S30: Efficient photosynthetic mechanisms in biofuel-producing microorganisms: what can metabolic analysis reveal?

Photosynthetic reduction of carbon by cyanobacteria, algae and plants is central to the production of renewable biofuels, but is often an inefficient process. The inefficiency arises primarily from the inability of photosynthetic cells to concentrate CO₂ around RuBisCO, the enzyme catalyzing the first step of carbon fixation, at concentrations sufficient enough to facilitate fast reaction. Certain unicellular photosynthetic microorganisms have been hypothesized to overcome this difficulty by operating biophysical or biochemical “CO₂ pumps” that concentrate substantial amounts of CO₂ around RuBisCO. However, little is known about the molecules and mechanisms involved in these processes. Metabolic flux and network analysis can provide significant, direct evidence about the operation and efficiencies of CO₂ concentrating mechanisms. We employed this technique to study carbon concentration in the model diatom (alga) Phaeodactylum tricornutum. The high photosynthetic productivity of diatoms despite their CO₂-deprived marine environments suggests efficient photosynthetic mechanisms. To investigate this, we performed isotope-assisted metabolic flux analysis by feeding isotopically labeled carbon sources and using mass spectrometry and metabolic modeling to quantify metabolic pathways and fluxes. The isotope labeling data from our experiments reveals distinct signatures of CO₂ concentrating pathways. In this presentation we will report these results and the insights they provide toward metabolic engineering of photosynthetic microorganisms for harnessing renewable energy.