The Role of bacterial EPS in Soil
Most conversations about drought start with climate. We know climate change results in higher temperatures and greater rainfall variability. Water scarcity is indeed a major stressor for farming systems, and sustainable solutions to this problem are increasingly needed. Fortunately, a solution may lie just beneath our feet.
When water becomes scarce, it is crucial to understand how farming systems respond to it. In agriculture, drought resilience depends largely on soil, with a healthy soil retaining more water and reducing crop vulnerability to water scarcity. This ability is closely linked to the soil microbiome, which helps organize soil structure in ways that improve water and nutrient storage over longer periods of time. For farmers, this translates into more stable yields, reduced losses during droughts and extreme weather events, and lower production costs.
But how do microorganisms help soils retain water? Scientists are increasingly interested in some substances secreted by bacteria and other microorganisms that are exceptionally good at holding water. They are known as extracellular polymeric substances, or EPS. In this article, we will break down what EPS are and how EPS in soil help build drought resilience and maintain soil and plant health, with a specific focus on EPS produced by soil bacteria.
What soil is made of, and why it matters for water
First things first, how can soils retain water? The answer to this question lies in soil composition and structure.
Soil is mainly made of mineral materials, organic matter, water, and living organisms. The mineral fraction includes clay, silt, and sand, whose size and chemical properties largely determine how soils behave. Sand particles are large and poorly reactive, leading to loose soils with poor water retention. Instead, clay particles are small and chemically active, making clay-rich soils dense and sticky, retaining too much water. However, most soils are not simply “sand” or “clay”.
In soils, small particles organize into aggregated structures. Small aggregates (microaggregates) have tiny pores where water can enter, acting like sponges where humidity is retained. These microaggregates interact with their surrounding soil matrix and, through biological binding agents, form macroaggregates, creating larger pores where water and air can move. This organization, known as soil structure, is key to balancing water retention and drainage in the soil.
These aggregates form through electric interactions between clay, organic matter, and other minerals. In addition, the soil microbiome plays an active role in building and stabilizing them. Within this complex community, bacteria contribute to this process by secreting EPS, which act as a biological glue in the soil, binding mineral particles and organic matter together. This strengthens and stabilizes microaggregates, giving EPS in soil a significant impact on water retention.
But what are EPS, and why are they so important for soil structure?
EPS-producing bacteria play a significant role in drought resistance through soil aggregation. Representation of soil structure showing how mineral particles, organic matter, microorganisms, fungal hyphae, and plant roots can organise into aggregates of different sizes. Adapted from Costa et al. 2018 Front Microbiol (doi: 10.3389/fmicb.2018.01636).
How do EPS help retain water in soils ?
Soil bacteria are becoming popular because of their roles in nutrient cycling and plant health. But for bacteria to have an impact in those areas, they need to be physically close to other microbes, soil particles and plant roots. And that’s where EPS in soil come into play.
EPS are made of long chains of sugars, called exopolysaccharides, together with proteins and extracellular DNA. These polysaccharides are not linear. Instead, they create a three-dimensional branching structure that expands in the soil, interacting with multiple surfaces.
On top of that, EPS contain negatively charged and polar chemical groups. This property allows EPS to adhere to mineral particles and plant roots and to bind water, nutrients, and other polar compounds. This sticky property of EPS is multiplied by its branching structure, creating a dense network where charged groups are highly abundant and closely spaced. As a result, EPS present many simultaneous interaction points, which increase their binding ability.
This way, not only do EPS in soil bind water itself, but they also promote the formation and stabilization of soil aggregates. This structural organization allows soils to retain water while maintaining aeration. Water retention supports plant health by reducing exposure to drought-related stress, making water available to plants over extended periods of time. At the same time, aerated soils drain excess water and prevent waterlogging during intense rainfall events.
Beneficial microbe-plant interactions mediated by EPS
Beyond directly improving soil structure, EPS in soil facilitate microbe-plant interactions. Microorganisms enhance plant drought tolerance through processes such as the production of plant hormones, nutrients, and protective compounds, but these functions require close attachment to the root surface. EPS facilitate this contact by promoting microbial adhesion and by protecting microbial cells themselves from stressors.
Moreover, EPS bind soil particles together and anchor them to the root surface, helping to create what’s known as root-adhering soil. This creates a more stable and hydrated microenvironment, retaining water, nutrients and microorganisms around the roots, improving the plant’s ability to cope with water scarcity.
Within the soil microbiome, bacteria have been particularly well studied for their contributions to plant drought tolerance. Using different experimental approaches, scientists have investigated the role of EPS in soil drought resilience, often using EPS-producing bacteria. In studies where EPS-producing bacteria were introduced into the soil, scientists observed that the soil was more stable and lost water more slowly as it dried. In greenhouse experiments, when these microorganisms were interacting with plants, results showed that more soil was attached to plant roots, which helps them maintain a better water balance during drought.
In order to understand if these benefits are produced by EPS or other mechanisms, researchers have designed experiments that allow them to pinpoint the role of EPS in soil. In several studies, bacteria were genetically modified so they could no longer produce EPS. When using these EPS-deficient strains, their beneficial roles in plant health, drought resilience and soil structure were strongly reduced.
Together, these results provide evidence on the potential of EPS to improve soil structure, support microbial survival, and enhance plant tolerance to water scarcity.
EPS as a buffer against soil stressors like salinity and heavy metals
While EPS in soil help build drought resilience, they do more than that, supporting plants to survive and resist other forms of stress. Much of our understanding of these protective functions comes from studies on EPS produced by soil bacteria.
Another major stress factor in agriculture is soil salinity. A high concentration of salt in soil, such as sodium chloride (NaCl), impairs plant water uptake, disrupts soil structure and causes oxidative stress in both plants and bacteria. In a similar way to drought, EPS help mitigate these effects by acting as a physically active matrix in the soil, binding salt cations like sodium, limiting the absorption of salt by plant roots and microbial exposure as well. At the same time, EPS promote bacterial adhesion to plants, allowing them to exert their beneficial roles in promoting plant tolerance to salinity.
This same binding capacity extends to other cations, such as heavy metals, which easily associate with the negatively charged sugars in EPS. By retaining these ions in the soil matrix, EPS reduce their bioavailability and toxicity, protecting both bacterial and plant cells.
Building climate-smart farming from the soil up
Climate change is a multifaceted challenge that needs a combination of approaches to reduce its impact on farming systems. The growing evidence on the benefits of EPS-producing bacteria for agriculture makes it a crucial piece of the puzzle for strengthening drought resilience and overall soil function, contributing to more climate-smart farming systems.
Yet, this body of research raises a major question: since EPS production provides so many advantages to soil and plant health, how can we leverage it to improve soil resilience to drought? Developing effective solutions requires a strong understanding of soil microbiome community functions. For example, identifying key members of the bacterial ecosystem regulating EPS production is crucial to foster their benefits. Hence, paying closer attention to bacterial processes belowground is key to building resilient and productive soils, supporting a more sustainable way of farming.
