Abstract
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.
Highlights
Biofilms represent the dominant microbial life in many aquatic systems and drive a number of important ‘‘ecosystem services’’ such as nutrient recycling, biodegradation and pollutant retention [1]
Microphytobenthos composition In the mixed assemblage, diatoms of the genera Achnanthes, Caloneis, Navicula and Nitzschia were the intial colonizers of the substratum at the beginning of the experiment
In the diatom assemblage (D), treated with antibiotics to inhibit bacterial colonization, the species composition was quite similar to the mixed assemblage with Achnanthes, Cylindrotheca, Cymbella, Navicula and Nitzschia species present but smaller Navicula species were dominant from the beginning
Summary
Biofilms represent the dominant microbial life in many aquatic systems and drive a number of important ‘‘ecosystem services’’ such as nutrient recycling, biodegradation and pollutant retention [1]. Pioneering work on the entrainment of a clay-water suspension by Dade et al [16] and on the stability of experimentally-derived biofilms by LeonMorales et al [17] indicated significant effects of bacterial exopolymers on the substratum. These studies inspired our recent work which has shown that natural benthic bacterial assemblages from estuarine areas significantly stabilized a test substratum, far exceeding our expectations, as based on the limited literature [18,19]. The question of the functional role and origin of EPS in microbial mats requires further interpretation and can initially be addressed by separate studies of the engineering potential of prokaryotic and eukaryotic assemblages
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