Evaluating microalgae attachment to surfaces: A first approach towards a laboratory integrated assessment

Recently, the challenge of producing algal biomass at low cost has been faced also with manufacturing systems based on immobilized microalgae. The growth of microalgae as a biofilm reduces the costs of harvesting and also the water demand, allowing at the same time a high biomass productivity. The initial adhesion to a surface is one of the key factors for the formation and maintenance of a stable microbial community, and although the physical properties of the surface have an influence on the adhesion process, the major role is played by species selection. Here we propose a simple procedure to establish on a benchscale the ability of an algal strain to form a stable biofilm on a surface. As a model organism was selected Scenedesmus vacuolatus (ACUF 053) and the progressive adhesion of the microalgae at the surface of two tissues, cotton and jute, was followed with a two-step protocol based on the combination of image analyses and fluorometric measurements. The growth and viability of the algal biofilm were followed through color measurements of inoculated carriers taken at different times using the Trainable Weka Segmentation (a plugin of Fiji) on digital photographs, whereas pulse amplitude modulation (PAM) fluorometry allowed the measurement of algal photochemical activity on two textures. The results indicate that the progressive adhesion of the microalgae reached 80% of the surface of cotton fabrics during the first four days, and that the indicators of cell photosynthetic performance, decreased during the time course of the experiment, suggesting that the reduction of the nutrients concentration in the media could be responsible of progressive decay of the photochemical activity. In conclusion, the method provides reliable data on the extent and metabolic efficiency of the algal attachment to a solid substrate.