Redirecting Photosynthetic Carbon Flux Toward Synthesis of Industrially Relevant Organic Acids

Abstract

Cyanobacteria are photosynthetic microbes capable of converting CO₂ into industrially relevant organic acids. This work investigated determinants of carbon allocation to two product classes: medium-chain (C8) free fatty acids, which serve as biofuel precursors, and lactate, a precursor for biodegradable plastics. These model products were used to probe mechanisms governing competition and coupling between heterologous product synthesis and total carbon fixation. Expressing enzymes targeting key reactions in fatty acid biosynthesis increased carbon flux toward C8-FFA and reduced the projected minimum selling price by four-fold, despite a substantial decrease in total carbon fixation. Metabolomic and proteomic profiling implicated carbon–nitrogen imbalance as a contributor to this metabolic burden. In this system, enhanced partitioning of carbon toward product formation (an NADPH sink) coincided with a partial recovery of carbon fixation. The investigation of lactate synthesis evaluated the hypothesis that insufficient NADPH demand limits total carbon fixation. ¹³C metabolic flux analysis (¹³C MFA) revealed lower-than-expected NADPH demand associated with lactate production. Increasing NADPH consumption capacity redirected greater relative flux toward lactate synthesis but did not enhance total carbon fixation. Together, these results demonstrate that relieving pathway bottlenecks can substantially enhance product synthesis, whereas increasing NADPH demand alone is insufficient to enhance carbon fixation, highlighting distinct physiological constraints governing carbon allocation in cyanobacteria.