Published online 17 May 2007
Published in Vadose Zone J 6:298-305 (2007)
DOI: 10.2136/vzj2006.0080
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
SPECIAL SECTION: SOIL BIOPHYSICS
Extracellular Polymeric Substances Affecting Pore-Scale Hydrologic Conditions for Bacterial Activity in Unsaturated Soils
Dani Ora,b,*,
Sachin Phutaneb and
Arnaud Dechesneb,c
a School of Architecture, Civil and Environmental Engineering, EPFL- Ecole Polytechnique Federale, Lausanne CH- 1016, Switzerland
b Dep. of Civil and Environmental Engineering, Univ. of Connecticut, Storrs, CT 06269
c Institute of Environment & Resources, Technical Univ. of Denmark, Kgs. Lyngby, DK-2800, Denmark
* Corresponding author (dani.or{at}epfl.ch).
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
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ABSTRACT
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Soil bacterial cells are often found embedded in biosynthesized extracellular polymeric substances (EPS) forming aggregates or stationary colonies attached to solid surfaces. Soil bacterial aggregation and pooling of resources offer a successful adaptation to variations in hydration status and in nutrient availability and enhance cooperative genetic and metabolic exchanges. The ubiquity of such microbially excreted exopolymeric substances across many different environmental conditions and habitats is attributed to their key role in environmental adaptation, including colony architecture and anchoring, nutrient entrapment, and maintenance of favorable hydration conditions. This review focuses on the hydrophysical properties of EPS and its primary constituent, exopolysaccharides, and their role as an interface between living cells and the harsh conditions common to the shallow vadose zone. We review water retention, diffusion, and hydraulic properties of EPS and postulate mechanisms conferring an advantage to embedded bacterial cells. The shrink–swell behavior of EPS for different water potentials affects mean pore size and passage of solutes and colloids of different sizes; we evaluate various water-related morphological transformations of EPS that influence diffusion behavior in unsaturated soils. We hypothesize that EPS low permeability results in hydraulic decoupling during rapid wetting or drying events, effectively shielding embedded bacterial cells from adverse effects of extreme fluctuations in hydration conditions. We show that the addition of minute amounts of EPS significantly alters the hydrological conditions experienced by microbial colonies and, in some case, may alter macroscopic hydrological and mechanical properties of the host porous medium.
Abbreviations: DDL, diffuse double layer • EPS, extracellular polymeric substances.
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INTRODUCTION
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In various environments, bacterial cells are often found embedded in a matrix of extracellular polymeric substances (EPS). The biosynthesized matrix is a complex mixture of macromolecules, primarily composed of polysaccharides but also containing various amounts of protein, lipid, DNA, and vitamins (Sutherland, 2001a; Flemming and Wingender, 2001). Depending on the microorganism and the environmental conditions, EPS can represent a significant weight compared to cells' own weight (Table 1), suggesting that its production constitutes an important investment in carbon and energy for the producers. Nevertheless, the presence of EPS is not essential for bacterial growth since mutants incapable of producing EPS were able to grow under standard laboratory conditions (Ophir and Gutnick, 1994).
Extracellular polymeric substances constitute a relatively minor but not negligible component of soils. Extracellular polymeric substances associated with living cells in soils constitute 0.1 to 1.5% of the soil organic matter (Chenu, 1995). According to Chenu (1993), typical values of EPS found in soils (assuming 109 cells g–1 of soil with total weight of 
1.1 mg) ranges between 0.1 to 5.0 mg EPS (or 10 to 500% of cells' biomass).
