Following up on PROMICON's #PromiconAsksPromiconAnswers campaign, we are introducing you to the Helmholtz Centre for Environmental Research (UFZ) and the “Reactor Concepts” team. Their primary goal is the development of novel bioreactors in order to cultivate microbial consortia by targeting two different processes - the production of biopolymers (PHACOS) and the production of hydrogen. This goal aims to broaden the application space of biobased processes to novel catalyst formats, like a cross-feeding coculture of two different organisms and biofilms. Currently, the most commonly used reactor system is the stirred tank reactor (STR), which is mainly operated with axenic (single species) suspended cell cultures.
The team’s objectives:
Here are some of the innovative bioreactors in development explained:
A two-chamber bioreactor, as the name suggests, consists of two cultivation chambers, each containing one species of the dual culture. Both chambers are connected via a semipermeable membrane, which allows certain molecules to freely pass between the chambers, while others are retained. Thereby, the growth conditions can be optimized for each of the two partners, without compromising the other, while metabolites can be exchanged.
The photobioreactor is designed for the cultivation of phototrophic organisms, in such a way that it supplies light to the phototrophic organisms growing within. Such organisms perform photosynthesis and use (sun)light as an energy source, making them dependable on light availability. The photobioreactors are thus made out of polymers and foils to ensure their transparency and light-transmissibility. To facilitate and accelerate their development, computer models are used to evaluate how certain process parameters will influence the growth and biocatalytic performance of the microbes without the necessity of work-intensive long-lasting experiments.
The process of validating a lab-scale photobioreactor in real conditions depends on the type of reaction. If the aim is to operate a photobioreactor in natural light, then it would at one point be transferred to the outside to be operated with natural light and climate dynamics instead of a highly controlled lab environment. Then the viability of the biocatalyst, its stability, and, most importantly, its catalytic performance will be closely monitored.
Capillary (photo-) biofilm reactor (CBR)
CBRs are tubes, characterized by a small (0.5-3 mm) inner diameter, leading to an exceptionally high surface area-to-volume ratio. This is important when working with surface-attached microbes like biofilms, as the surface area directly correlates with the overall performance of the system. The organisms will grow on the inside of this tube directly on the tube wall, while being constantly flushed with medium, which is pumped through the tube, normally at rather low flow rates in the range of 50 - 150 µL / min. Thus the system experiences a laminar flow condition. In PROMICON the focus is on hydrogen production, however, this system can also be combined with other reactions, creating other products as well.
CBRs are usually monitored by looking at the biomass forming within the reactor and also determining the fraction of biomass leaving the reactor system. This way the gas formation depletion, and substrate consumption can also be continuously observed. For a more comprehensive analysis, the experiment is transferred into a so-called flow cell, which fits under a confocal scanning laser microscope (CLSM) and allows in vivo to be scanned through the 3D structure of the biofilm, telling us more about its composition, architecture, and viability. Certain chemical gradients can also be visualized via CLSM, while chemical composition can be assessed by Raman spectroscopy. Upscaling remains an unsolved question, wherein numbering up is an approach from the chemical industry for capillary reactors, but it has so far not been looked into in more detail.
One of the main challenges for operating the CBR outdoors is the weather dynamics, causing completely different reaction conditions. Temperature, day-night cycles and changing light intensities depending on the weather and time of year all influence the performance of the organisms in the reactor. However, the respective infrastructure for testing has already been established and the first experiments were conducted, showcasing the light intensity has to be reduced, as the organisms encounter light inhibition rather than light limitation when growing outside.
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