Degradation of 2,3,4,6-tetrachlorophenol at low temperature and low dioxygen concentrations by phylogenetically different groundwater and bioreactor bacteria.

Abstract

Effects of low temperature and low oxygen partial pressure on the occurrence and activity of 2,3,4,6-tetrachlorophenol degrading bacteria in a boreal chlorophenol contaminated groundwater and a full-scale fluidized-bed bioreactor were studied using four polychlorophenol degrading bacterial isolates of different phylogenetic backgrounds. These included an alpha-proteobacterial Sphingomonas sp. strain MT1 isolated from the full-scale bioreactor and three isolates from the contaminated groundwater which were identified as beta-proteobacterial Herbaspirillum sp. K1, a Gram-positive bacterium with high G + C content Nocardioides sp. K44 and an alpha-proteobacterial Sphingomonas sp. K74. The Sphingomonas strains K74 and MT1 and Nocardioides sp. K44 degraded 2,4,6-trichlorophenol and 2,3,4,6-tetrachlorophenol as the sole carbon and energy sources. Close to stoichiometric inorganic chloride release with the 2,3,4,6-tetrachlorophenol removal and the absence of methylation products indicated mineralization. Tetrachlorophenol degradation by the Herbaspirillum sp. K1 was enhanced by yeast extract, malate, glutamate, pyruvate, peptone and casitone. At 8 degrees C, Sphingomonas sp. K74 had the highest specific degradation rate (mu(max) = 4.9 x 10(-2) mg h(-1) cell(-1)) for 2,3,4,6-tetrachlorophenol. The Nocardioides strain K44 had the highest affinity (K(s) = 0.46 mg l(-1)) fortetrachlorophenol. K1 and MT1 grew microaerophilically in semisolid glucose medium. Furthermore, the growth of MT1 was inhibited in liquid glucose medium at high oxygen partial pressure indicating sensitivity to accumulating toxic oxygen species. On the other hand, trichlorophenol degradation was not affected by oxygen concentration (2-21%). The isolates K44, K74 and MT1, with optimum growth temperatures between 23 and 25 degrees C, degraded tetrachlorophenol faster at 8 degrees C than at room temperature indicating distinctly different temperature optima for chlorophenol degradation and growth on complex media. These results show efficient polychlorophenol degradation by the isolates at the boreal groundwater conditions, i.e., at low temperature and low oxygen concentrations. Differences in chlorophenol degradation and sensitivities to chlorophenols and oxygen among the isolates indicate that the phylogenetically different chlorophenol degraders have found different niches in the contaminated groundwater and thus potential for contaminant degradation under a variety of saturated subsurface conditions.