Characterization of a Chlamydomonas reinhardtii mutant strain with improved biomass production under low light and mixotrophic conditions ☆ Y. Zhou a , L.C. Schideman a, ⁎, D.S. Park b , A. Stirbet c , Govindjee d,e,f , S.I. Rupassara g , J.D. Krehbiel h , M.J. Seufferheld i a Department of Agricultural and Biological Engineering, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA b Center for Biophysics and Quantitative Biology, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA c 204 Anne Burras Lane, Newport News, VA 23606, USA d Department of Plant Biology, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA e Department of Biochemistry, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA f Center of Biophysics, and Quantitative Biology, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA g Institute for Genomic Biology, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA h Department of Mechanical Sciences and Engineering, University of Illinois at Urbana–Champaign, IL 61801, USA i Department of Entomology—Illinois National History Survey, University of Illinois at Urbana–Champaign, Urbana, IL 61801, USA abstract article info Article history: Received 24 June 2014 Received in revised form 6 May 2015 Accepted 4 June 2015 Available online 25 June 2015 Keywords: Chlamydomonas reinhardtii Algal biofuels Chlorophyll fluorescence transient Metabolite profiling Knock-out mutant Non-photochemical quenching Mixotrophic condition Biophysical and biochemical characteristics of a spontaneous “mutant” strain (IM) of Chlamydomonas reinhardtii were quantified and compared with its progenitor (KO), a “knock-out” mutant with defects in phototaxis, and to its wild-type (WT); defects were shown to be preserved in the IM mutant. Growth curves showed that IM cultivated under mixotrophic conditions (TAP medium) and low light (10 and 20 μmol photons m -2 s -1 ), had 5–27% higher dry cell weight than WT and KO. This advantage was most likely attributable to increased acetate metabolism because it was not observed under purely photoautotrophic conditions using high salt minimal medium. Further characterization of these strains grown under mixotrophic conditions revealed several other unique features for the KO and IM mutant strains. Specifically, the IM and KO cells, grown under 60 μmol photons m -2 s -1 , showed higher rates of net oxygen evolution and respiration than the WT cells. Further, the slow (minute range) SM rise phase of chlorophyll a fluorescence transient was much reduced in IM cells, which has been ascribed to a regulatory event, labeled as “state 2 to state 1 transition”. Additionally, modulated fluorescence measurements showed that, when the IM strain is grown under low light, non- photochemical quenching of excited chlorophyll rises faster and recovers faster than in the other strains. Finally, compared to the WT, IM cells had a higher amount of metabolites related to carbon metabolism and protection against oxidative stress. These results suggest that the IM strain of C. reinhardtii has unique features that may be advantageous for improving algal biofuel production under mixotrophic conditions, such as algae cultivated in conjunction with wastewater treatment. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Algae represent a promising new source of feedstock for the produc- tion of various renewable liquid biofuels [1,2] or hydrogen [3–5] with a low carbon footprint. Their diverse metabolic capability also makes algae a unique and versatile “crop” to produce various food ingredients, nutraceuticals, pharmaceuticals and animal feed [6]. Algae have several key advantages, including higher growth rates than terrestrial plants, the ability to grow on marginal lands and in low quality water sources, as well as the ability to take up excess nutrients from wastewater and eutrophic water sources [7], which provides important water quality benefits. Despite these significant advantages, the promise of algae for clean energy resources remains largely unfulfilled due to several practi- cal bottlenecks in the production process. One important issue for the success of large-scale algal biomass production is maximizing biomass production under light limited conditions [8–10]. Due to rapid light attenuation in dense algal cultures, resulting from light absorption and scattering, significant spatial heterogeneity of light intensities occurs in- side most photobioreactors. Cells at the lighted surface can be damaged Algal Research 11 (2015) 134–147 ☆ This paper is dedicated to the memory of Robert M. Clegg (July 18, 1945–October 15, 2012), under whose direction David Park did the measurements shown in Fig. 5. Clegg was a pioneer of the “Physics of the Living Cells”, and of Fluorescence Lifetime Imaging Microscopy (FLIM), and a very dear friend to all of us. ⁎ Corresponding author. E-mail address: schidema@illinois.edu (L.C. Schideman). http://dx.doi.org/10.1016/j.algal.2015.06.001 2211-9264/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Algal Research journal homepage: www.elsevier.com/locate/algal