Cell Surface Energy Affects the Structure of Microalgal Biofilm

Qualitative Assessment of Microalgae-Bacteria Biofilm Development on K5 Carriers: Photoheterotrophic Growth in Wastewater

Wastewater (WW) treatment using biofilms harboring bacteria and microalgae is considered a promising polishing solution to improve current treatment technologies present in wastewater treatment plants (WWTPs), but their interaction in a sessile community remains to be understood. In this work, multi-species biofilms of Chlorella vulgaris, Chlorella sorokiniana, or Scenedesmus obliquus were selected as representative microalgae species of interest for WW bioremediation, and Rhodococcus fascians, Acinetobacter calcoaceticus, or Leucobacter sp. were selected as the bacteria for co-cultivation in a synthetic WW since they are normally found in WW treatment processes. The attached consortia were developed in specific carriers (K5 carriers) for 168 h, and their biofilm formation ability was evaluated in a profilometer and via scanning electron microscopy (SEM) imaging. From the selected microorganisms, C. sorokiniana was the microalga that adapted best to co-cultivation with R. fascians and A. calcoaceticus, developing a thicker biofilm in these two consortia (3.44 ± 0.5 and 4.51 ± 0.8 µm, respectively) in comparison to the respective axenic cultures (2.55 ± 0.7 µm). In contrast, Leucobacter sp. did not promote biofilm growth in association with C. vulgaris and C. sorokiniana, while S. obliquus was not disturbed by the presence of this bacterium. Some bacterial clusters were observed through SEM, especially in A. calcoaceticus cultures in the presence of microalgae. In some combinations (especially when C. vulgaris was co-cultivated with bacteria), the presence of bacteria was able to increase the number of microalga cells adhered to the K5 carrier. This study shows that biofilm development was distinctly dependent on the co-cultivated species, where synergy in biofilm formation was highly dependent on the microalgae and bacteria species. Moreover, profilometry appears to be a promising method for biofilm analyses.

Elucidating bacterial coaggregation through a physicochemical and imaging surface characterization

Bacterial coaggregation is a highly specific type of cell-cell interaction, well-documented among oral bacteria, and involves specific characteristics of the cell surface of the coaggregating strains. However, the understanding of the mechanisms promoting coaggregation in aquatic systems remains limited. This gap is critical to address, given the broad implications of coaggregation for multispecies biofilm formation, water quality, the performance of engineered systems, and diverse biotechnological applications. Therefore, this study aims to comprehensively characterize the cell surface of the coaggregating strain Delftia acidovorans 005P, isolated from drinking water, alongside a non-coaggregating strain, D. acidovorans 009P. By analyzing two strains of the same species, we aim to identify the factors contributing to the coaggregation ability of strain 005P. To achieve this, we employed a combination of physicochemical characterization, Fourier-transform infrared spectroscopy (FTIR), and advancing imaging techniques [transmission electron microscopy and cryo-electron tomography (cryo-ET)]. The coaggregating strain (005P) exhibited higher surface hydrophobicity, negative surface charge, and cell surface and co-adhesion energies than the non-coaggregating strain (009P). The chemical characterization of bacterial surfaces through FTIR revealed subtle differences, particularly in spectral regions linked to carbohydrates and phosphodiesters/amide III of proteins (860-930 cm-1 and 1212-1240 cm-1, respectively). Cryo-ET highlighted significant differences in pili structures between the strains, such as variations in length, frequency, and arrangement. The pili in the 005P strain, identified as pili-like adhesins, serve as key mediators of coaggregation. By integrating physicochemical analyses and high-resolution imaging techniques, this study conclusively links the coaggregation ability of D. acidovorans 005P to its unique pili characteristics, emphasizing their crucial role in microbial coaggregation in aquatic environments.

Improving biomass yields of microalgae biofilm by coculturing two microalgae species via forming biofilms with uniform microstructures and small cell-clusters

Microalgae coculture has the potential to promote microalgae biofilm growth. Herein, three two-species cocultured biofilms were studied by determining biomass yields and detailed microstructure parameters, including porosity, average pore length, average cluster length, etc. It was found that biomass yields could reduce by 21-53 % when biofilm porosities decreased from about 35 % to 20 %; while at similar porosities (∼20 %), biomass yields of cocultured biofilms increased by 37 % when they possessed uniform microstructure and small cell-clusters (pores and clusters of 1 ∼ 10 μm accounted for 96 % and 68 %, respectively). By analyzing morphologies and surface properties of cells, it was found that cells with small size, spherical shape, and reduced surface polymers could hinder the cell-clusters formation, thereby promoting biomass yields. The study provides new insights into choosing cocultured microalgae species for improving the biomass yield of biofilm via manipulating biofilm microstructures.

