Cyanobacteria
are of great biotechnological interest for their potential to produce biofuels
driven by oxygenic photosynthesis. In other words, you can make biofuels from
sunlight, water, and CO2. One approach is to produce ethanol: in
order to do this a couple of enzymes need to be introduced using genetic engineering
to metabolize pyruvate to ethanol. Arguably, the most successful of this is
that by company Algenol were they introduced pyruvate decarboxylase and alcohol
dehydrogenase from the alpha-proteobacterium Zymomonas mobilis into the marine unicellular cyanobacterium Synechococcus sp. PCC 7642. This
approach was patented and in the patent it is said that rates of 1.7 µmoles of
ethanol / mg chlorophyll-a / hour were obtained, which in my opinion is a very modest
rate. Certainly, it is below 1% of the rates of Photosystem II activity under saturating
light. Algenol is now producing 10000 gallons / acre / year, what appears to be
a promising yield... and if the life cycle analysis they published is correct, the energy balance is positive: in other words, there is more energy in the ethanol produced than it was invested to drive the company and purify the ethanol.
Here I
want to propose an alternative approach that I think will generate better yields.
This is based on quantitative proteomic results in multicellular cyanobacteria
capable of differentiating heterocysts. Under nitrogen starvation multicellular
filamentous cyanobacteria differentiate 5-10% of their cells into a cell type
specialized in atmospheric nitrogen fixation, nitrogen-fixing cells are called heterocysts. Heterocysts contain nitrogenase and other oxygen
intolerant enzymes and for that reason photosynthetic oxygen evolution is
inactivated or slowed down in the heterocysts. However, the surrounding cells
are still capable of oxygenic photosynthesis and they transfer reductant to the
heterocysts in exchange for fixed nitrogen in the form of glutamine.
A, Filamentous cyanobacteria, the arrow points to a heterocysts. B, Isolated heterocyts.
The
proteomic work by Ow et al. (2009) indicated that heterocysts from Nostoc punctifurme contained very large
amounts of the enzyme pyruvate kinase
which uses phosphoenolpyruvate to generate ATP and pyruvate. In this study it
was shown that pyruvate kinase was at least 3.8 times more abundant in the
heterocysts compared to the vegetative cells. This predicts that heterocysts
might have naturally higher concentrations of pyruvate. The reason why pyruvate
is in higher concentrations in heterocysts is because it is the precursor of
2-oxoglutarate, which is the precursor of glutamate. In heterocysts glutamate
reacts with ammonia (the product of atmospheric nitrogen reduction by nitrogenase) to produce
glutamine.
My idea
is to express pyruvate decarboxylase and alcohol dehydrogenase from Zymomonas or Saccharomyces in the heterocysts of a multicellular cyanobacterium,
such as the fresh water Nostoc sp. PCC 7120, Nostoc punctiforme or Anabaena variabilis or a marine version like Anabaena sp. 90. The technology to
do this is already in existence. Since the metabolism of heterocysts is
super-ramped up to provide nitrogen for 90-95% of the cells, I am pretty sure
that the yields of ethanol could be pretty high.
It does
not come without challenges because probably the diversion of pyruvate to
generate ethanol might impair to certain extent nitrogen-fixation, however
it has been shown that heterocysts can compensate a loss of reducing
equivalents by boosting up cyclic photosynthesis and probably their own
metabolism. It might be that a lack of pyruvate could enhance carbon-fixation, a
bonus. In any case, we will not know if this is a sound idea until we try it out.
