The engineering potential of natural benthic bacterial assemblages in terms of the erosion resistance of sediments

Decrypting the stream periphyton physical habitat of recently deglaciated floodplains

The rapid recession of glaciers is exposing large zones to the development of embryonic phototrophic ecosystems and eventual ecological succession. Traditionally, succession patterns in glacial forefields have been seen as a response to time since deglaciation, but nowadays forefield exposure is so rapid that this theory may be less applicable. In this succession process, periphyton are potential pioneer organisms because of their role in modifying the local environment (e.g. access to water) to create conditions conducive to plant colonization. In this paper, we aim to decrypt the physical properties of the habitats that define the spatial and temporal assemblage of periphyton during the melt-season of an Alpine temperate glacier in the context of rapid climate change. We show that periphyton develop in glacial floodplains throughout the melt-season and could extend to cover significant surfaces. However, development is only possible when the combined conditions of stability and water accessibility are met. In glacial floodplains, stable zones exist and are typically located on terraces; but they can also be locally found for much shorter periods in the more active, glacial-stream reworked zone. On terraces, water accessibility can be a limit due to well-drained sediments, but when present, often aided by the role that biofilms play in creating an impermeable layer, it provides a stable and clear water source that biofilms could exploit. In the active part of the braid plain, whilst water availability is very high, the water is harsh (low temperature, high turbidity) and erosive. Therein, periphyton can rapidly exploit short windows of opportunity but the habitat conditions rarely remain stable for long enough for continuous periphyton cover to develop. Thus, the role of periphyton in ecosystem succession is strongly conditioned by the spatial extent of the active zone, itself a function of high rates of glacier melt and sediment supply associated with rapid glacier retreat.

Biological Layer in Household Slow Sand Filters: Characterization and Evaluation of the Impact on Systems Efficiency

Schmutzdecke, the biofilm formed on the top of the sand bed in household slow sand filters (HSSF) is a key factor for the filters’ high efficiency in removing particles and microorganisms from water. This paper aims to investigate the extracellular polymeric substances composition (carbohydrates and proteins), biomass, dissolved oxygen, and microbial community in two types of HSSFs and identify a correlation between them and their efficiency. A continuous- and an intermittent-HSSF (C-HSSF and I-HSSF) were studied to treat river water for 48 days. Their efficiencies for bacteria (E. coli and total coliforms), turbidity, and apparent color removals were analyzed. Results clearly showed an increase of carbohydrates (from 21.4/22.5 to 101.2/93.9 mg·g−1 for C-/I-HSSF) and proteins (from 34.9 to 217/307.8 mg g−1 for C-/I-HSSF), total solids (from 0.03/<0.03 to 0.11/0.19 g L−1 for C-/I-HSSF), dissolved oxygen depletion inside the filter (6.00 and 5.15 mg L−1 for C- and I-HSSF) and diversity of microorganisms over time, pointing out the schmutzdecke development. A clear improvement on the HSSFs’ efficiency was observed during operation, i.e., E. coli removal of 3.23 log and 2.98 log for total coliforms, turbidity from 60 to 95%, and apparent color from 50 to 90%.

Spatiotemporal variation of extracellular polymeric substances (EPS) associated with the microphytobenthos of tidal flats in the Yellow Sea

The physical functions of extracellular polymeric substances (EPS), viz., by-product of microphytobenthos (MPB), in tidal flat system are well documented, but some ecological aspects remain unknown. We investigated MPB biomass (Chl-a), EPS, diatom assemblage, and erodibility in two contrasting tidal flat environments (megatidal vs. macrotidal flat) in the Yellow Sea. Thick biofilms were observed when MPB bloomed, with high Chl-a and increased EPS concentrations. Among diatom genera, Navicula was the most dominant taxa found over the year (mean 41%) in both areas. Compared with non-bloom periods, the erodibility decreased by 54-73% as biofilm thickened during the blooms. It was attributed to the elevated abundance of large-sized (>40 μm) Navicula, which was expected to secrete large amounts of EPS. Overall, we successfully demonstrated spatiotemporal differences of sediment stabilization that significantly related to ecological variations of MPB, and identified the key diatom genus as a "sediment stabilizer" in the typical tidal flats of the Yellow Sea.

