A lumped pathway metabolic model was developed for carbon-accumulating phototrophic microorganisms. The model (denoted the Phototrophic Process Model, or PPM) was developed using the metabolic pathways of Chlamydomonas reinhardtii (a green alga), which were used to solve for model stoichiometry. The PPM was structured in a manner consistent with the Activated Sludge Models (ASMs), and is presented as a stoichiometric matrix and a set of transformation processes. State variables include functional biomass (XCPO), stored lipids (as triacylglycerol; XTAG), and stored carbohydrates (as polyglucose; XPG) among others. Model structure was refined and kinetic parameters were calibrated and validated based on experiments with a mixed community of phototrophic microorganisms in flat panel photobioreactors (PBRs) operated as cyclostats (i.e., chemostats subjected to a day:night light regime). The model was a reasonable predictor of growth and organic carbon storage/mobilization, but requires additional refinement to enable wastewater treatment modeling.