The effect of light intensity on microalgae biofilm structures and physiology under continuous illumination

The interest by biofilm-based microalgae technologies has increased lately due to productivity improvement, energy consumption reduction and easy harvesting. However, the effect of light, one key factor for system's operation, received less attention than for planktonic cultures. This work assessed the impact of Photon Flux Density (PFD) on Chlorella vulgaris biofilm dynamics (structure, physiology, activity). Microalgae biofilms were cultivated in a flow-cell system with PFD from 100 to 500 [Formula: see text]. In the first stage of biofilm development, uniform cell distribution was observed on the substratum exposed to 100 [Formula: see text] while cell clusters were formed under 500 [Formula: see text]. Though similar specific growth rate in exponential phase (ca. 0.3 [Formula: see text]) was obtained under all light intensities, biofilm cells at 500 [Formula: see text] seem to be ultimately photoinhibited (lower final cell density). Data confirm that Chlorella vulgaris showed a remarkable capability to cope with high light. This was marked for sessile cells at 300 [Formula: see text], which reduce very rapidly (in 2 days) their chlorophyll-a content, most probably to reduce photodamage, while maintaining a high final cell density. Besides cellular physiological adjustments, our data demonstrate that cellular spatial organization is light-dependent.

The extracellular polysaccharide determine the physico-chemical surface properties of Microcystis

Microcystis possesses the capacity to form colonies and blooms in lakes and reservoirs worldwide, causing significant ecological challenges in aquatic ecosystems. However, little is known about the determining factors of physico-chemical surface properties that govern the competitive advantage of Microcystis. Here, The physico-chemical surface properties of Microcystis wesenbergii and Microcystis aeruginosa, including specific surface area (SSA), hydrophobicity, zeta potential, and functional groups were investigated. Additionally, the extracellular polysaccharide (EPS) were analyzed. Laboratory-cultured Microcystis exhibited hydrophilic, a negative zeta potential and negatively charged. Furthermore, no significant relationship was shown between these properties and the cultivation stage. Microcystis wesenbergii exhibited low free energy of cohesion, high surface free energy, high growth rate, and high EPS content during the logarithmic phase. On the other hand, M. aeruginosa displayed lower free energy of cohesion, high surface free energy, high EPS content, and high growth rate during the stationary phase. These characteristics contribute to their respective competitive advantage. Furthermore, the relationship between EPS and surface properties was investigated. The polysaccharide component of EPS primarily influenced the SSA and total surface energy of Microcystis. Likewise, the protein component of EPS influenced hydrophobicity and surface tension. The polysaccharide composition, including glucuronic acid, xylose, and fructose, mainly influenced surface properties. Additionally, hydrophilic groups such as O-H and P-O-P played a crucial role in determining hydrophobicity in Microcystis. This study elucidates that EPS influenced the SSA, hydrophobicity, and surface free energy of Microcystis cells, which in turn impact the formation of Microcystis blooms and the collection.

Mechanistic behaviour of Chlorella vulgaris biofilm formation onto waste organic solid support used to treat palm kernel expeller in the recent Anthropocene

The capacity to maximize the proliferation of microalgal cells by means of topologically textured organic solid surfaces under various pH gave rise to the fundamental biophysical analysis of cell-surface attachment in this study. The substrate used in analysis was palm kernel expeller (PKE) in which the microalgal cells had adhered onto its surface. The findings elucidated the relevance of surface properties in terms of surface wettability and surface energy in relation to the attached microalgal growth with pH as the limiting factor. The increase in hydrophobicity of PKE-microalgae attachment was able to facilitate the formation of biofilm better. The pH 5 and pH 11 were found to be the conditions with highest and lowest microalgal growths, respectively, which were in tandem with the highest contact angle value at pH 5 and conversely for pH 11. The work of attachment (Wcs) had supported the derived model with positive values being attained for all the pH conditions, corroborating the thermodynamic feasibility. Finally, this study had unveiled the mechanism of microalgal attachment onto the surface of PKE using the aid of extracellular polymeric surfaces (EPS) from microalgae. Also, the hydrophobic nature of PKE enabled excellent attachment alongside with nutrients for microalgae to grow and from layer-by-layer (LbL) assembly. This assembly was then isolated using organosolv method by means of biphasic solvents, namely, methanol and chloroform, to induce detachment.

