Lumped Pathway Metabolic Model of Organic Carbon Accumulation and Mobilization by the Alga Chlamydomonas reinhardtii

Phototrophic microorganisms have significant potential as bioenergy feedstocks, but the sustainability of large-scale cultivation will require the use of wastewater as a renewable resource. A key barrier to this advancement is a lack of bioprocess understanding that would enable the design and implementation of efficient and resilient mixed community, naturally lit cultivation systems. In this study, a lumped pathway metabolic model (denoted the phototrophic process model or PPM) was developed for mixed phototrophic communities subjected to day/night cycling. State variables included functional biomass (<i>X</i>_CPO), stored carbohydrates (<i>X</i>_CH), stored lipids (<i>X</i>_LI), nitrate (<i>S</i>_NO), phosphate (<i>S</i>_P), and others. PPM metabolic reactions and stoichiometry were based on Chlamydomonas reinhardtii, but experiments for model calibration and validation were performed in flat panel photobioreactors (PBRs) originally inoculated with biomass from a phototrophic system at a wastewater treatment plant. PBRs were operated continuously as cyclostats to poise cells for intrinsic kinetic parameter estimation in batch studies, which included nutrient-available conditions in light and dark as well as nitrogen-starved and phosphorus-starved conditions in light. The model was calibrated and validated and was shown to be a reasonable predictor of growth, lipid and carbohydrate storage, and lipid and carbohydrate mobilization by a mixed microbial community