Abstract
Biofouling represents an important limitation in photobioreactor cultures. The biofouling propensity of different materials (polystyrene, borosilicate glass, polymethyl methacrylate and polyethylene terephthalate glycol-modified) and coatings (two spray-applied and nanoparticle-based superhydrophobic coatings and a hydrogel-based fouling release coating) was evaluated by means of a short-term protein test, using bovine serum albumin (BSA) as a model protein, and by the long-term culture of the marine microalga Nannochloropsis gaditana under practical conditions. The results from both methods were similar, confirming that the BSA test predicts microalgal biofouling on surfaces exposed to microalgae cultures; these secrete macromolecules, such as proteins, that have a high capacity for forming a conditioning film prior to cell adhesion. The hydrogel-based coating showed significantly reduced BSA and N. gaditana adhesion, whereas the other surfaces failed to control biofouling. Microalgal biofouling was associated with an increased concentration of sticky extracellular proteins at low N/P ratios (below 15).
Keywords: microalgae, cell adhesion, extracellular proteins, N/P ratio, extracellular organic substances
Introduction
Biofouling represents a major limitation for the long-term culture of microalgae in photobioreactors (Zeriouh et al., 2019a; Zeriouh et al., 2017b). The ubiquity of cells and extracellular organic substances (EOS) secreted by microalgae in industrial photobioreactors (PBRs) creates a competition in both to colonize the PBR walls in contact with the culture. Since the adhesion process timescale for EOS, such as proteins, is shorter (seconds to hours) than that for cell adhesion (hours to days), the initial conditioning film covering the PBR walls mainly consists of EOS. Nonetheless, this conditioning film accelerates cell adhesion on surfaces. The biofouling that forms is extremely difficult to prevent or eliminate. The use of in-situanti-biofouling approaches, such as increasing the turbulence in the culture, generally leads to cells that are more resistant to detachment being selected. This results in the development of very sticky biofilms.
A wide variety of anti-biofouling materials and coatings considered for use in PBRs has been studied (Talluri et al., 2020; Wang et al., 2017; Wang et al., 2020; Zeriouh et al., 2019a). All of them have failed to completely prevent long-term biofouling. They were selected based on how their surface properties might interact with the cells’ surface properties, not with those of the EOS. Even though proteins and proteinaceous materials are thought to be the most abundant class of EOS components in marine microalgae cultures, protein adsorption has hardly been considered as the origin of cell adhesion. Therefore, anti-biofouling studies looking at microalgae-based bioprocesses should evaluate at least two aspects: (i) the propensity of surfaces to protein adherence; and (ii) the effect of the culture conditions on EOS production. The quantity, composition, structure, and properties of the EOS produced are influenced by a wide variety of abiotic factors (Xiao and Zheng, 2016). However, the nitrogen to phosphorous ratio (N/P) in the microalgae culture medium exerts a remarkable modulating effect on EOS production (Xiao and Zheng, 2016).
Small-scale culture devices in orbital shakers are commonly used to study microorganism biofilms and to assess the effect exerted by shear stress on cell proliferation and attachment. Nonetheless, they present a complex three-dimensional flow pattern, varying in both time and space. Understanding flow topology has been related to the shear stress distribution in these culture devices, which has helped explain the cell biofilm distributions observed (Salek et al., 2011). CFD simulations have also been used to resolve the flow structures in these devices (Berson et al., 2008).
This work aims to evaluate the biofouling susceptibility of different materials and coatings immersed in cultures of the marine microalgaNannochloropsis gaditana , a species used as model in studies on biofouling formation in photobioreactors (Zeriouh et al., 2019a; Zeriouh et al., 2019b; Zeriouh et al., 2017a). As a first step, the surfaces were tested in short-term batch adhesion experiments with the model protein BSA (Wang et al., 2017). In the second step, microalgal adhesion experiments on long-term cultures were carried out in an orbitally agitated transparent laboratory vessel acting as a simulator of real flow conditions. The long-term cultures were performed at different initial N/P ratios in the culture medium to alter the cells’ EOS production. The results allowed us to assess the suitability of the short-term BSA adhesion test as a predictor of biofouling and the effectiveness of selected surfaces for biofouling control.