Microbial mineralisation of benzene and characterisation of microbial biomass in soil above hydrocarbon-contaminated groundwater

Rates of microbial mineralisation of benzene were measured in a soil profil e above gasoline-contaminated groundwater. The fastest mineralisation rate of benzene occurred in soil with overlapping concentration gradients of hyd rocarbon volatile organic carbon and oxygen, in which neither oxygen nor hy drocarbon volatile organic carbon was at a concentration considered to be r ate limiting (13.8% oxygen and 1.1 mg l(-1) hydrocarbon volatile organic ca rbon). Al our site, the fastest I ate of microbial mineralisation of [C-14] benzene was 83 +/- 13 mu mol kg(-1) day(-1) with a half-life (t(1/2)) of 11 +/-1 days in soil from a depth of 0.25 m below the ground surface. Microbia l mineralisation rates were slower in soil from a depth of 0.5 m (27+/-6 mo l kg(-1) day(-1) t(1/2) = 72+/-16 days) as were mineralisation rates in sur face soils (52+/-13 mu mol kg(-1) day(-1), t(1/2) = 26+/-7 days). In the an oxic soils just above the groundwater table, microbial mineralisation rates were extremely slow (0.04+/-0.01 mu mol kg(-1) day(-1) t(1/2) = 173+/-31 y ears). Microbial biomass at 0.25 m below the ground was comparable to the m icrobial biomass in surface soils (2.0+/-0.2X10(8) and 1.4+/-0.4X10(8) stat ionary phase Escherichia coli equivalent cells g(-1) dry wt., respectively) . The monounsaturated fatty acids in the phospholipids in the microbiota in soils from 0.25 and 0.5 m below the ground surface were greatly enriched i n their trans isomers. As the surface soil microbiota was poor in trans iso mers of mono-unsaturated fatty acids and showed no better mineralisation ra te than microbes from similar environments that were not previously exposed to hydrocarbons, it would seem likely that aromatic hydrocarbons do not pa ss through the surface of the soil due to microbial mineralisation in the s oil profile. This hypothesis was supported by the absence of hydrocarbon vo latile organic carbons in air collected from above the soil surface and a c omparison of the measured mineralisation rates with the mass flux of aromat ic hydrocarbons. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.