PHOTANOL - Photofermentative ethanol production through metabolic engineering
of the cyanobacterium Synechocystis sp. PCC 6803
In order to meet future demands for liquid fuels in the face of declining fossil fuel
reserves, biofuels such as ethanol are becoming increasingly important. Novel biofuel
production methods are required to overcome some of the flaws of previous generations
of biofuels, most notably the requirement for fertile soils and the destruction of biomass
during processing. The photanol approach uses metabolic engineering to link
photosynthesis of the cyanobacterium Synechocystis sp. PCC 6803 to fermentative
pathways by introducing heterologous genes; hereby a variety of products can be
biosynthesised using light as the energy source, carbon dioxide as substrate and water as
the ultimate electron donor.
In this study, the photanol approach was used to create an ethanol producing mutant
Synechocystis strain by introducing pyruvate decarboxylase and alcohol dehydrogenase
from Zymomonas mobilis. Physiological analysis revealed a detrimental effect of glucose,
increased production rates at high light intensities and a yet to be resolved carbon leak
through acetate. Observed ethanol production rates were highest, 0.51 mM g-1 h-1 or 4.08
mM OD730nm-1 day-1, in late exponential phase photoautotrophic cultures grown at a light
intensity of 38 µE and reached a maximum concentration of ~8 mM. This indicated a rate
of photofermentative ethanol formation competitive with other studies with relatively
more efficient conversion of light energy to chemical energy in the form of a liquid
biofuel compound.