Exploring flow-biofilm-sediment interactions: Assessment of current status and future challenges

Biofilm activities and their interactions with physical, chemical and biological processes are of great importance for a variety of ecosystem functions, impacting hydrogeomorphology, water quality and aquatic ecosystem health. Effective management of water bodies requires advancing our understanding of how flow influences biofilm-bound sediment and ecosystem processes and vice-versa. However, research on this triangle of flow-biofilm-sediment is still at its infancy. In this Review, we summarize the current state of the art and methodological approaches in the flow-biofilm-sediment research with an emphasis on biostabilization and fine sediment dynamics mainly in the benthic zone of lotic and lentic environments. Example studies of this three-way interaction across a range of spatial scales from cell (nm - µm) to patch scale (mm - dm) are highlighted in view of the urgent need for interdisciplinary approaches. As a contribution to the review, we combine a literature survey with results of a pilot experiment that was conducted in the framework of a joint workshop to explore the feasibility of asking interdisciplinary questions. Further, within this workshop various observation and measuring approaches were tested and the quality of the achieved results was evaluated individually and in combination. Accordingly, the paper concludes by highlighting the following research challenges to be considered within the forthcoming years in the triangle of flow-biofilm-sediment: i) Establish a collaborative work among hydraulic and sedimentation engineers as well as ecologists to study mutual goals with appropriate methods. Perform realistic experimental studies to test hypotheses on flow-biofilm-sediment interactions as well as structural and mechanical characteristics of the bed. ii) Consider spatially varying characteristics of flow at the sediment-water interface. Utilize combinations of microsensors and non-intrusive optical methods, such as particle image velocimetry and laser scanner to elucidate the mechanism behind biofilm growth as well as mass and momentum flux exchanges between biofilm and water. Use molecular approaches (DNA, pigments, staining, microscopy) for sophisticated community analyses. Link varying flow regimes to microbial communities (and processes) and fine sediment properties to explore the role of key microbial players and functions in enhancing sediment stability (biostabilization). iii) Link laboratory-scale observations to larger scales relevant for management of water bodies. Conduct field experiments to better understand the complex effects of variable flow and sediment regimes on biostabilization. Employ scalable and informative observation techniques (e.g., hyperspectral imaging, particle tracking) that can support predictions on the functional aspects, such as metabolic activity, bed stability, nutrient fluxes under variable regimes of flow-biofilm-sediment.

Factors affecting the spatial and temporal distribution of E. coli in intertidal estuarine sediments

Microbiological water quality monitoring of bathing waters does not account for faecal indicator organisms in sediments. Intertidal deposits are a significant reservoir of FIOs and this indicates there is a substantial risk to bathers through direct contact with the sediment, or through the resuspension of bacteria to the water column. Recent modelling efforts include sediment as a secondary source of contamination, however, little is known about the driving factors behind spatial and temporal variation in FIO abundance. E. coli abundance, in conjunction with a wide range of measured variables, was used to construct models to explain E. coli abundance in intertidal sediments in two Scottish estuaries. E. coli concentrations up to 6 log₁₀ CFU 100 g dry wt^-1 were observed, with optimal models accounting for E. coli variation up to an adjusted R² of 0.66. Introducing more complex models resulted in overfitting of models, detrimentally affected the transferability of models between datasets. Salinity was the most important single variable, with season, pH, colloidal carbohydrates, organic content, bulk density and maximum air temperature also featuring in optimal models. Transfer of models, using only lower cost variables, between systems explained an average deviance of 42%. This study demonstrates the potential for cost-effective sediment characteristic monitoring to contribute to FIO fate and transport modelling and consequently the risk assessment of bathing water safety.

Analysis of growth and lipid production characteristics of Chlorella vulgaris in artificially constructed consortia with symbiotic bacteria

The aim was to study the effect of artificially constructed consortia of microalgae-bacterial symbionts on growth and lipid production by Chlorella vulgaris (C. vulgaris), as well as the inter-relationship between microalgae and bacterial in a photoautotrophic system. The results showed that compared to an axenic culture of C. vulgaris, H1 co-culture system (axenic C. vulgaris-Stenotrophomona smaltophilia) had the strongest effect on the C. vulgaris growth. The biomass, specific growth rate and maximum productivity of C. vulgaris were increased by 21.9, 20.4, and 18%, respectively. The bacteria in co-culture system had a significant effect on the accumulation of lipid and fatty acid components of C. vulgaris: the content of lipid was increased by 8.2-33.83%, and the components of the saturated fatty acids and oleic acids also had an obvious improvement. The results indicate that the microalgae-bacterial co-culture system can improve microalgal biomass and the quality of biodiesel.

