Analyzing the Effects of Increased Salinity on Microbial Interactions in Activated Sludge Microbial Communities

Biological treatment processes play a crucial role in sewage purification, as they rely on microbial interactions for effective treatment. While previous studies have explored the impact of environmental factors, such as salinity, on microbial communities’ diversity and composition, the interactions among microorganisms have been overlooked. To bridge this gap, an international team of researchers conducted a comprehensive analysis of microbial interactions in activated sludge systems under elevated salinity conditions.

Biological treatment processes are extensively used for pollutant removal due to their cost-effectiveness and high efficiency. However, elevated salinity can negatively affect the performance of biological wastewater treatment plants (WWTPs) due to the harmful effects of high osmotic pressure on microbes and enzyme activity inhibition. As a result, wastewater salinity is a significant concern in biological treatment processes worldwide.

An activated sludge system is a complex micro-ecosystem, where various bacterial taxa engage in energy transfer, substance exchange, and information sharing, forming a large and intricate ecological network that efficiently removes organic matter and nutrients. Previous studies have demonstrated that salinity reduces bacterial community diversity, alters the composition of nitrifiers and denitrifiers, and further inhibits nitrifying bacteria activity.

Although extensive research has been conducted on the activated sludge microbial community, 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, including heterotrophic bacteria, nitrifiers, denitrifiers, and polyphosphate accumulating organisms (PAO), ultimately impacting the efficiency of activated sludge systems. However, information on microbial interactions and their response to elevated salinity is scarce.

To address these gaps, researchers from Beijing University of Chemical Technology and Beijing Technology and Business University examined microbial interactions in response to elevated salinity in an activated sludge system through association network analysis. Their study revealed that higher salinity led to reduced microbial diversity and smaller, more complex, and competitive overall networks, resulting in poor treatment process 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 analyzing 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 interaction under 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%) resulted in 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, exhibited significant differences when exposed to elevated salinity. The connection between NOB species Nitrospira and other species was severely inhibited under salinity levels of 1% to 3%, leading to a significant increase in NAR (nitrite-ammonium ratio) over 99.72%. Additionally, the researchers observed that keystone species, which act as hubs and connectors, exhibited dynamic behavior in response 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. The findings contribute to our understanding of the relationship between system performance and microbial interaction dynamics in activated sludge microbial communities under elevated salinity conditions.

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