Microbial Exudates: Functions and Environmental Impact in Aquatic Systems

Microbial exudates are a chemically diverse group of biomolecules actively secreted by bacteria, fungi, archaea, and algae into their surrounding environment. These secretions have a profound impact on biological communities and the chemical properties of diverse aqueous systems, ranging from oceans and surface waters to groundwater and engineered environments. Conceptually, any molecule within a microbe—whether synthesized or taken up—can be released, leading to an overwhelming chemical diversity that makes comprehensive characterization challenging. Despite this complexity, major classes of particularly abundant compounds can be identified, and specific functions—such as surface attachment, communication, and nutrient acquisition—can be assigned to them. This operational classification provides a functional framework for understanding the critical roles these molecules play in microbial ecology and biogeochemistry.

Extracellular Polymeric Substances (EPS): The Framework of Microenvironments.A primary category of microbial exudates is Extracellular Polymeric Substances (EPS), which are high-molecular-weight polymers secreted by microbes. These substances include polymers composed of sugars (polysaccharides), amino acids (proteins), fatty acids (lipids), and nucleic acids. The production of EPS is fundamental to modifying the immediate environment of the microbe, creating a structured microenvironment that is key to survival in both natural settings (like surface and soil waters) and engineered systems (such as water distribution networks and industrial bioreactors).

The most significant function of EPS is its central role in the formation of biofilms and flocs—aggregate structures that are critical microbial habitats (Figure 1). It is now widely recognized that most bacteria and many other microbes in natural environments reside within these biofilms, which are slimy layers that form on surfaces and at interfaces. Biofilms are complex, patchy assemblages of cells, proteins, and polysaccharides, where EPS biopolymers play a central role in adhesion. A variable but significant portion of biofilm biomass, typically 50-90%, is composed not of cells but of the EPS matrix, which can also incorporate other components like humic substances and biominerals.

Figure 1. (a) Biofilms and aggregates associated with iron oxidizing bacteria in Rocky Branch Creek, a small restored stream in Raleigh, North Carolina [1, 2]. (b) Schematic depicting the architecture of polymers and cells in biofilms [3]

The advantages conferred by EPS and biofilm formation are multifaceted. Biofilms allow microbes to create a controlled microenvironment that enhances their ability to manipulate local conditions and provides a shield against environmental stresses like desiccation, antibiotics, and predation. Furthermore, the EPS matrix can concentrate resources, including nutrients and metabolites, increasing their bioavailability to the embedded microbial community. It also serves to concentrate other functional exudates, such as metallophores, extracellular enzymes, and signaling compounds, thereby increasing their efficacy. The collective benefits of EPS secretion make biofilm formation a vital survival strategy and a major focus of research in environmental science, engineering, and biomedicine.