Date Published: April 29, 2019
Publisher: Public Library of Science
Author(s): Christoph Westerwalbesloh, Carl Brehl, Sophie Weber, Christopher Probst, Janka Widzgowski, Alexander Grünberger, Christian Pfaff, Ladislav Nedbal, Dietrich Kohlheyer, Arum Han.
Microalgae are an ubiquitous and powerful driver of geochemical cycles which have formed Earth’s biosphere since early in the evolution. Lately, microalgal research has been strongly stimulated by economic potential expected in biofuels, wastewater treatment, and high-value products. Similar to bacteria and other microorganisms, most work so far has been performed on the level of suspensions which typically contain millions of algal cells per millilitre. The thus obtained macroscopic parameters average cells, which may be in various phases of their cell cycle or even, in the case of microbial consortia, cells of different species. This averaging may obscure essential features which may be needed for the correct understanding and interpretation of investigated processes. In contrast to these conventional macroscopic cultivation and measuring tools, microfluidic single-cell cultivation systems represent an excellent alternative to study individual cells or a small number of mutually interacting cells in a well-defined environment. A novel microfluidic photobioreactor was developed and successfully tested by the photoautotrophic cultivation of Chlorella sorokiniana. The reported microbioreactor facilitates automated long-term cultivation of algae with controlled temperature and with an illumination adjustable over a wide range of photon flux densities. Chemical composition of the medium in the microbioreactor can be stabilised or modulated rapidly to study the response of individual cells. Furthermore, the algae are cultivated in one focal plane and separate chambers, enabling single-cell level investigation of over 100 microcolonies in parallel. The developed platform can be used for systematic growth studies, medium screening, species interaction studies, and the thorough investigation of light-dependent growth kinetics.
Microalgae are a diverse group of photosynthetic organisms, including eukaryotic algae and prokaryotic cyanobacteria, which has formed our oxygen-containing atmosphere and remains responsible for a major fraction of carbon cycling which stabilises the Earth’s climate . In addition to this immense ecological relevance, microalgae are also gaining importance as potential source of biofuels [2–4] and high value products, [5–7] or as tool to clean wastewater .
The presented microfluidic cultivation system for photoautotrophic cultivation of microalgae, with single-cell resolution under precisely controlled conditions regarding illumination, medium supply and temperature, was tested with the green algae C. sorokiniana under different light conditions. We were able to show how the system allows to investigate the influence of light on growth, division behaviour and cell death. The system facilitates future studies imitating light dark cycles as they can be found in nature or more complex dynamic light conditions found in macroscopic photo bio reactors, helping to verify computational models  and simplify the scale-up process . As the system is coupled to an inverted microscope, other studies could make use of fluorescent bio markers to investigate population heterogeneity. The excellent medium control could help screen for optimal nutrient conditions. It also showed promise for the analysis of mixed cultures, in this case specifically parasite-algae interactions. The presented systems allows to gain a deeper understanding of algae bioprocesses at lab- and large-scale, via cultivation and observation with well-defined illumination and medium composition.