Microbial Extracellular Polymeric Substances (EPS) in Soil: From Interfacial Behaviour to Ecological Multifunctionality
Soil is the product of terrestrial biogeochemical processes and an essential foundation for human survival. Microorganisms endow soil with life properties and drive the biogeochemical cycles within it. Microorganisms play a crucial role in improving soil structure, enhancing fertility, controlling pollution, and responding to global climate change. In soil, microorganisms mainly adhere to the surfaces of soil minerals and organic matter in the form of microcolonies or biofilms. Biofilms are microbial colonies (bacteria, algae, fungi, and/or archaea) embedded in self-produced extracellular polymeric substances (EPS) and attached to organic-inorganic interfaces. EPS serves as the carrier of the structural integrity of biofilms, determining their physicochemical properties and functional complexity. In soil, EPS contributes to soil health through its properties such as adhesion, hygroscopicity, and complexing ability.
However, current research on EPS in soil is still insufficient, particularly in understanding its ecological functions and interfacial behaviors. To better understand and manage biologically mediated nutrient cycling and soil health, in-depth research on the ecological functions of EPS and its potential applications in environmentally friendly agriculture is of great significance.
Source of the Paper
The review paper titled “Microbial Extracellular Polymeric Substances (EPS) in Soil: From Interfacial Behaviour to Ecological Multifunctionality” was co-authored by Ming Zhang, Yichao Wu, Chenchen Qu, Qiaoyun Huang, and Peng Cai from the National Key Laboratory of Agricultural Microbiology, College of Resources and Environment, Huazhong Agricultural University. The paper was accepted on August 5, 2024, and published in the journal Geo-Bio Interfaces with the DOI 10.1180/gbi.2024.4.
Main Content of the Paper
1. Concept, Composition, and Properties of EPS
EPS are high molecular weight polymers secreted by microorganisms during their growth and metabolism, primarily composed of polysaccharides, proteins, lipids, and extracellular DNA (eDNA). Polysaccharides are the most studied components of EPS, classified into homopolysaccharides (e.g., dextran, curdlan, cellulose) and heteropolysaccharides (e.g., alginate, xanthan gum, hyaluronic acid). Proteins in EPS include structural proteins and extracellular enzymes, while eDNA plays a crucial role in the spatial shaping and structural stability of biofilms. The secretion and composition of EPS are influenced by strain type, growth stage, substrate availability, and environmental parameters.
2. Ecological Functions of EPS in Soil
EPS plays multiple ecological roles in soil, including the following aspects:
a) Promoting Soil Aggregate Formation and Stabilization
Through its adhesive and bridging properties, EPS promotes the formation and stabilization of soil aggregates. Studies have shown a positive correlation between EPS polysaccharides and soil aggregate stability, particularly in the rhizosphere, where EPS secretion significantly enhances aggregate stability.
b) Enhancing Soil Water Retention Capacity
EPS has high hygroscopicity, protecting microorganisms from desiccation stress. Research indicates that the water-holding capacity of EPS can be 15 to 20 times its own mass, significantly improving the porosity and water retention capacity of sandy soils.
c) Mediating Nutrient Storage and Trapping
EPS can capture and store nutrients, providing carbon and energy sources for microorganisms under nutrient-limited conditions. Studies have found that the degradation of EPS in soil involves multiple enzymes, and the degradation products can be utilized by other microorganisms.
d) Regulating Contaminant Sequestration and Transformation
EPS contains rich functional groups that can adsorb and immobilize heavy metals, influencing their environmental behavior. Additionally, EPS can act as an electron transfer medium, promoting the degradation of organic pollutants.
3. Extraction and Characterization Techniques for EPS
The extraction and characterization of EPS are key to understanding its ecological functions in soil. Common extraction methods include the cation exchange resin (CER) method, which minimizes intracellular contamination and co-extraction of non-target organic matter. The polysaccharide, protein, and uronic acid contents of EPS are quantified using the phenol-sulfuric acid method, Lowry assay, and meta-hydroxydiphenyl assay, respectively.
4. Future Research Directions
The paper proposes several key directions for future research:
a) Source and Quantitative Analysis of Soil EPS
The vague delineation of the sources of biochemical substances in soil hinders the development of soil science. Specific extraction and precise analysis of soil EPS can reveal the adaptability of microbial communities to environmental conditions and their correlation with soil functions.
b) EPS as a Soil Biochemical Indicator
EPS, located at the interface between microbial cells and the soil matrix, is considered an ideal biochemical indicator for assessing soil health. Analyzing soil EPS can uncover microbial responses to environmental changes.
c) Applications of EPS in Environmentally Friendly Agriculture
EPS can be used to encapsulate microbial strains, preparing novel biofilm-based biofertilizers to enhance the colonization and survival of inoculated microorganisms in soil while improving soil structure and nutrient conditions.
Significance and Value of the Paper
This review paper systematically summarizes the research progress on microbial EPS in soil, detailing its interfacial behaviors and ecological functions. It not only provides a theoretical basis for understanding the role of EPS in soil ecosystems but also offers new insights for the development of environmentally friendly agriculture. By deepening the understanding of the ecological functions of EPS, future technologies based on EPS for soil improvement and pollution remediation can be developed, promoting sustainable agriculture.
Highlights
- Comprehensiveness: The paper provides a comprehensive review of EPS, from its concept, composition, and extraction methods to its ecological functions in soil, covering the latest research advancements.
- Application Prospects: The paper highlights the potential applications of EPS in environmentally friendly agriculture, offering directions for future research and practice.
- Future Research Directions: The paper identifies key areas for future research, providing a clear framework for subsequent studies.
The publication of this paper offers valuable references for researchers in soil microbiology and environmental science and makes significant contributions to advancing soil health management and sustainable agricultural development.