and Yavitt, 1997; 1998), and progress has been made in unravelling the pathway for isoprene degradation in Rhodococcus (van Hylckama Vlieg et al., 1998; 2000). This work may signal new interest in bacterial roles in biosphere±atmosphere exchange processes. Why is there interest in biogenic isoprene? Isoprene is a reactive compound that is emitted to the atmosphere from terrestrial plants in large amounts, similar to global annual methane emissions (i.e. 500 million tons per year), and in smaller amounts from phytoplankton sources in the oceans (Guenther et al., 1995). Atmospheric chemists have been interested in the chemical reactions of isoprene for some time, and it is now known that, in the presence of sunlight and nitrogen oxides, isoprene reacts to form ozone, organic peroxides and carbon monoxide (Fehsenfeld et al., 1992). Indeed, in many ecosystems and even some urban areas, isoprene may dominate photochemical reactions in the atmospheric boundary layer (Chameides et al., 1988; Goldstein et al., 1998). Here, we review the emerging role of soil bacteria as sources or sinks of this important atmospheric hydrocarbon.