E. coli Surface Properties Differ between Stream Water and Sediment Environments

The importance of E. coli as an indicator organism in fresh water has led to numerous studies focusing on cell properties and transport behavior. However, previous studies have been unable to assess if differences in E. coli cell surface properties and genomic variation are associated with different environmental habitats. In this study, we investigated the variation in characteristics of E. coli obtained from stream water and stream bottom sediments. Cell properties were measured for 77 genomically different E. coli strains (44 strains isolated from sediments and 33 strains isolated from water) under common stream conditions in the Upper Midwestern United States: pH 8.0, ionic strength 10 mM and 22°C. Measured cell properties include hydrophobicity, zeta potential, net charge, total acidity, and extracellular polymeric substance (EPS) composition. Our results indicate that stream sediment E. coli had significantly greater hydrophobicity, greater EPS protein content and EPS sugar content, less negative net charge, and higher point of zero charge than stream water E. coli. A significant positive correlation was observed between hydrophobicity and EPS protein for stream sediment E. coli but not for stream water E. coli. Additionally, E. coli surviving in the same habitat tended to have significantly larger (GTG)₅ genome similarity. After accounting for the intrinsic impact from the genome, environmental habitat was determined to be a factor influencing some cell surface properties, such as hydrophobicity. The diversity of cell properties and its resulting impact on particle interactions should be considered for environmental fate and transport modeling of aquatic indicator organisms such as E. coli.

Biostabilization of cohesive sediments: revisiting the role of abiotic conditions, physiology and diversity of microbes, polymeric secretion, and biofilm architecture

In aquatic habitats, micro-organisms successfully adhere to and mediate particles, thus changing the erosive response of fine sediments to hydrodynamic forcing by secreting glue-like extracellular polymeric substances (EPS). Because sediment dynamics is vital for many ecological and economic aspects of watersheds and coastal regions, biostabilization of cohesive sediments is one of the important ecosystem services provided by biofilms. Although the research on biostabilization has gained momentum over the last 20 years, we still have limited insights principally due to the complex nature of this topic, the varying spatial, temporal, and community scales examined, oversimplified ecohydraulic experiments with little natural relevance, and the often partial views of the disciplines involved. This review highlights the current state of our knowledge on biostabilization and identifies important areas for future research on: (A) the influence of abiotic conditions on initial colonization and subsequent biofilm growth, focusing on hydrodynamics, substratum, salinity, nutrition, and light climate; (B) the response of microbes in terms of physiological activity and species diversity to environmental settings as well as biotic conditions such as competition and grazing; and (C) the effects of the former on the EPS matrix, its main constituents, their composition, functional groups/substitutes, and structures/linkages. The review focuses specifically on how the numerous mutual feedback mechanisms between abiotic and biotic conditions influence microbial stabilization capacity, and thus cohesive sediment dynamics.

Nutrient release, recovery and removal from waste sludge of a biological nutrient removal system

The uncontrolled release of nutrients from waste sludge results in nitrogen and phosphorus overloading in wastewater treatment plants when supernatant is returned to the inlet. A controlled release, recovery and removal of nutrient from the waste sludge of a Biological Nutrient Removal system (BNR) are investigated. Results showed that the supernatant was of high mineral salt, high electrical conductivity and poor biodegradability, in addition to high nitrogen and phosphorus concentrations after the waste sludge was hydrolysed through sodium dodecyl sulphate addition. Subsequently, over 91.8% of phosphorus and 10.5% of nitrogen in the supernatants were extracted by the crystallization method under the conditions of 9.5 pH and 400 rpm. The precipitate was mainly struvite according to X-ray diffraction and morphological examination. A multistage anoxic-oxic Moving Bed Biofilm Reactor (MBBR) was then adopted to remove the residual carbon, nitrogen and phosphorus in the supernatant. The MBBR exhibited good performance in simultaneously removing carbon, nitrogen and phosphorus under a short aeration time, which accounted for 31.25% of a cycle. Fluorescence in situ hybridization analysis demonstrated that nitrifiers presented mainly in floc, although higher extracellular polymeric substance content, especially DNA, appeared in the biofilm. Thus, a combination of hydrolysis and precipitation, followed by the MBBR, can complete the nutrient release from the waste sludge of a BNR system, recovers nutrients from the hydrolysed liquor and removes nutrients from leftovers effectively.

