Vol.:(0123456789) 1 3 Bioprocess and Biosystems Engineering https://doi.org/10.1007/s00449-017-1870-3 CRITICAL REVIEW Application of phototrophic bioflms: from fundamentals to processes D. Strieth 2  · R. Ulber 2  · K. Mufer 1 Received: 22 July 2017 / Accepted: 24 November 2017 © Springer-Verlag GmbH Germany, part of Springer Nature 2017 Abstract Biotechnological production of valuables by microorganisms is commonly achieved by cultivating the cells as suspended solids in an appropriate liquid medium. However, the main portion of these organisms features a surface-attached growth in their native habitats. The utilization of such bioflms shows signifcant challenges, e.g. concerning control of pH, nutrient supply, and heat/mass transfer. But the use of bioflms might also enable novel and innovative production processes addressing robustness and strength of the applied biocatalyst, for example if variable conditions might occur in the process or a feedstock (substrate) is changed in its composition. Besides the robustness of a bioflm, the high density of the immobilized biocatalyst facilitates a simple separation of the catalyst and the extracellular product, whereas intracellular target compounds occur in a concentrated form; thus, expenses for downstream processing can be drastically reduced. While phototrophic organisms feature a fabulous spectrum of metabolites ranging from biofuels to biologically active compounds, the low cell density of phototrophic suspension cultures is still limiting their application for production processes. The review is focusing on pro- and eukaryotic microalgae featuring the production of valuable compounds and highlights requirements for their cultivation as phototrophic bioflms, i.e. setup as well as operation of bioflm reactors, and modeling of phototrophic growth. Keywords Phototrophic bioflms · Terrestrial cyanobacteria · Microalgae · Valuable products · Commodities · Renewable resources Introduction Since van Leuwenhoek made the frst illustrations of bac- teria in the late seventeenth century, microorganisms are mainly associated with single cells or cell clusters which occur as suspended particles in a liquid medium. Although microorganisms commonly feature a growth associated with an interface, i.e. liquid–solid, liquid–gas as well as liq- uid–solid–gas, industrial production processes are almost exclusively utilizing suspended microorganisms. This is due to minimization of mass and heat transfer problems which occur in clusters of aggregated cells; moreover, control of important process parameters such as dissolved oxygen, pH, or temperature is a challenging task when clustered cells are investigated. The term “bioflm” comprises organisms growing attached to a surface as well as organisms which grow as aggregates, whereupon the cells are sticked together by extracellular polymeric substances, abbreviated as EPS. Such a matrix shelters diferent microbial species in a natural environment; however, production processes in biotechnol- ogy are rather utilizing single species biocatalysts, due to the processes reproducibility. The matrix is playing a pivotal protective role for its “residents” endowing the cells with a higher robustness towards critical physical and chemical conditions (e.g. salt condition, pH, disinfectants). Whereas such properties are detrimental in the medicinal area (e.g. on wounds and on surgical instruments), they may be ben- efcial in biotechnological production processes (cf. reviews by Rosche et al. [1] and Mufer et al. [2]), utilizing feedstock streams containing inhibiting compounds or having a pH which is not tolerated by suspended cells. A second impor- tant reason for implementing bioflms in bioprocesses is the ability to switch the production from batch to a continuous mode; thus, more efcient processes may be arranged. More- over, some properties of microbial strains are regulated via * K. Mufer k.mufer@th-bingen.de 1 Department of Life Sciences and Engineering, University of Applied Sciences Bingen, Berlinstr. 109, 55411 Bingen, Germany 2 Institute of Bioprocess Engineering, University of Kaiserslautern, Gottlieb-Daimler-Str. 49, 67663 Kaiserslautern, Germany