Relevance and intellectual merit. Photosynthesis converts solar energy and atmospheric CO2 into food, fuel, renewable carbon and molecular oxygen. A rapidly growing human population and an increasing need for renewable fuel and carbon demand a greater increase in photosynthetic output. In this context, understanding the metabolism of photosynthetically efficient organisms will enable metabolic engineering for enhanced photosynthetic productivity.
Photosynthetic carbon fixation is typically inefficient because CO2 is sparingly soluble in water and highly diffusible, which prevents it from accumulating at a high concentration around ribulose bisphosphate carboxylase/oxygenase (RuBisCO), the enzyme that catalyzes the first step of photosynthesis. This reduces both the rate and yield of photosynthetic carbon assimilation. A small number of photosynthetic organisms overcome this problem by concentrating or pumping CO2 around RuBisCO through biochemical or biophysical mechanisms. One such biochemical mechanism known as the C4 pathway is found in a small number of crop plants including maize and certain grasses. However, recent circumstantial evidence has suggested that diatoms, a class of unicellular, marine algae, may also operate a C4 pathway. This may explain why diatoms are responsible for 20% to 40% of global photosynthetic output despite surviving in CO2 -depleted environments. This project takes advantage of the availability of isotope-assisted metabolic flux analysis (isotope MFA) and recently established genomic data and reverse genetic tools to investigate the C4 pathway in the model diatom species Phaeodactylum tricornutum. Specifically, the project will employ isotope MFA to measure carbon flow through the CO2 assimilation pathways and identify whether this organism concentrates CO2 exclusively through the C4 pathway. Furthermore, the project will assess the effects of varying environmental conditions on the efficiency of CO2 assimilation and finally, genetically engineer P. tricornutum by silencing key genes in the C4 pathways and identify rate-limiting steps.
Broader impacts. The work will reveal the key metabolic steps and the organization of the C4 pathway in diatoms, which may underlie the high photosynthetic efficiency of these unicellular algae. Such knowledge will help pinpoint target genes and rate-limiting steps for future metabolic engineering towards improved photosynthetic productivity in algae and perhaps plants. A long-term goal will be to construct CO2-sequestering devices via synthetic biology approaches. This project will train students at the graduate, undergraduate and high school levels in the interdisciplinary fields of metabolic engineering, biochemistry, plant biology, and systems biology. Additionally, this project will develop metabolic pathway teaching modules for undergraduate and graduate courses in chemical and biomolecular engineering as well as biology, many of which will be disseminated to BioEMB, an NSF-funded repository for biology-intensive course materials in engineering.