Microbial assemblages as ecosystem engineers of sediment stability

Abstract

Purpose Sediment erosion and transport is a governing factor in the ecological and commercial health of aquatic ecosystems from the watershed to the sea. There is now a general consensus that biogenic mediation of submersed sediments contributes significantly to the resistance of the bed to physical forcing. This important ecosystem function has mainly been linked to microalgae ("ecosystem engineers") and their associated extracellular polymeric substances (EPS), yet little is known about the impact of bacterial assemblages and how their varying interactions with microalgae affect the overall biostabilization potential of the combined community. Materials and methods Natural assemblages of bacteria and diatoms-originating from sediment and water samples from the Eden Estuary (Scotland, UK)-were growing on noncohesive glass beads over 5 weeks. The adhesion and the stability of the biofilm was determined by magnetic particle induction (MagPI) and by Cohesive Strength Meter (CSM), respectively, and related to EPS (spectrophotometric determination of carbohydrates and proteins), bacterial cell numbers (flow cytometry), bacterial community (fluorescence in situ hybridization (FISH)), diatom biomass (spectrophotometric determination of chlorophyll a), and diatom assemblage composition (microscopy). Results and discussion The adhesive properties and stability of the biofilm were significantly enhanced over time as compared to controls. The diatoms profited from additional nutrients, while bacteria dominated in nutrient-limited cultures. Subsequent shifts in the microbial population at a species level resulted in varying patterns of EPS production which moderated the biostabilization capacity: Cultures with strong diatom development were less stable than cultures dominated by bacteria (MagPI: ×8.5 and ×10.8, CSM: ×2.5 and ×5.7, respectively). The data also suggested synergistic effects between proteins and carbohydrates, which enhanced adhesion and stability. Conclusions Bacteria populations under these conditions can be regarded as "ecosystem engineers" since their role in sediment stabilization is more important than previously recognized. Abiotic factors such as nutrients altered the interactions between bacteria and microalgae to influence the overall microbial stabilization potential ("engineering web") by affecting the quantity and quality of EPS. Data from MagPI and CSM correlated well (R 2 = 0.82, P < 0.0001), and the new technique, MagPI, is to be recommended for studies on growing biofilms since it determines subtle changes in sediment/biofilm properties with high sensitivity.Recommendations and perspectives Further studies should examine the highly species-specific interactions between microalgae and bacteria and their effects on EPS secretion to impact stability as well as postentrainment of sediments under varying abiotic scenarios. Our growing understanding of the ecosystem functionality of "bioengineering" will have wider implications for water framework directive and sediment/pollutant management strategi...

The RNA Processing Enzyme Polynucleotide Phosphorylase Negatively Controls Biofi lm Formation by Repressing Poly-N-Acetylglucosamine (PNAG) Production in Escherichia coli C

“…After only one week, the stability of the biofilm exposed to the highest TCS concentration (T5: 100 mg L -1 ) decreased significantly; the same effect was observed much later (day 14 -day 17) in the other treatments (CB, T1 -T4: 2 -50 mg L -1 ). In former experiments, without a continuous nutrient supply, decreasing microbial substratum stabilization was observed after time and deemed as a typical ''batch culture effect'' caused when the initial culture nutrients have been used up [32,34]. In the present experiment, the exposure to TCS seemed to have additionally impeded the stabilization potential in nutrient depleted cultures.…”

“…The adsorption for EPS carbohydrates and proteins was read by a spectrophotometer (CECIL CE3021) at the wavelengths 488 nm and 750 nm and calibrated versus defined concentration ranges (0 -200 mg/L) of glucose and bovine albumin, respectively. For more details please see [32,34]. The EPS carbohydrates and proteins concentrations are given in microgram per cubic centimeter (mg cm -3 ).…”

Endogenous versus environmental control of vertical migration by intertidal benthic microalgae

“…Microphytobenthic biofilms play an important role in estuarine ecosystem functioning, contributing to up to 50% of primary production (MacIntyre et al, 1996;Underwood & Kromkamp, 1999). They constitute an essential food source for grazers Middleburg et al, 2000), regulate the exchange of nutrients across the sediment-water interface (MacIntyre et al, 1996;Tyler et al, 2003) and promote sediment biostabilization (Paterson, 1986;Yallop et al, 1994;de Brouwer et al, 2000;Gerbersdorf et al, 2009;Andersen et al, 2010;Stal, 2010).…”

Form, function and physics: the ecology of biogenic stabilisation

“…However, the examples of ecosystem engineers that are given are often larger charismatic species (elephants, beavers, otters, etc.) while organisms such as bacteria and protists (including the algae) equally, if not more important, attract less attention (Boogert et al 2006;Gerbersdorf et al 2009). There is a further interesting twist to the Becosystem engineering^debate.…”

“…The physical properties of natural sediments are also variable; they contain mixtures of particles sizes, varying cohesive fractions or mineral types. Furthermore, natural sediments are colonised by both macrofauna and microorganisms that change the bed properties and behaviour under erosive stress (Paterson 1994;Gerbersdorf et al 2009;Passarelli et al 2012). It is therefore not surprising that the relationship between grain size and sediment erosion loses predictive power under natural conditions, and this is the area where interdisciplinary investigations are required.…”