Hindered erosion: The biological mediation of noncohesive sediment behavior

Abstract

Extracellular polymeric substances (EPS) are ubiquitous on tidal flats but their impact on sediment erosion has not been fully understood. Laboratory-controlled sediment beds were incubated with Bacillus subtilis for 5, 10, 16, and 22 days before the erosion experiments, to study the temporal and spatial variations in sediment stability caused by the bacterial secreted EPS. We found the biosedimentary systems showed different erosional behavior related to biofilm maturity and EPS distribution. In the first stage (5 days), the biosedimentary bed was more easily eroded than the clean sediment. With increasing growth period, bound EPS became more widely distributed over the vertical profile resulting in bed stabilization. After 22 days, the bound EPS was highly concentrated within a surface biofilm, but a relatively high content also extended to a depth of 5 mm and then decayed sharply with depth. The biofilm increased the critical shear stress of the bed and furthermore, it enabled the bed to withstand threshold conditions for an increased period of time as the biofilm degraded before eroding. After the loss of biofilm protection, the high EPS content in the sublayers continued to stabilize the sediment (hindered erosion) by binding individual grains, as visualized by electron microscopy. Consequently, the bed strength did not immediately revert to the abiotic condition but progressively adjusted, reflecting the depth profile of the EPS. Our experiments highlight the need to treat the EPS-sediment conditioning as a bed-age associated and depth-dependent variable that should be included in the next generation of sediment transport models. Plain Language Summary Sedimentology and geomorphology have traditionally been seen as fields in which physical and chemical processes dominate. However, microbial communities should never be bystanders, because they suffuse all sedimentary environments on earth. Under hydrodynamic forces, they take part in an impressive range of sediment processes and thus exercising a formative influence on coastal evolutions. Bio-sediments exhibit more complex characteristics than abiotic systems, and lead to different modelling methods compared to those in traditional settings. For instance, the thresholds for sediment initiation and subsequent erosion rates are no longer solely related to particle properties (e.g., particle size, the most widely used), but mediated by glue-like extracellular polymeric substances (EPS) secreted by microbes. From this point of view, it is easy to understand why sediments in field observations behave differently from predictions, usually appearing considerably strengthened. Our results indicate that the EPS mediation in sediment stability may vary with the rhythms of microbial growth, and re-profile the sediment stability during different stages of cementing processes. A conceptual framework for sediment erosion is hence put forward to transform traditional sediment system to EPS-sediment system.

The effect of cyclic variation of shear stress on non‐cohesive sediment stabilization by microbial biofilms: the role of ‘biofilm precursors’

“…The device is an improved version of the UMCES (University of Maryland Centre for Environmental Science) Gust Erosion Microcosm System (U-GEMS) Figure 1. Surface sediment at Site S7 in winter (see site information in Chen et al, 2017b). A tidal flat surface with three different features: (a) natural sediment surface covered with biofilms forming extensive brown mats of bacteria and microphytobenthos; (b) eroded but not fully detached (flipped-over) biofilm; and (c) the exposed sediment beneath the biofilm.…”

“…A tidal flat surface with three different features: (a) natural sediment surface covered with biofilms forming extensive brown mats of bacteria and microphytobenthos; (b) eroded but not fully detached (flipped-over) biofilm; and (c) the exposed sediment beneath the biofilm. Details of the benthic chamber can be found in Chen et al (2017b). (Thomsen and Gust, 2000) and Core Mini Flume (CMF) (Thompson et al, 2013).…”

“…(Thomsen and Gust, 2000) and Core Mini Flume (CMF) (Thompson et al, 2013). Calm low shear referred to a constant BSS of 0.06 Pa. A BSS of 0.06 Pa was selected as a reasonable value during neap tide, allowing biofilm growth and was lower than the critical shear stress to avoid the re-suspension of the constituent sand grains (Shi et al, 2016;Chen et al, 2017b). Details of the benthic chamber can be found in Chen et al (2017b).…”

“…The chamber was used for both biosedimentary bed cultivation and for determining sediment erosion thresholds by adjusting the rotation speed of the paddle. After incubation, the developed bio-sedimentary bed was immediately exposed to a series of stepwise increments of BSS (IS from 0.06 to 0.33 Pa) for approximately 14 minutes at each level (see full details of the shear stress applied in Chen et al, 2017b). One chamber was used as a control in which the treated clean sediment was eroded in artificial sea water (ASW, salinity of 23 ‰); five were used for single-cycle scenarios with different cultivation periods (four for incubation days of 5, 10, 16 and 22, respectively, and one for sediment sample extraction and analyses); and another one was for the repeatedcycle scenario.…”

“…As influenced by microbial metabolism, the bio-sedimentary matrix responds to both ecological (light, nutrients, pollutants) and physical drivers (hydrodynamic conditions and meteorological forcing) and exhibits more complex and variable characteristics than abiotic systems (Montague, 1986;Celmer et al, 2008;Paterson et al, 2008;Stone et al, 2008;Fang et al, 2017;Chen et al, 2017b;Schmidt et al, 2018). For instance, field observations on a tidal flat show that natural biofilms are heterogeneously distributed and that biofilm thickness varies across the flat, leading to a highly complex surface structure ( Figure 1).…”