Particle aggregation induced by microorganisms is a key mechanism for the biostabilization of coastal sediment

Abstract

Biofilm surrounding sediment particles can significantly enhance the stability of coastal sediment. This phenomenon is known as biostabilization. Biofilm mainly consists of extracellular polymeric substances (EPS) and microorganisms. While the role of EPS in biostabilization is well-established, microbial contribution requires further exploration. Herein, an experiment was conducted to compare the stability and morphological characteristics of sediment under the effects of biofilm (with microorganisms) and pure EPS extracted from the biofilm (without microorganisms). The microorganisms used for biofilm culture are composed of diatoms and chlorella. The sediment used is a mixture of well-sorted muddy sand (D₅₀ = 77 μm). Erosion tests showed that in the early stage of biofilm culture (within 17 days), biofilm and pure EPS had a similar ability to stabilize sediment, suggesting that in this stage, biofilm stabilized sediment relying on EPS. However, after 17 days of culture, biofilm showed a stronger ability to stabilize sediment than pure EPS, implying that in this stage, biofilm stabilized sediment by both EPS and microorganisms. These results indicate that microorganisms can also contribute to biostabilization, but during the early stage of culture, microorganisms in biofilm cannot make such contributions. Optical observations found that when the sediment-stabilizing ability of biofilm and pure EPS was comparable, the morphological characteristics of sediment particles under the effects of them were similar, both appearing as scattered single particles. However, when biofilm exhibited a stronger sediment-stabilizing ability, sediment particles with biofilm connected into large-size aggregates, whereas sediment particles with pure EPS remained single and scattered. These results suggest that in the later stage of biofilm culture, microorganisms play a role in the aggregation of sediment particles and hence contribute to biostabilization. Based on these findings, a conceptual framework has been proposed for predicting sediment stability under the effects of biofilm.