The large amount of EPS produced in soil and the taxonomic diversity of its producers suggest that these substances contribute to microbial fitness in soil. Soils form complex habitats in which bacteria must cope with various stresses, especially in the shallow vadose zone. A recent review by Or et al. (2007) discusses key physical constraints encountered by bacteria in the vadose zone, especially the fragmentation of aquatic habitats as soil become unsaturated and water forms thin and discontinuous films. Microorganisms are critically dependent on water for nutrient supply and to sustain physiological functions, as their interior is almost entirely comprised of water and cell membrane integrity relies on well-hydrated environment. Desiccation thus has a deleterious effect on bacterial cells that can result in growth arrest and eventually the cell's death. Even bacteria with physiological adaptation to desiccation (Potts, 1994) benefit from avoiding or delaying the onset of desiccating conditions to make the necessary physiological adjustments. Bacteria benefit from well-hydrated environment as water mediates diffusion of substrates to the cell. The reduction of aquatic pathways (thin films) under low matric potentials has been associated with reduced bacterial growth (Skopp et al., 1990; Rivkina et al., 2000; Koch, 1990).
Most studies on EPS function in natural systems considered water-saturated environments such as aquifers (Allison, 1998; Lewandowski et al., 1995), sludge (Houghton and Stephenson 2002), flocs (Sutherland, 2001b), or marine (Bhaskar and Bhosle, 2005) environments. Despite the importance of the vadose zone in supporting microbial diversity, plant life, and nutrient cycling, little is known about EPS behavior and function under unsaturated conditions (Roberson and Firestone, 1992; Holden et al., 1997; Chenu and Roberson, 1996; Rockhold et al., 2004).
The primary objective of this study is to review EPS characteristics and function in the unsaturated zone where hydraulic connectivity is maintained through thin water films that control processes ranging from nutrient fluxes to cell-to-cell signaling and progressively suppressing microbial activity and survivability (Potts, 1994). Such highly transient hydration conditions are known to trigger an array of biological responses, including the production of copious amounts of EPS (Roberson and Firestone, 1992). A key question pertains to the characteristics of EPS that make it a "universal" interface between microbes and their environments (Chenu, 1995), especially its capacity to improve microbial existence under transient desiccation conditions in the vadose zone (Potts, 1994; Tamaru et al., 2005).
In the following sections, we review EPS properties, morphology and behavior that facilitate microbial survival in nutrient-poor and unsaturated porous media. Among the numerous constituents of natural EPS, we emphasize structure and function of exopolysaccharides due to their abundance in natural EPS and availability of a wealth of information. We examine morphological changes in the EPS matrix under varying hydration states, its role in maintenance of aquatic microhabitats and facilitating nutrient diffusion under desiccated conditions, and potential modification of macroscopic hydrologic properties of host porous media.
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Experimental Evidence for the Role of EPS in Desiccation Resistance
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Although the role of EPS in the protection of bacteria from desiccation in variably saturated environment was hypothesized several decades ago, direct experimental evidence has been acquired only recently (Tamaru et al., 2005). The pioneering studies of Roberson and Firestone (1992) showed that Pseudomonas in sand increased its EPS production in response to desiccation, suggesting that EPS may offer potential advantages or protection. Higher survival rates observed in air-dried soil of EPS-producing nitrifiers compared with a nonproducing strain provide an additional support for the potential protective role of EPS (Allison and Prosser, 1991), although the strains compared may have had other ecological or physiological differences in addition to EPS production. In a series of more convincing experiments, Ophir and Gutnick (1994) used nonmucoid isogenic mutants belonging to Escherichia, Erwinia, and Acinetobacter genera, which showed a reduced tolerance to desiccation compared with their wild-type counterpart. Nevertheless, whereas EPS production has been demonstrated to be important for Pseudomonas fluorescens survival in dry conditions, highly mucoid mutants, overproducing EPS, did not tolerate desiccation stress better than the wild type (Schnider-Keel et al., 2001).
Some of the most definitive evidence for the desiccation tolerance and protective role of EPS was presented in a recent study featuring cyanobacterial cells that were mechanically EPS depleted (Tamaru et al., 2005). Whereas about 100% of EPS-coated cells were active after an overnight exposure to air drying, only about 30% EPS-depleted cells produced O2 after the same treatment. Results from other scenarios of desiccation stress confirm that EPS contributes significantly to desiccation resistance and enhances recovery after desiccation for a diverse group of prokaryotes both in vitro and in terrestrial habitats.