Spatial configuration of extracellular polymeric substances of Bacillus megaterium TF10 in aqueous solution

Configuration of extracellular polymeric substances (EPS) excreted by microorganisms is related greatly to the inherent properties of EPS, and has a significant effect on the physicochemical characteristics of microbial aggregates, such as activated sludge for wastewater treatment, as well as their interaction with other substances in aqueous systems. In this work, the spatial configuration of microbial EPS is characterized using laser light scattering (LLS) technique, with EPS extracted from Bacillus megaterium TF10 as an example. The combined utilization of static light scanning (SLS) and dynamic light scanning (DLS) offers an effective avenue to explore the EPS configuration in aqueous solution, thus enables a better understanding about the physicochemical properties of EPS. The results show that EPS exist in the form of colloids in neutral aqueous solution (pH 7.0) and that their shape is random coil with incompletely extending chains. The attraction interaction between EPS colloids is related with the high flocculability of B. megaterium TF10. The cryo-electron microscopy image further confirms the spherical shape of EPS colloids. The LLS approach offers a powerful and convenient tool for characterizing microbial EPS configuration and understanding their behaviors in biological wastewater treatment systems.

Impairment of the bacterial biofilm stability by triclosan

The accumulation of the widely-used antibacterial and antifungal compound triclosan (TCS) in freshwaters raises concerns about the impact of this harmful chemical on the biofilms that are the dominant life style of microorganisms in aquatic systems. However, investigations to-date rarely go beyond effects at the cellular, physiological or morphological level. The present paper focuses on bacterial biofilms addressing the possible chemical impairment of their functionality, while also examining their substratum stabilization potential as one example of an important ecosystem service. The development of a bacterial assemblage of natural composition – isolated from sediments of the Eden Estuary (Scotland, UK) – on non-cohesive glass beads (<63 µm) and exposed to a range of triclosan concentrations (control, 2 – 100 µg L⁻¹) was monitored over time by Magnetic Particle Induction (MagPI). In parallel, bacterial cell numbers, division rate, community composition (DGGE) and EPS (extracellular polymeric substances: carbohydrates and proteins) secretion were determined. While the triclosan exposure did not prevent bacterial settlement, biofilm development was increasingly inhibited by increasing TCS levels. The surface binding capacity (MagPI) of the assemblages was positively correlated to the microbial secreted EPS matrix. The EPS concentrations and composition (quantity and quality) were closely linked to bacterial growth, which was affected by enhanced TCS exposure. Furthermore, TCS induced significant changes in bacterial community composition as well as a significant decrease in bacterial diversity. The impairment of the stabilization potential of bacterial biofilm under even low, environmentally relevant TCS levels is of concern since the resistance of sediments to erosive forces has large implications for the dynamics of sediments and associated pollutant dispersal. In addition, the surface adhesive capacity of the biofilm acts as a sensitive measure of ecosystem effects.

The stabilisation potential of individual and mixed assemblages of natural bacteria and microalgae

It is recognized that microorganisms inhabiting natural sediments significantly mediate the erosive response of the bed ("ecosystem engineers") through the secretion of naturally adhesive organic material (EPS: extracellular polymeric substances). However, little is known about the individual engineering capability of the main biofilm components (heterotrophic bacteria and autotrophic microalgae) in terms of their individual contribution to the EPS pool and their relative functional contribution to substratum stabilisation. This paper investigates the engineering effects on a non-cohesive test bed as the surface was colonised by natural benthic assemblages (prokaryotic, eukaryotic and mixed cultures) of bacteria and microalgae. MagPI (Magnetic Particle Induction) and CSM (Cohesive Strength Meter) respectively determined the adhesive capacity and the cohesive strength of the culture surface. Stabilisation was significantly higher for the bacterial assemblages (up to a factor of 2) than for axenic microalgal assemblages. The EPS concentration and the EPS composition (carbohydrates and proteins) were both important in determining stabilisation. The peak of engineering effect was significantly greater in the mixed assemblage as compared to the bacterial (x 1.2) and axenic diatom (x 1.7) cultures. The possibility of synergistic effects between the bacterial and algal cultures in terms of stability was examined and rejected although the concentration of EPS did show a synergistic elevation in mixed culture. The rapid development and overall stabilisation potential of the various assemblages was impressive (x 7.5 and ×9.5, for MagPI and CSM, respectively, as compared to controls). We confirmed the important role of heterotrophic bacteria in "biostabilisation" and highlighted the interactions between autotrophic and heterotrophic biofilm consortia. This information contributes to the conceptual understanding of the microbial sediment engineering that represents an important ecosystem function and service in aquatic habitats.