Estimation of "Dehalococcoides" Populations in Lake Sediment Contaminated with Low Levels of Polychlorinated Dioxins

Abstract

Vertical profiles of polychlorinated dioxins and microbial biomass including "Dehalococcoides" populations in cores of sediment of Lake Suwa, Japan, were investigated. The core samples were analyzed in 3-cm intervals at 0-15 cm and 50 cm, where a sharp gradient of Eh from 5 to −110 mV with depth occurred. The concentration of polychlorinated dioxins was relatively constant at 0-15 cm, ranging from 7.6 to 8.3 ng (7.0-9.2 pg-TEQ [toxic equivalent]) g-1 dry wt, but decreased sharply at 50 cm. The total bacterial count was in the order of 108 to 109 g-1 dry wt, being highest at 3-6 cm and decreasing in the deeper sediment. A similar vertical profile was found for respiratory quinones with larger amounts of ubiquinones than menaquinones at 0-15 cm. Quantitative real-time PCR with a specific primer set showed that "Dehalococcoides" and its phylogenetic relatives occurred in the order of 104 g -1 (dry wt) at 0-12 cm but were absent at 50 cm. The amplified clones showed 91-100% similarity (mostly <94%) in sequence to a well-known dioxin-dechlorinating organism, "Dehalococcoides" sp. strain CBDB1. These results suggested that the surface sediment up to a depth of 12 cm provided favorable conditions for the growth and activity of both aerobic and anaerobic microorganisms including "Dehalococcoides". It is likely that a wide variety of "Dehalococcoides" and phylogenetic relatives thereof are omnipresent even in sediment with low levels of dioxins and play the primary role in dechlorinating organohalorides over a relatively wide range of Eh.