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EPS Components and Structure
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Bacterial extracellular matrices exhibit complex composition and structure that vary with environmental conditions or the producer's physiological state (Tease and Walker, 1987; Hu et al., 2003; Steinberger and Holden, 2004). An EPS matrix is composed primarily of polysaccharides but contains other biopolymers such as proteins, lipids, DNA, and humic substances, along with amino acids, polypeptides, amides, vitamins, and growth regulators (Flemming and Wingender, 2001; Sutherland, 2001b; Hu et al., 2003). Typical molecular size of biopolymers in EPS range from 103 to 108 kDa (Allison, 1998).
Extracellular polymeric substance strands are typically negatively charged due to glucuronic, galacturonic, or mannuronic acids or to nonsugar units such as pyruvic acids and succinic acids. Variations in EPS structure, including the degree of acetylation of polysaccharide molecules, molecular composition, and/or the molecular mass may reflect changes in environmental conditions (e.g., pH) (Sutherland, 2001b). Deacetylation of the bacterial EPS has been shown to improve pseudoplasticity in aqueous solution as well as to increase the cooperativity of the order–disorder transition. Moreover, it also contributes to the increased water-binding capacity of EPS that reduces self-diffusion coefficient of water in deacetylated xanthan (Hart et al., 1999). On the other hand, deacetylation of some polysaccharides may lead to loss of any ordered conformation (Sutherland, 2001b). The structure of polysaccharides in EPS produced by Xanthomonas campestris is depicted in Fig. 1.
Physical properties such as water solubility, ion binding ability, and gelation of the EPS depend on the polymer composition and structure and also on the interactions with other macromolecules and ions (Sutherland 2001b). The structure of the repeating monomer varies between different strains, and the introduction of branched polysaccharide chains and substitution of both organic and inorganic molecules adds to structural complexity. The components that determine the physical properties of EPS are listed in Table 2.
The physical structure of many EPS consists of strands a few nanometers wide and >100 nm in length that may be extensively interconnected (Chenu, 1995). Extracellular polymeric substances have been described as forming polymeric strands that wrap around the cell surface or interact with one another to form helical duplexes (Erlandson et al., 2004; Chenu, 1995). The extracellular matrix can be seen as a compact three-dimensional network formed by the cross-linking of arrays of the polymeric molecules, such as polysaccharides and proteins, with a high density of cross-linking conferring its rigidity to the matrix (Xia et al., 2000; Strathmann et al., 2001). Three major kinds of forces that bind the EPS molecules can be distinguished as electrostatic interactions, hydrogen bonds, and London dispersion forces (Flemming and Wingender, 2001).
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Hydration and Transport Properties of EPS
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Hydration of EPS
The network of EPS strands and their surface properties act as a sponge with considerable capacity to absorb water. Water is attracted to the EPS matrix by surface, osmotic, and capillary forces resulting in swelling of the matrix and progressive increase in spaces between the biopolymeric strands (Fig. 2). (Israelachvili, 1991; Goerke et al., 2000; Shaw et al., 2003). This cross-linked and compact structure of EPS matrix is capable of binding more than 15 to 20 g of water per gram of EPS near saturation (Chenu and Roberson, 1996; Roberson and Firestone, 1992). Experimental studies by Chenu (1993) show water-retention curves for pure polysaccharides holding more than 50 to 70 g of water per gram of polysaccharide while maintaining structural coherence. Water retention varies with the type of polysaccharides (Fig. 3), but EPS possesses water-retention capacity that far exceeds that of most earth materials (e.g., clays).
