The developed lumped pathway metabolic model consists of 15 reactions (Reactions 1-15; R1-R15) representing the relationships between the core metabolites of simplified microalgal metabolic processes (Figure , Table ). The model builds on the original PPM with distinct pathway combinations under nutrient-replete and nutrient-deplete conditions, 17 but has been updated to include the potential for heterotrophic metabolism in microalgae. for all nutrient availability conditions. The yellow (R1, R6) and blue (R5, R7, R11) arrows are only active in the presence of light and extracellular organic carbon, respectively. The green arrows are shared by photoautotrophic and heterotrophic metabolisms. These reactions were compiled into systems of linear equations to determine stoichiometric coefficients for PM 2 . The inclusion of a given reaction does not imply it is meaningfully active under all environmental conditions (e.g., the assimilation of N and P into biomass precursors will not occur if no nutrients are available to the cell); rate expressions are used to control model kinetics.