1 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Biotechnol. J. 2016, 11 DOI 10.1002/biot.201500357 www.biotechnology-journal.com Biotechnology Journal 1 Introduction To realize long-distance manned space missions or build extraterrestrial habitats on other planets, reliable, closed and autonomous life support systems are needed. Micro- algae as fast growing photosynthetic organisms have already been in the focus of research as possible basic modules of such systems [1, 2]. Energy-effciency is one of the crucial characteristics of such a system also concern- ing lighting of photosynthetic modules housing microal- gae or even higher plants. Energy effciency has to be realized concerning both biology and technology. For higher plants it is well known that red and blue photons are effcient for light to biomass conversion [3]. However for microalgae cultivation exact values for photo conver- sion effciency (PCE) and knowledge on optimal wave- lengths are still lacking. For the moss Physcomitrella patens [4] studies with different LED light colors have been conducted. Blue and red lasers were used for grow- ing the alga Chlamydomonas reinhardtii achieving best results with dichromatic illumination compared to white control [5]. However, Lee et al. [6] only used red photons at 680 nm in a photoautotrophic cultivation of Chlorella vulgaris to achieve high biomass concentrations. Only recently, studies focusing on photobioreactor productivi- ty as function of light quality in continuous microalgae cultures have been published [7, 8]. Even the addition of Research Article Mono- and dichromatic LED illumination leads to enhanced growth and energy conversion for high-effciency cultivation of microalgae for application in space Ines Wagner, Christian Steinweg and Clemens Posten Dept. of Bioprocess Engineering, KIT, Karlsruhe Institute of Technology, Karlsruhe, Germany Illumination with red and blue photons is known to be effcient for cultivation of higher plants. For microalgae cultivation, illumination with specifc wavelengths rather than full spectrum illumina- tion can be an alternative while there is a lack of knowledge about achievable biomass yields. This study deals with the usage of color LED illumination to cultivate microalgae integrated into closed life support systems for outer space. The goal is to quantify biomass yields using color illumination (red, blue, green and mixtures) compared to white light. Chlamydomonas reinhardtii was cultivated in plate reactors with color compared to white illumination regarding photo conversion effciency (PCE), specifc pigment concentration and cell size. Highest PCE values were achieved under low photon fux densities (PFDs) with a red/blue illumination (680 nm/447 nm) at a 90 to 10% molar ratio. At higher PFDs saturation effects can be observed resulting from light absorption character- istics and the linear part of the photosynthesis-irradiance curve. Cell size and aggregation are also infuenced by the applied light color. Red/blue color illumination is a promising option applicable for microalgae-based modules of life support systems under low to saturating light intensities and double-sided illumination. Results of higher PCE with addition of blue photons to red light indicate an infuence of sensory pigments. Keywords: Green gap · Illumination · Photobioreactor · Photo conversion effciency · Sensory pigments Correspondence: Ines Wagner, Institute of Engineering in Life Sciences, Department, Bioprocess Engineering, Karlsruhe Institute of Technology KIT, Fritz-Haber-Weg 2, Building 30.44, 76131 Karlsruhe, Germany E-mail: ines.wagner@kit.edu Abbreviations: Car, carotenoids; CDW, cell dry weight; Chl a, chlorophyll a; Chl b, chlorophyll b; LED, light emitting diode; μ, specifc growth rate; OD, optical density; PAR, photosynthetically active radiation; PBR, photobiore- actor; PCE, photo conversion effciency; PFD, photon fux density; PI curve, photosynthesis-irradiance curve Received 25 SEP 2015 Revised 08 FEB 2016 Accepted 26 APR 2016 Accepted article online 11 MAY 2016 Supporting information available online