Cell Surface Energy Affects the Structure of Microalgal Biofilm

Abstract

Microalgae biofilm-based culture systems have wide applications in environmental engineering and biotechnology. Biofilm structure is critical for the transport of nutrients, gas, and signaling molecules in a microalgal biofilm. This work aims to understand the influence of cell surface energy (SE) on the microalgal biofilm structure. Three microalgae species were used as model cells in the study: Chlorella sp., Nannochloris oculata, and Chlorella pyrenoidosa. First, by mediating biofilm culture conditions, we obtained Chlorella sp. cells with SEs of 40.4 ± 1.5, 44.7 ± 1.0, and 62. 7 ± 1.2 mJ/m2, N. oculata cells with SEs of 47.7 ± 0.5, 41.1 ± 1.0, and 62.6 ± 1.2 mJ/m2, and C. pyrenoidosa cells with SEs of 64.0 ± 0.6, 62.1 ± 0.7, and 62.8 ± 0.6 mJ/m2. Then, based on the characterizations of biofilm structures, we found that cell SE can significantly affect the microalgae biofilm structure. When the cell SEs ranged from 40 to 50 mJ/m2, the microalgae cells formed heterogeneous biofilms with a large number of open voids, and the biofilm porosity was higher than 20%. Alternatively, when the cell SEs ranged from 50 to 65 mJ/m2, the cells formed a flat, homogeneous biofilm with the porosity lower than 20%. Finally, the influencing mechanism of cell SE on biofilm structure was interpreted based on the thermodynamic theory via analyzing the co-adhesion energy between cells. The study has important implications in understanding factors that influence the biofilm structures.

Wetting of Dehydrated Hydrophilic Pseudomonas fluorescens Biofilms under the Action of External Body Forces

“…Biofilm formation is considered as a multistage process starting with microbial surface attachment and adhesion, followed by the production and accumulation of EPS that leads to the growth of sessile populations into mature biofilms and, eventually, concluded by cell detachment and dispersion . It is known that chemical (e.g., nutrient availability, temperature, pH fluctuations) and mechanical stimuli (e.g., shear stress, − electric or magnetic fields or gravity effect, i.e., surface curvature) play a key role in biofilm development at different stages of growth, by regulating microbial transport, attachment on substrata and their subsequent adhesion, nutrients and oxygen delivery, and metabolites disposal through biofilm matrix and cell detachment . In addition, environmental conditions are also responsible for altering bacteria phenotype and genotype, , bacterial quorum sensing, and morphology of biofilms. , …”

How Interfacial Properties Affect Adhesion: An Analysis from the Interactions between Microalgal Cells and Solid Substrates

“…Therefore, for microalgal biofilm cultures, reducing γ m – or γ s – is the preferred operation to promote microalgal cell adhesion. Concretely, a higher content of hydrophobic functional groups on the microalgal surface usually results in a lower γ m – . ,, Meanwhile, surface modifications of solid substrates, including surface oxidation, hydrophobic treatment, and reduction of hydrophilic functional groups, are effective methods to reduce γ s – . , When algal cells adhere to the solid substrates with a positive surface potential (ξ s > 0), an improvement in ξ s and a reduction in γ s – are both effective means of improving the microalgal adhesion onto solid substrates. The solid substrate grafted by some polymers, such as polyethylenimine, diethylamine, etc., could present a positive surface potential.…”

“…In this case, the method of increasing the absolute value of ξ m is to improve the pH of the solution or to decrease the ionic concentration of the solution . Second, increasing γ s – could contribute to reducing the microalgal cell adhesion by reducing the content of hydrophobic functional groups on the solid surface. , Third, for different microalgal cell adhesion onto the same solid substrate, microalgal cells with higher γ m – , that is, higher contents of hydrophilic functional groups (C–OH, C–(O, N), P–O–C, and C–O–C) in the microalgal cell surface, would experience bad adhesion on the solid surface. ,, …”

“…Concretely, a higher content of hydrophobic functional groups on the microalgal surface usually results in a lower γ m − . 34,40,41 Meanwhile, surface modifications of solid substrates, including surface oxidation, hydrophobic treatment, and reduction of hydrophilic functional groups, are effective methods to reduce γ s − . 42,43 When algal cells adhere to the solid substrates with a positive surface potential (ξ s > 0), an improvement in ξ s and a reduction in γ s − are both effective means of improving the microalgal adhesion onto solid substrates.…”

“…42,43 Third, for different microalgal cell adhesion onto the same solid substrate, microalgal cells with higher γ m − , that is, higher contents of hydrophilic functional groups (C−OH, C−(O, N), P−O−C, and C−O− C) in the microalgal cell surface, would experience bad adhesion on the solid surface. 34,40,41 Essentially, the predictive approach is based on the analytical results of eDLVO theory, in which the surfaces of algal cells and solid substrates are typically assumed to be smooth and homogeneous. Spherical particles are usually considered in traditional eDLVO models, while the particle shape must be considered in the interaction energy calculations to correctly predict adhesion behavior for nonspherical particles.…”