New insights into developing antibiofouling surfaces for industrial photobioreactors.

The biofouling formation of the marine microalga Nannochloropsis gaditana on nontoxic surfaces was quantified on rigid materials, both coated (with fouling release coatings and nanoparticle coatings) and noncoated, to cover a wide range of surface properties from strongly hydrophobic to markedly hydrophilic under conditions similar to those prevailing in outdoor massive cultures of marine microalgae. The effect of seawater on surfaces that presented the best antibiofouling properties was also evaluated. The adhesion intensity on the different surfaces was compared with the predictions of the biocompatibility theories developed by Baier and Vogler using water adhesion tension (τ0 ) as the quantitative parameter of surface wettability. For the most hydrophobic surfaces, τ0 ≤ 0, the microalgae adhesion density increased linearly with τ0 , following the Baier's theory trend. However, for the rest of the surfaces, τ0 ≥ 0, a tendency toward minimum adhesion was observed for amphiphilic surfaces with a τ0 = 36 mJ/m2 , a value close to that which minimizes cell adhesion according to Vogler's theory. The understanding and combination of the two biocompatibility theories could help to design universal antibiofouling surfaces that minimize the van der Waals forces and prevent foulant adsorption by using a thin layer of hydration.

Biofouling in photobioreactors for marine microalgae.

The economic and/or energetic feasibility of processes based on using microalgae biomass requires an efficient cultivation system. In photobioreactors (PBRs), the adhesion of microalgae to the transparent PBR surfaces leads to biofouling and reduces the solar radiation penetrating the PBR. Light reduction within the PBR decreases biomass productivity and, therefore, the photosynthetic efficiency of the cultivation system. Additionally, PBR biofouling leads to a series of further undesirable events including changes in cell pigmentation, culture degradation, and contamination by invasive microorganisms; all of which can result in the cultivation process having to be stopped. Designing PBR surfaces with proper materials, functional groups or surface coatings, to prevent microalgal adhesion is essential for solving the biofouling problem. Such a significant advance in microalgal biotechnology would enable extended operational periods at high productivity and reduce maintenance costs. In this paper, we review the few systematic studies performed so far and applied the existing thermodynamic and colloidal theories for microbial biofouling formation in order to understand microalgal adhesion on PBR surfaces and the microalgae-microalgae cell interactions. Their relationship to the physicochemical properties of the solid PBR surface, the microalgae cell surfaces, and the ionic strength of the culture medium is discussed. The suitability and the applicability of such theories are reviewed. To this end, an example of biofouling formation on a commercial glass surface is presented for the marine microalgae Nannochloropsis gaditana. It highlights the adhesion dynamics and the inaccuracies of the process and the need for further refinement of previous theories so as to apply them to flowing systems, such as is the case for PBRs used to culture microalgae.