Upon drying, morphological changes resulting from electrostatic interactions between EPS strands take place within the extracellular matrix (Israelachvili, 1991; Chun and Ladd, 2004). A diffuse double layer (DDL) forms around charged EPS surfaces due to the ionization or dissociation of protons from surface groups. Changes in the extent of DDL result from repulsive osmotic pressure between the counterions (Israelachvili, 1991) that forces them away from the surface and from each other so as to increase their configurational entropy. As matric potential decreases, the EPS remain water-saturated over a wide range of matric potential (Chenu and Roberson, 1996). The distance between polymeric strands becomes smaller by shrinking the extent of the DDL by forcing the dissociated ions closer to surfaces. The interplay between water potential and mean distance between strands can be calculated according to Langmuir (as cited in Israelachvili, 1991), and the distance between strands varies from a few micrometers for fully saturated EPS (at zero water potential) to a minimum distance of a few nanometers under complete desiccation.
The cross-linking of EPS strands confers the ability of the extracellular matrix to regain its morphology after a swelling and drying cycle (Strathmann et al., 2001, Karlsson et al., 1998). These properties enable hydrated EPS gel to undergo morphological collapse (Fig. 2) (Roberson and Firestone 1992) to form the weblike glassy film structure.
EPS Aqueous Diffusion Properties
The diffusion of aqueous solutions through EPS biopolymers has been the subject of numerous investigations (Lekhlif et al., 1994; Stoodley et al., 1994). The resulting diffusion patterns vary from Fickian diffusion with mass uptake as a function of square root of time to mass uptake linear with time due to viscoelastic properties of the biopolymers (Fouad and Bhargava, 2005; Rittmann and McCarty, 1980; Suiden et al., 1987). The capability of EPS to bind and immobilize water reduces self-diffusion rates relative to that of free water. The water diffusion coefficient in EPS drops from its value in free water (2.5 x 10–5 cm2 s–1) to the 2.5 to 7.5 x 10–6 cm2 s–1 range when the polymer concentration reaches 50%, with deacetylated xanthan presenting a higher water-binding capacity than its acetylated counterpart (Hart et al., 1999).
Numerous reports provide direct evaluation of diffusion fluxes in biofilms for a wide range of solutes. It is customary to express the reduction in diffusion coefficient due to EPS on a relative basis as D/Daq, where D is the diffusion coefficient of species in EPS and Daq is the diffusion coefficient of species in pure water. The value of De/Daq (where De is the effective diffusion coefficient) in saturated biofilms (or in well-hydrated EPS) ranges from 0.2 to 0.8. More specifically, values of 0.6 and 0.25 are suggested for dissolved gases (e.g., oxygen, nitrous oxide, carbon dioxide, or methane) and for most organic solutes, respectively (Stewart, 2003). The effective diffusion coefficient of solutes in well-hydrated EPS as a function of solute molecular weight (MW) is shown in Fig. 4a.
As water potential decreases, a structural transformation of EPS morphology takes place from a well-hydrated bioweb to highly cross-linked gel structure, with the mean pore size of EPS matrix reduced by three to four orders of magnitude. Various diffusion regimes can be observed in the gelated EPS, ranging from normal diffusion to trapped diffusion depending on the solute hydrodynamic radius and the solute–EPS matrix physical interactions (Fatine-Rouge et al., 2004). A typical diffusion behavior of various solutes in agarose gel, an analog of EPS material, is shown in Fig. 4b (Fatine-Rouge et al., 2004). The largest size of the trapped molecule was found to be 70 nm, consistent with the estimated mean pore-size of the gel (77 nm). This suggests that solute molecules like glucose with hydrodynamic radius of 4 Å (Chenu and Roberson, 1996) can very easily pass through gelated EPS. Nevertheless, a linear decrease in diffusion coefficient with the EPS water content suggests a dependency of diffusion coefficient on the hydrated cross-section available for diffusion (Chenu and Roberson, 1996).
