D. Strieth

Application of Biofilm Bioreactors in White Biotechnology

The production of valuable compounds in industrial biotechnology is commonly done by cultivation of suspended cells or use of (immobilized) enzymes rather than using microorganisms in an immobilized state. Within the field of wastewater as well as odor treatment the application of immobilized cells is a proven technique. The cells are entrapped in a matrix of extracellular polymeric compounds produced by themselves. The surface-associated agglomerate of encapsulated cells is termed biofilm. In comparison to common immobilization techniques, toxic effects of compounds used for cell entrapment may be neglected. Although the economic impact of biofilm processes used for the production of valuable compounds is negligible, many prospective approaches were examined in the laboratory and on a pilot scale. This review gives an overview of biofilm reactors applied to the production of valuable compounds. Moreover, the characteristics of the utilized materials are discussed with respect to support of surface-attached microbial growth.

A new photobioreactor concept enabling the production of desiccation induced biotechnological products using terrestrial cyanobacteria

Cyanobacteria offer great potential for the production of biotechnological products for pharmaceutical applications. However, these organisms can only be cultivated efficiently using photobioreactors (PBR). Under submerged conditions though, terrestrial cyanobacteria mostly grow in a suboptimal way, which makes this cultivation-technique uneconomic and thus terrestrial cyanobacteria unattractive. Therefore, a novel emersed photobioreactor (ePBR) has been developed, which can provide the natural conditions for these organisms. Proof of concept as well as first efficiency tests are conducted using the terrestrial cyanobacteria Trichocoleus sociatus as a model organism. The initial maximum growth rate of T. sociatus (0.014±0.001h(-1)) in submerged systems could be increased by 35%. Furthermore, it is now possible to control desiccation-correlated product formation and related metabolic processes. This is shown for the production of extracellular polymeric substances (EPS). In this case the yield of 0.068±0.006g of EPS/g DW could be increased by more than seven times.

A semi-continuous process based on an ePBR for the production of EPS using Trichocoleus sociatus

Biodiversity forms the basis for a large pool of potential products and productive organisms offered by terrestrial cyanobacteria. They are stuck together by EPS (extracellular polymeric substances) that can obtain antiviral, antibacterial or anti-inflammatory substances. Most stress conditions, e.g. drought, induce the production of protective EPS or biotechnological-products for pharmaceutical application. However, the growth of a phototrophic biofilm is limited under submerged conditions. Therefore, a semi-continuous process to produce EPS by cyanobacteria was developed in an aerosol-based ePBR (emerse photobioreactor) that imitates the natural habitat of terrestrial cyanobacteria. The process consists of a growth-phase (biomass production), followed by a dry-phase (EPS-production) and a consecutive extraction. The EPS-productivities of Trichocoleus sociatus (ranging from 0.03 to 0.04gL-1d-1) were 32 times higher than described in topic-related literature. In comparison to submerge cultivations in shaking flasks, the EPS-productivities were sevenfold higher. To ensure that the extraction solvent has no influence on cell viability, a cell-vitality-test was performed. However, no statistically significant difference between the amount of living and dead cells before and after the extraction was detected. A bioactivity assay was then performed to identify antimicrobial activity within EPS extracts from emerse and submerge cultivations. The EPS revealed an antibacterial effect against gram-negative bacteria (E. coli) which was two times higher than EPS from a submerged cultivation.

Terrestrial Microalgae: Novel Concepts for Biotechnology and Applications

In the emerging field of algal biotechnology, optimization of algal production, engineering challenges, and scale-up of photobioreactors are urgent demands. Emphasis is placed on reducing cultivation limitations regarding, e.g., mass transfer, thermostability, photoinhibition, as well as expenses for energy and resource investments. Until now, almost all cultivation techniques are processed under submerged conditions with aquatic microalgae. The biotechnological implementation of terrestrial microalgae, however, exhibits several physiological and technological advantages for an efficient production in biofilm photobioreactors. Their outstanding performance and considerable advantages for biotechnology may reduce several of the current limitations and provide new principles in bioengineering. Can they outcompete the capacity of commercial algal strain due to their thermostability, light utilization, or desiccation tolerance? How terrestrial microalgae could highly contribute to technological and economic improvement of microalgal biotechnology is discussed reviewing their species diversity, physiology, valuable products, bioengineering processes, biofilm photobioreactors, and some visions of potential developments. Moreover, the overview may allow choosing interesting organisms for further studies.

Application of phototrophic biofilms: from fundamentals to processes

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 biofilms shows significant challenges, e.g. concerning control of pH, nutrient supply, and heat/mass transfer. But the use of biofilms 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 biofilm, 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 biofilms, i.e. setup as well as operation of biofilm reactors, and modeling of phototrophic growth.