Analyzing Effects of Higher Salinity on Microbial Interactions in Activated Sludge Microbial Community

Biological treatment processes play a crucial role in purifying sewage by relying on microbial interactions. While previous studies have examined the impact of environmental factors, like salinity, on microbial community diversity and composition, they often overlook the connections between microorganisms. To bridge this gap, an international team of researchers conducted an in-depth analysis of microbial interactions in activated sludge systems under elevated salinity conditions.

Biological treatment processes are widely employed for pollutant removal due to their cost-effectiveness and efficiency. However, high salinity levels can adversely affect the performance of biological wastewater treatment plants (WWTPs), as it poses a threat to microbial survival and inhibits enzyme activity. Therefore, wastewater salinity is a significant issue in biological treatment processes worldwide.

The activated sludge system is a complex micro-ecosystem where various bacterial taxa interact through energy transfer and substance exchange, forming a large and intricate ecological network for efficient organic matter and nutrient removal. Past studies have indicated that salinity reduces bacterial community diversity, alters the composition of nitrifiers and denitrifiers, and further disrupts the activity of nitrifying bacteria.

While the activated sludge microbial community has been extensively studied, little attention has been given to the interactions among microbial taxa during system operation.

It is hypothesized that physicochemical changes, such as influent salinity, may disrupt microbial interactions among different functional populations, such as heterotrophic bacteria, nitrifiers, denitrifiers, and polyphosphate accumulating organisms (PAO), and consequently impact the efficiency of activated sludge systems. However, there is limited information on microbial interactions and their response to elevated salinity.

To address these gaps, researchers from Beijing University of Chemical Technology and Beijing Technology and Business University conducted an association network analysis to describe microbial interactions in activated sludge systems under elevated salinity. Their study revealed that higher salinity levels resulted in lower microbial diversity and the formation of smaller, more complex, and more competitive networks, leading to poor treatment performance.

This study, titled “Responses of microbial interactions to elevated salinity in activated sludge microbial community,” is published in Frontiers of Environmental Science & Engineering.

In this study, the research team explored the following questions by examining the dynamic variation of molecular ecological networks (MENs): 1) How does the overall network structure respond to elevated salinity? 2) How does the subnetwork structure of different phylogenetic taxa respond to elevated salinity? 3) How do functional bacteria and keystone species respond to elevated salinity? This study provides novel insights into the dynamic changes of microbial interactions during physicochemical changes.

The researchers found that 3% salinity inhibited TN removal and reduced microbial community diversity. Network analysis revealed that higher salinity conditions (2% and 3%) led to the formation of more complex, tighter networks with increased bacterial competition.

Subnetworks of bacteria with similar functions, such as ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), and denitrifiers, differed significantly when exposed to elevated salinity. The connection between Nitrospira (NOB) and other species was severely inhibited under 1%-3% salinity, resulting in an elevation of NAR (nitrate removal ratio) over 99.72%. Additionally, the study identified keystone species (hubs and connectors) that dynamically responded to different salinity levels and played crucial roles in maintaining system stability, despite their low abundances in the microbial community.

This study successfully evaluated the effects of elevated salinity on the ecological networks of activated sludge systems using a novel RMT-based network analysis. It enhances our understanding of the relationship between system performance and the dynamics of microbial interactions in activated sludge microbial communities under elevated salinity conditions.

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