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EPS-Mediated Hydrological Processes in Unsaturated Porous Media
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EPS Water Retention and Hydraulic Conductivity
The high water-retention capacity of EPS is expected to alter microhydrological conditions experienced by EPS-encapsulated microbial cells or microbial colonies occupying soil pores. Extracellular polymeric substances maintain higher water contents in the microenvironment of bacterial colonies that is almost 15 to 20 times larger than the bulk soil under dry conditions (Wiebe, 1996).
Considering that EPS at –1.5 MPa holds five times its weight in water, and at –0.5 MPa holds 10 times its weight in water, it is not surprising that the addition of small amounts of EPS to soil may result in disproportionably large increases in its macroscopic water-retention capacity. In a study by Chenu and Roberson (1996) it was observed that 1% (w/w) addition of EPS results in a significant increase in water-retention capacity of unsaturated media, as seen in Fig. 5. The increased water retention can be also attributed to the creation of a more open pore space, soil solid particles being held separated by the EPS fibrous network.
Additionally, the reduced water permeability in the EPS matrix compared with that of most host porous media contributes to the gradual water loss from EPS-containing pores, providing microorganisms with time for physiological adjustments and deployment of survival strategies. Small amounts of EPS may significantly reduce the hydraulic conductivity of the porous medium (Vandevivere and Baveye. 1992). The results depicted in Fig. 6 show a four order of magnitude reduction in saturated hydraulic conductivity of sand with addition of 1% xanthan (Yen, 2001). We obtained similar results using glass bead columns amended with various percentages of EPS analog (xanthan). The hydraulic conductivity of the glass beads column was reduced from 1.5 x 10–5 m s–1 (without EPS) to 5.4 x 10–10 m s–1 (with 1% EPS amendment). This value is very close to the permeability of 1% EPS matrix (2.55 x 10–10 m s–1). This confirms that minute amounts of EPS may reduce the hydraulic conductivity of a porous medium by five orders of magnitude and be almost as effective as EPS alone in reducing the permeability.
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FIG. 6. Effects of xanthan gum concentration on the saturated hydraulic conductivity of sand medium (modified from Yen, 2001), and of a pack of glass beads with mean diameter of 0.5 to 1.0 mm.
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EPS-Enhanced Substrate Diffusion in Unsaturated Soil
Desaturation affects diffusion processes at a continuum of scales ranging from individual microbial colony and pore scales to soil profile scale. Diffusion rates at low matric potentials are significantly greater within EPS-rich microbial microenvironments than in surrounding soil pore spaces (Chenu and Roberson, 1996). The diffusion rates of glucose in xanthan even at water potential of –1 MPa are as high as diffusion rates found in saturated silt loam soil (Fig. 7), indicating that EPS remains hydrated and maintains open matrix, enabling passage of glucose molecules with hydrodynamic radius of 4 Å (Chenu and Roberson, 1996). The high diffusivity sustained by the presence of EPS should thus ensure large diffusive fluxes of nutrients to the EPS-producing cells even under low matric potentials.
EPS and Hydraulic Decoupling
We postulate that EPS, by virtue of its special hydraulic properties, offers a potential advantage to embedded microbial cells through hydraulic decoupling with the surrounding porous medium. The combination of low hydraulic conductivity and high water retention relative to host porous medium would be conducive to formation of hydraulically isolated and hydrated islands of EPS-rich zones during rapid drainage events. Conversely, during rapid rewetting, the slow hydraulic conductivity would moderate the rewetting rates and enable gradual osmotic adjustment of microbial cells to rainwater with low salt content.
