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.