(462g) Isotope-Assisted Metabolic Flux Analysis Reveals Efficient Photosynthetic Pathways in the Unicellular Diatom (alga) Phaeodactylum Tricornutum

Photosynthesis is crucial in converting solar energy and atmospheric CO₂ into fuel and renewable carbon. A rapidly growing human population and an increasing need for renewable fuel and carbon warrant a substantial increase in photosynthetic output over the next few decades. This motivates research into metabolic engineering for enhanced photosynthetic productivity. However, the overall efficiency of native photosynthesis is low because of the low processing ability of its central enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO). This is primarily because of the difficulty of concentrating CO₂ around RuBisCO at high enough concentrations to facilitate fast reaction. A small number of photosynthetic organisms overcome this problem by concentrating or “pumping” CO₂ around RuBisCO through biophysical and biochemical mechanisms. Although the orchestration of such a mechanism requires a multicellular architecture (as in maize or certain grasses), recent circumstantial evidence including genome sequences, transcriptome profiling and biophysical measurements has suggested that diatoms, a class of unicellular, marine algae may operate an efficient photosynthetic pathway. This may explain why diatoms are responsible for 20% to 40% of global photosynthetic output despite surviving in CO₂ -depleted environments. However, direct, metabolic evidence of such a pathway and the measurements of flux through it are lacking.

We have employed metabolic engineering and systems biology tools such as isotope-assisted metabolic flux analysis to address this gap in knowledge. We performed isotope-assisted metabolic flux analysis on the model diatom Phaeodactylum tricornutum and used mass spectrometry to measure incorporation of these carbon sources into soluble metabolites and biomass components. We then employed metabolic network modeling to convert the isotopomer abundances to metabolic flux maps. The results of these analyses reveal signatures of efficient photosynthetic pathways by which P. tricornutum apparently assimilates HCO₃^– (the abundant form of CO₂ in marine environments) into its biomass. In this presentation, we will discuss how our isotope labeling data and its analysis led to this conclusion. We anticipate that the knowledge obtained from this investigation will have important implications toward metabolic engineering of diatoms and other organisms for improved CO₂ fixation, and ultimately toward engineering synthetic CO₂-sequestering devices.