Microalgae as Soft Permeable Particles

The colloidal stability of non-motile algal cells in water drives their distribution in space. An accurate description of the interfacial properties of microalgae is therefore critical to understand how microalgae concentrations can change in their biotope or during harvesting processes. Here, we probe the surface charges of three unicellular algae─Chlorella vulgaris, Nannochloropsis oculata, and Tetraselmis suecica─through their electrophoretic mobility. Ohshima's soft particle theory describes the electrokinetic properties of particles covered by a permeable polyelectrolyte layer, a usual case for biological particles. The results appear to fit the predictions of Ohshima's theory, proving that all three microalgae behave electrokinetically as soft particles. This allowed us to estimate two characteristic parameters of the polyelectrolyte external layer of microalgae: the volume charge density and the hydrodynamic penetration length. Results were compared with transmission electron microscopy observations of the algal cells' surfaces, and in particular of their extracellular polymeric layer, which was identified with the permeable shell evidenced by electrophoretic measurements. Noticeably, the algal surface potentials estimated from electrophoretic mobility using the soft particle theory are less negative than the apparent zeta potentials. This finding indicates that electrostatics are expected to play a minor role in phenomena of environmental and industrial importance, such as microalgae aggregation or adhesion.

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

Microalgal biofilm, a stable community of many algal cells attached to a solid substrate, plays a significant role in the efficient accumulation of renewable energy feedstocks, wastewater treatment, and carbon reduction. The adhesion tendency of microalgal cells on solid substrates is the basis for controlling the formation and development of microalgal biofilm. To promote the adhesion of microalgal cells on solid substrates, it is necessary to clarify which surface properties have to be changed in the most critical factors affecting the adhesion. However, there have been few systematic discussions on what surface properties influence the adhesion tendency of algal cells on solid substrates. In this study, the essential principle of microalgal cell adhesion onto solid substrates was explored from the perspective of the interaction energy between microalgal cells and solid substrates. The influence of surface properties between microalgal cells and solid substrates on interaction energies was discussed via extended Derjaguin-Landau-Verwey-Overbeek (eDLVO) theory and a sensitivity analysis. The results showed that surface properties, including surface potential (ξ) and surface free energy components, significantly affect the adhesion tendency of microalgal cells on different solid substrates. When the solid surface possesses positive charges (ξ > 0), reducing ξ or the electron donor components of the solid substrate (γ_s^-) is an effective measure to promote microalgal cell adhesion onto the solid substrate. When the solid surface possesses negative charges (ξ < 0), an increase in either γ_s^- or the absolute value of ξ should be avoided in the process of microalgae adhesion. Overall, this research provides a direction for the selection of solid substrates and a direction for surface modification to facilitate the adhesion tendency of microalgal cells on solid substrates under different scenarios.

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

Wetting of dehydrated Pseudomonas fluorescens biofilms grown on glass substrates by an external liquid is employed as a means to investigate the complex morphology of these biofilms along with their capability to interact with external fluids. The porous structure left behind after dehydration induces interesting droplet spreading on the external surface and imbibition into pores upon wetting. Static contact angles and volume loss by imbibition measured right upon droplet deposition indicate that biofilms of higher incubation times show a higher porosity and effective hydrophilicity. Furthermore, during subsequent rotation tests, using Kerberos device, these properties dictate a peculiar forced wetting/spreading behavior. As rotation speed increases a long liquid tail forms progressively at the rear part of the droplet, which stays pinned at all times, while only the front part of the droplet depins and spreads. Interestingly, the experimentally determined retention force for the onset of droplet sliding on biofilm external surface is lower than that on pure glass. An effort is made to describe such complex forced wetting phenomena by presenting apparent contact angles, droplet length, droplet shape contours, and edges position as obtained from detailed image analysis.

Analyzing microalgal biofilm structures formed under different light conditions by evaluating cell-cell interactions

Biofilm structure plays an important role in microalgae biofilm-based culture. This work aims to understand microalgal biofilm structures formed under different light conditions. Here, Scenedesmus obliquus was biofilm cultured under the light spectra of white, blue, green, and red, and the photoperiods of 5:5 s, 30:30 min, and 12:12 h (light : dark period). Biofilms were observed with confocal laser scanning microscopes and profilometry, then the porosity and roughness of biofilm were determined. We found that cells under white light formed a heterogeneous biofilm with many voids, high porosity, and roughness. While under red and blue lights, cells formed homogeneous biofilms with low porosity. Biofilm structures formed under different photoperiods were different. The mechanism of forming different biofilm structures under different light conditions was interpreted from the aspect of cell-cell interactions. Moreover, the results revealed that biomass accumulation increased with the increasing biofilm porosity due to the high effective diffusion coefficient.