Small Minority Microbes in Activated Sludge Influence the Overall Wastewater Treatment Performance

– New analysis approach for understanding reaction mechanisms in the environment based on the microbial genetic diversity –

Researchers: SATO Yuya, Senior Researcher, HORI Tomoyuki, Senior Researcher, and HABE Hiroshi, Leader, Environmental Microbiology Research Group, Environmental Management Research Institute, and KOIKE Hideaki, Bio-Design Research Group, Bioproduction Research Institute

Key points

The researchers discovered that when degrading heavy oil in wastewater, nitrifying bacteria with a population as low as less than 0.25% in the complex wastewater treatment microbiomes (activated sludge) influence the heavy oil degradation performance of the overall activated sludge.

Comparison of nitrifying bacteria activity, and promotion of heavy oil degradation by nitrate supply
(Left) A nitrifying bacteria produces nitrate from ammonia as the respiratory substrate for the denitrifying bacteria that degrades heavy oil components. (Right) Differences in the gene expression levels of nitrifying bacteria that govern heavy oil degradation efficiency (wastewater treatment performance).


Activated sludge is the most familiar and important biotechnology and has been used around the world for more than 100 years. However, activated sludge is a complex microbiome consisting of thousands of species, so there remain many mysteries with the wastewater treatment mechanism and trouble with unknown causes continues to occur.

New results

The researchers combined a new analytical approach to evaluate genetic diversity with metatranscriptome analysis, applied it to an activated sludge reactor for wastewater treatment, and extracted the key reactions for heavy oil degradation from complex metabolic pathways mediated by various types of microorganisms. As a result, they discovered that the heavy oil degradation performance of the overall wastewater treatment system is influenced not mainly by the heavy oil degrading bacteria themselves, but by the small minority of nitrifying bacteria that supply the respiratory substrate to the heavy oil degrading bacteria.

Future research plans

Metatranscriptome analysis is currently being performed for various phenomena that are issues for actual wastewater treatment systems with the goal of improving the efficiency of the treatment processes from a microbiological viewpoint. In addition, the developed method can be applied to many different environment samples, so the researchers plan to expand the scope of the analysis to address various environmental issues in which microbes are involved but causal relationships are unknown.

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