Laboratory batch and column experiments were conducted to investigate the role of microbial exudates, e.g., exopolymeric substance (EPS) and alginic acid, on microbial Cr(VI) reduction by two different Pseudomonas strains (P. putida P18 and P. aeuroginosa P16) as a method for treating subsurface environment contaminated with Cr(VI). Our results indicate that microbial exudates significantly enhanced microbial Cr(VI) reduction rates by forming less toxic and highly soluble organo-Cr(III) complexes despite the fact Cr(III) has a very low solubility under the experimental conditions studied (e.g., pH 7). The formation of soluble organo-Cr(III) complexes led to the protection of the cells and chromate reductases from inactivation. In systems with no organic ligands, soluble organo-Cr(III) end products were formed between Cr(III) and the EPS directly released by bacteria due to cell lysis. Our results also provide evidence that cell lysis played an important role in microbial Cr(VI) reduction by Pseudomonas bacteria due to the release of constitutive reductases that intracellularly and/or extracellularly catalyzed the reduction of Cr(VI) to Cr(III). The overall results highlight the need for incorporation of the release and formation of organo-Cr(III) complexes into reactive transport models to more accurately design and monitor in situ microbial remediation techniques for the treatment of subsurface systems contaminated with Cr(VI).

Thermophilic soil geobacilli isolated from cool temperate geographical zone environments have been shown to be metabolically inactive under aerobic conditions at ambient temperatures (-5 to 25 degrees C). It is now confirmed that a similar situation exists for their anaerobic denitrification activity. It is necessary therefore to determine the mechanisms that sustain the observed significant viable populations in these soils. Population analysis of thermophiles in rainwater and air samples has shown different species compositions which support the view that long distance global transport and deposition in rainwater is a possible source of replenishment of the soil thermophile populations. Survival experiments using a representative Geobacillus isolate have indicated that while cells lose viability rapidly at most temperatures, populations can increase only when the temperature allows growth to take place at a rate which exceeds death rate. Long term (9-month) experiments at 4 degrees C show population increases which can be accounted for by very slow growth rates complemented by negligible death rates. These results are interpreted in the context of current hypotheses on the biogeography patterns of bacteria.

No abstract

No abstract

No abstract