Modelling photon absorption rate for predicting optimal carotenoid production in purple phototrophic bacteria under...

Purple phototrophic bacteria (PPB) are emerging platforms for producing natural carotenoids from organic waste, yet light delivery in their cultivation is still set empirically. This is difficult for two reasons. First, PPB rely on two pigment systems that absorb in different spectral windows: carotenoids in the visible, and bacteriochlorophyll a (BChl a) in the near-infrared, where standard photosynthetically active radiation sensors are blind. Second, the Beer-Lambert law overestimates attenuation in dense, scattering cultures. This study aimed to specify light on an absorbed-photon basis and to identify the photon absorption rate that maximises carotenoid productivity. The Modified Cornet two-flux model was adapted to six PPB strains under seven light sources, separating absorption from scattering and resolving photon capture into carotenoid and BChl a contributions. Carotenoid-specific absorption varied up to 7.6-fold across sources; scattering dominated the visible band (80-85 % of extinction) and absorption the near-infrared. BChl a content fell 3- to 4-fold from low to high irradiance in strong acclimators, reshaping this partitioning. Under a halogen reference, maximum specific growth rates were 0.04-0.12 h^-1, and carotenoid productivity peaked at a strain-specific local volumetric rate of photon absorption (LVRPA 56-147 µmol photons m^-3 s^-1; log-normal fits, R² = 0.90-0.97), reaching 27-149 µg L^-1h^-1 at photon efficiencies up to 2.17 µg carotenoid per µmol photon. Because this optimum is defined on absorbed rather than incident photons, it anchors production to a controllable quantity that transfers across lamps and reactor geometries, providing a practical key to scaling PPB carotenoid production.