Studies on the in situ physiology of Thiothrix spp. present in activated sludge

The in situ physiology of the filamentous sulphur bacterium Thiothrix spp. was investigated in an industrial wastewater treatment plant with severe bu lking problems as a result of overgrowth of Thiothrix. Identification and e numeration using fluorescence in situ hybridization (FISH) with species-spe cific 16S and 23S rRNA probes revealed that 5-10% of the bacteria in the ac tivated sludge were Thiothrix spp. By using a combination of FISH and micro autoradiography it was possible to study the in situ physiology of probe-de fined Thiothrix filaments under different environmental conditions. The Thi othrix filaments were very versatile and showed incorporation of radiolabel led acetate and/or bicarbonate under heterotrophic, mixotrophic and chemoli thoautotrophic conditions. The Thiothrix filaments were active under anaero bic conditions (with or without nitrate) in which intracellular sulphur glo bules were formed from thiosulphate and acetate was taken up. Thiothrix-spe cific substrate uptake rates and growth rates in activated sludge samples w ere determined under different conditions. Doubling times of 6-9 h under mi xotrophic conditions and 15-30 h under autotrophic conditions were estimate d. The key properties that Thiothrix might be employing to outcompete other microorganisms in activated sludge were probably related to the mixotrophi c growth potential with strong stimulation of acetate uptake by thiosulphat e, as well as stimulation of bicarbonate incorporation by acetate in the pr esence of thiosulphate.