We conducted a simple experiment to qualitatively illustrate the hydraulic decoupling hypothesis. We placed 2% (w/w) EPS blobs in a glass-bead filled column (glass bead size 0.5 mm diam.), as depicted in Fig. 8a. We then saturated the column with water dyed with brilliant blue. During the first 15 min, very little dye diffused into the visible EPS blobs (Fig. 8b). After 12 h the dye finally diffused to the center of the EPS blobs (10-mm radius), as shown in Fig. 8c. Subsequently, the column was drained and flushed with tap water, which visually enhanced the extent of dye penetration into the EPS blobs (Fig. 8d). The column was then rapidly drained, and the water content in the EPS blobs and in the glass beads matrix was quantified in triplicates. The results show that EPS blobs (mimicking microbial microenvironments) remained fully saturated and contained almost 3.5 times more water than the surrounding matrix. The porosity of the EPS–glass beads mixture (38%) was fully water saturated (0.34 g/g), unlike the neighboring glass beads matrix which retained only 0.09 g water per gram solids. This simple exploratory experiment demonstrates salient features of hydraulic decoupling during rapid drainage of a coarse porous medium in which fully hydrated EPS microenvironments are left behind the drainage front. The occurrence and importance of such a phenomenon in real systems awaits experimental confirmation.
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FIG. 8. (a) Glass beads column (0.5–1.0 mm diam.) containing blobs of extracellular polymeric substance (EPS) prior addition of dyed water, (b) column saturated with dyed water for 15 min (no dye diffusion in EPS), (c) saturated column after 12 h (including saturated EPS blobs), and (d) column drained and flushed with water (retention of water by EPS blobs after drainage).
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Unlike soil drainage and subsequent drying, which are relatively slow and gradual processes, the wetting of a dry soil leads to a rapid increase in soil water potential from values in dry soil of –20 MPa to nearly 0 MPa within a few seconds. Such rapid changes may subject many near-surface soil organisms to severe osmotic stresses (Halverson et al., 2000). Because of gel to liquid–crystal transformation of cell membrane, rapid wetting of viable, dry cells may result in extensive leakage and release of a substantial fraction (17–70%) of the biomass (Potts, 1994). This can cause cell lysis and lead to cell death. Extracellular polymeric substances in these conditions can gradually absorb large amount of water and protect microbial colonies from rewetting stresses (Ophir and Gutnick, 1994; Chenu and Roberson, 1996; Roberson and Firestone, 1992). The embedding in EPS, therefore, may be as important to bacterial survival during wetting as during the desiccation that precedes it.
EPS and Evaporation Retardation
Chenu and Roberson (1996) provided evidence suggesting that evaporative water losses from EPS-amended sand were slower than losses from control without EPS. Similar speculations concerning the potential of EPS forming an evaporation-retarding barrier were made by others (Sutherland 2001b; Holden et al., 1997). The implications for microbial survivability of evaporation retardation by EPS are significant: Chenu and Roberson (1996) claim to have documented a delay of 3 to 4 h in attaining a critical drop in matric potential values (Fig. 9), a delay that could enable a certain degree of physiological adaptation. The results, however, rely on determining matric potential from water-retention information rather than through direct measurements of water-potential evolution.
The evaporation retardation and time lag observed for EPS-amended soil can be explained by the drying mechanism of EPS. Under drying conditions, EPS responds by adjusting its morphology (Sutherland, 2001a): when subjected to desiccation, an EPS network shrinks dramatically and remains water-saturated over a wide range of water potentials. As the drying continues, cracks may appear at the outermost layer of the EPS matrix, where differential rates of water loss form a glassy EPS crust that may slow down desiccation of inner layers and embedded bacterial cells (Allison, 1998; Holden et al., 1997; Xia et al., 2000). Formation of a biopolymer crust may also reduce diffusion rates (Burke, 1986). Similar to the hydraulic decoupling hypothesis, present experimental evidence is anecdotal, and additional definitive tests and direct quantification are required to establish such a potential desiccation protection mechanism.
