A methodological study of adhesion dynamics in a batch culture of the marine microalga Nannochloropsis gaditana

This paper addresses a complete set of procedures to study and to better understand the emerging problem of the biofouling formation in photobioreactors (PBRs). Methodologies are described in detail for: (i) PBR and microalgae mat surface preparation, (ii) contact angles (CA) and zeta potential (ZP) measurements for both microalgal cells and PBR surfaces, and (iii) microscopic methods for studying the evolution of adhesion intensity. The impact that these methodologies may have on the photosynthetic apparatus of the cells, the biomass concentration and cell viability are also quantified. A lab-scale flat-plate PBR was used to perform a long-term batch culture of the marine microalgae Nannochloropsis gaditana, in which a devised rack of 25 PBR glass surfaces were submerged. To study the cell-to-cell and cell-to-PBR surface interactions, the existing surface thermodynamics and colloidal theories (XDLVO) were used. The major outcomes were: (1) that N. gaditana has a exposure time threshold to the electric field produced by the ZP meter; (2) a linear equation is provided for predicting the PBR surface potential as a function of the culture medium's ionic strength; (3) the biofouling growth curve on the PBR surface varied in line with the growth kinetic followed by the freely suspended culture cells; (4) the devised PBR slide rack system offers a versatile experimental platform for generating biofouling results, making it suitable for the in situ efficiency evaluation of antibiofouling coatings; (5) there was a significant variation in the surface free energy of the PBR surfaces and algal mats with respect to that present at the beginning of the culture, and, consequently, the application of thermodynamic theories failed to predict cell adhesion over long-term cultivation. However, the XDLVO model satisfactorily explained the dynamics of the adhesion studied. The reported results might be useful for research in the microalgal production and PBR engineering area.