Mimicking lichens: incorporation of yeast strains together with sucrose-secreting cyanobacteria improves survival, growth, ROS removal, and lipid production in a stable mutualistic co-culture production platform.
BACKGROUND
The feasibility of heterotrophic-phototrophic symbioses was tested via pairing of yeast strainsCryptococcus curvatus,Rhodotorula glutinis, orSaccharomyces cerevisiaewith a sucrose-secreting cyanobacteriumSynechococcus elongatus.


RESULTS
The phototrophS. elongatusshowed no growth in standard BG-11 medium with yeast extract, but grew well in BG-11 medium alone or supplemented with yeast nitrogen base without amino acids (YNB w/o aa). Among three yeast species,C. curvatusandR. glutinisadapted well to the BG-11 medium supplemented with YNB w/o aa, sucrose, and various concentrations of NaCl needed to maintain sucrose secretion fromS. elongatus, while growth ofS. cerevisiaewas highly dependent on sucrose levels.R. glutinisandC. curvatusgrew efficiently and utilized sucrose produced by the partner in co-culture. Co-cultures ofS. elongatusandR. glutiniswere sustained over 1 month in both batch and in semi-continuous culture, with the final biomass and overall lipid yields in the batch co-culture 40 to 60% higher compared to batch mono-cultures ofS. elongatus.The co-cultures showed enhanced levels of palmitoleic and linoleic acids. Furthermore, cyanobacterial growth in co-culture withR. glutiniswas significantly superior to axenic growth, asS. elongatuswas unable to grow in the absence of the yeast partner when cultivated at lower densities in liquid medium. Accumulated reactive oxygen species was observed to severely inhibit axenic growth of cyanobacteria, which was efficiently alleviated through catalase supply and even more effectively with co-cultures ofR. glutinis.


CONCLUSIONS
The pairing of a cyanobacterium and eukaryotic heterotroph in the artificial lichen of this study demonstrates the importance of mutual interactions between phototrophs and heterotrophs, e.g., phototrophs provide a carbon source to heterotrophs, and heterotrophs assist phototrophic growth and survival by removing/eliminating oxidative stress. Our results establish a potential stable production platform that combines the metabolic capability of photoautotrophs to capture inorganic carbon with the channeling of the resulting organic carbon directly to a robust heterotroph partner for producing biofuel and other chemical precursors.
Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, MD 21218 USA.
Tingting
T
Li
Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, MD 21218 USA.
Chien-Ting
CT
Li
Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218 USA.
Kirk
K
Butler
Department of Systems Biology, Harvard Medical School, Boston, MA 02115 USA.|Wyss Institute for Biologically Inspired Engineering, Harvard University, Boston, MA 02115 USA.
Stephanie G
SG
Hays
National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401 USA.
Michael T
MT
Guarnieri
Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, MD 21218 USA.
George A
GA
Oyler
Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, MD 21218 USA.
Michael J
MJ
Betenbaugh
Artificial lichen; Co-culture; Cyanobacteria; Hydrogen peroxide; Lipid production; ROS; Sucrose; Yeasts