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Effects of EPS on Soil Structure
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In addition to macroscopic effects on soil water retention and solute diffusion, EPS may also alter microscale interfaces between microbes and grain surfaces. Extracellular polymeric substance fibers bind to clay particles or other mineral particles surrounding the bacteria and penetrate the surrounding mineral matrix, thereby establishing a continuum between the cells and other soil constituents (Czarnes et al., 2000; Chenu and Stotzky, 2002). Thus, excretion of EPS establishes a porous and well-hydrated continuum between the cell and its mineral surroundings (Chenu, 1995)
The enmeshing of soil particles with EPS has a dual role in the formation of soil microaggregates and, more important, in their stabilization (Czarnes et al., 2000; Chenu and Stotzky, 2002). As a cementing and stabilizing agent, EPS enhances open structure among soil and clay particles, creating conditions for favorable liquid and gaseous transport property. The combined effects of adsorption and cross-linking between different EPS particle strands set up a very effective bridging, leading to an organo-mineral network. We expect that soil strength and structural stability acquired by accumulation of EPS would reflect some of the mechanical properties of EPS forming the bacterial colonies. Thwaites and Mendelson (1991) have shown that in response to changes in hydration status, EPS-like biopolymers exhibit orders of magnitude increase in tensile strength and Young's modulus. Moreover, biopolymers change from soft and ductile under saturated conditions to stiff and brittle at low saturation. Following intense colonization of rhizosphere of wheat (Triticum aestivum L.) by EPS-producing bacteria, Amellal et al. (1998) observed significant increase in soil aggregation and concluded that P. agglomerans plays an important role in soil water regulation by improving aggregation.
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Summary and Conclusions
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Large fluctuations in hydration conditions and associated variations in water and nutrient fluxes are common in the shallow vadose zone. The ubiquity and properties of EPS, and the embedding of living cells in this interfacial bi-porous matrix, suggest that EPS may play a role in sheltering and dampening the effects of fluctuations in hydrated environment. The links between desiccation stress and the production of extracellular polymeric substances have received considerable attention, although the genetic regulation of the process is not yet resolved (Roberson and Firestone, 1992; Schnider-Keel et al., 2001). Nevertheless, direct evidence of the protective role of EPS has been obtained recently (Tamaru et al., 2005). The retention, transport, mechanical, and morphological properties of EPS make the matrix exceptionally suitable for bacterial protection against fluctuating hydration conditions in soil.
The EPS forms a compact three-dimensional network of cross-linking polymeric strands that can retain water more than 15 times its weight. This property extends to unsaturated conditions; at water potential of –0.5 MPa, for example, it retains 10 times its weight. This exceptional hydration capacity suggests that even small amounts of EPS in unsaturated soil (1%) would not only form hydrated microenvironments but could also enhance overall water-retention capacity of the host soil. The EPS coat or interface also plays an important role in preventing the direct exposure and rupture of microbial membranes due to sudden drop in water potentials during rapid wetting. The capacity of EPS coating to maintain inner-microbial colony hydration prevents protein denaturation and provides microbial colonies with the necessary time for physiological and metabolic adaptations. The low hydraulic conductivity and high water retention of EPS promote formation of hydraulically isolated and hydrated microbial islands during rapid drainage. Additionally, the slow and gradual hydration rates of EPS during rapid wetting of the porous medium protect embedded cells from osmotic shocks and help their gradual reactivation.
The EPS matrix interfaces with solid pores and surfaces, providing a continuum between microbes and their host porous medium. The EPS remains hydrated and retains gel-like morphology even under low water potentials, which provides open structures with sufficiently large pores that effectively facilitate diffusion of higher molecular weight solutes. Thus, higher solute diffusion in EPS extends the range of uninterrupted biological activity for individual cells and for microbial community-level functions even during large fluctuations in water potentials. It appears that bacteria engineer their own microenvironment in the form of a biosynthesized porous medium that intimately interacts with soil particles, providing buffered and predictable hydration and transport properties.
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ACKNOWLEDGMENTS
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The authors gratefully acknowledge the partial support of BARD, US–Israel Binational Agricultural Research and Development Fund, under grant US-3377-02, and the National Science Foundation (Hydrologic Sciences) under grant EAR-0409364.
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ABSTRACT
INTRODUCTION
