Chlorella vulgaris (SAG 211-12) biofilm formation capacity and proposal of a rotating flat plate photobioreactor for more sustainable biomass production Miguel Melo 1 & Sílvia Fernandes 2,3 & Nídia Caetano 4,5 & Maria Teresa Borges 6,7 Abstract Difficulties and cost of suspended microalgal bio- mass harvest and processing can be overcome by cultivating microalgae as biofilms. In the present work, a new photoau- totrophic biofilm photobioreactor, the rotating flat plate photobioreactor (RFPPB), was developed aiming at a cost- effective production of Chlorella vulgaris (SAG 211-12), a strain not frequently referred in the literature but promising for biofuel production. Protocols were developed for evaluat- ing initial adhesion to different materials and testing the con- ditions for biofilm formation. Polyvinyl chloride substrate promoted higher adhesion and biofilm production, followed by polypropylene, polyethylene, and stainless steel. The new RFPPB was tested, aiming at optimizing incident light utili- zation, minimizing footprint area and simplifying biomass harvesting. Tests show that the photobioreactor is robust, pro- motes biofilm development, and has simple operation, small footprint, and easy biomass harvest. Biomass production (dry weight) under non-optimized conditions was 3.35 g m -2 , and areal productivity was 2.99 g m -2 day -1 . Lipid content was 10.3% (dw), with high PUFA content. These results are prom- ising and can be improved by optimizing some operational parameters, together with evaluation of long-term photobioreactor maximum productivity. Keywords Biofilm . Chlorella vulgaris SAG 211-12 . Footprint . Harvest . Rotating flat plate photobioreactor Introduction Microalgae have a multiplicity of uses, from biochemical to biofuel production, and high-density culture is of primary in- terest to the algal industry (Mata et al. 2010). Microalgae are predominantly cultivated under photoautotrophic conditions either in open algal ponds or closed photobioreactors (PBRs) (Borowitzka 2013). Although having advantages, PBRs pres- ent many problems, including high production costs, low light utilization, and difficulty in scale-up (Wang et al. 2014). Thus, research aiming at the maximization of photobioreactor per- formance is of utmost importance. New designs, combining improved production with more performing operation, can boost microalgal exploitation. Among the factors of concern, light collection, land occupation, and harvesting process are crucial for optimization (Mata et al. 2010; Chini Zittelli et al. 2013). In fact, one of the most expensive steps in large-scale microalgal production relates to harvesting and dewatering, as * Maria Teresa Borges mtborges@fc.up.pt 1 Faculty of Sciences, Department of Geoscience, Environment and Territory Management, University of Porto, Rua do Campo Alegre, S/N, 4169-007 Porto, Portugal 2 Institute for Research and Innovation in Health, University of Porto, Rua Alfredo Allen, 4200-135 Porto, Portugal 3 School of Health – Polytechnic of Porto (ESS-P. Porto), Rua Dr. António Bernardino de Almeida, 400, 4200-072 Porto, Portugal 4 LEPABE, Faculty of Engineering of University of Porto (FEUP), Rua Dr. Roberto Frias, 4200-465 Porto, Portugal 5 School of Engineering (ISEP), Polytechnic of Porto (P. Porto), Rua Dr. António Bernardino de Almeida 431, 4249-015 Porto, Portugal 6 Faculty of Sciences, Department of Biology, University of Porto, Rua do Campo Alegre, S/N, 4169-007 Porto, Portugal 7 CIIMAR – Interdisciplinary Centre of Marine and Environmental Research of the University of Porto, Novo Edifício do Terminal de Cruzeiros do Porto de Leixões, Avenida General Norton de Matos, S/N, 4450-208 Matosinhos, Portugal