Joint Meeting - Genomics - U.S. Department of Energy

24
Systems Biology for DOE Energy and Environmental Missions
sion vector based on this novel signaling system whereby
varying the amount of signaling molecule provided as the
inducer can control gene expression levels over a dynamic
range. We anticipate that we will be able to combine
new information about mechanisms of electron flow
to nitrogenase with the use of our new gene expression
system and other tools to improve rates of H 2 production
by R. palustris.
References
1. Oda, Y., S. K. Samanta, F. E. Rey, L. Wu, X. Liu, T.
Yan, J. Zhou, and C. S. Harwood. 2005. Functional
genomic analysis of three nitrogenase isozymes in
the photosynthetic bacterium Rhodopseudomonas
palustris. J Bacteriol 187:7784-94.
2. Rey, F. E., E. K. Heiniger, and C. S. Harwood. 2007.
Redirection of metabolism for biological hydrogen
production. Appl Environ Microbiol 73:1665-71.
GTL
29
Modeling Electron Flow in
Rhodobacter sphaeroides to
Quantitatively Identify Approaches to
Maximize Hydrogen Production
L. Safak Yilmaz 1, * (yilmaz@cae.wisc.edu), Yi-Kai Su, 1
Wayne S. Kontur, 2 Timothy J. Donohue 2 (tdonohue@
bact.wisc.edu), and Daniel R. Noguera 1 (noguera@
engr.wisc.edu)
1Department of Civil and Environmental Engineering
and 2Department of Bacteriology, University of
Wisconsin, Madison, Wisconsin
Project Goals: To determine the impact of metabolic
and regulatory networks on light-powered hydrogen
production by Rhodobacter sphaeroides. Our experiments
will determine the quantitative contribution of
networks that are known or predicted to impact solarpowered
hydrogen production. We will use genetic,
genomic, and modeling approaches to quantify the role
of previously uncharacterized networks, ultimately
using this information to design microbial bioreactors
to efficiently produce hydrogen from solar energy.
This is a new project aimed at quantifying electron flow
within networks that impact solar-powered H 2 production
by the photosynthetic bacterium Rhodobacter sphaeroides.
We plan to use genetic, genomic, and modeling
approaches to quantify the role of known and previously
uncharacterized networks in solar-powered H 2 production
by this organism. Ultimately, this information can
* Presenting author
be used to design microbial bioreactors that efficiently
generate H 2 or other alternative fuels from solar energy.
The initial work has centered on the development of a
mathematical model to represent the electron flow from
an organic substrate to the different and competing electron
accepting reactions, during anaerobic photosynthetic
growth. As a preliminary step, the model describes the
quantitative apportionment of electrons derived from
the oxidation of an organic substrate to pathways related
to cell mass synthesis, polyhydroxyalkanoate (PHA)
formation, hydrogen production, and accumulation of
soluble organic products. Experimental measurements of
substrate, biomass accumulation, nitrogen and phosphorus
consumption, and pH are used to calculate best-fit
estimates of the fraction of electrons consumed in each
pathway.
When the model is fit to data from batch experiments
with exponentially growing R. sphaeroides 2.4.1. and succinate
as the sole organic substrate, it predicts that 47% of
the electron flow is related to biomass formation, 41% to
PHA accumulation, and 12% to soluble microbial products.
In experiments with stationary phase cultures with
nitrogen limitation, 26% of the electrons are predicted to
be used in PHA synthesis, 26% in hydrogen production,
and 48% in the formation of reduced soluble organic
substrates. When propionate is used as the sole organic
substrate, the predicted apportionment of electrons
during exponential growth is similar to that seen with
succinate, but under nitrogen limiting conditions, the
model predicts lower PHA formation, higher hydrogen
production, and a higher formation of soluble organic
substrates.
These preliminary evaluations are being used to inform
genetic strategies to optimize hydrogen production.
Finally, as more complete experimental methods are
developed, we expect to be able to experimentally validate
the model predictions regarding PHA formation and
hydrogen production, as well as to identify the type of
soluble organic substrates produced by R. sphaeroides.