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In microalgal bioprocesses, pH is a key operational parameter that substantially influences growth and photosynthesis. While pH control is well established in photobioreactor operation, quantitative and model-based analyses of pH influence on physiological parameters remain limited. In particular, the modeling of photosynthesis-related parameters as specific functions of pH has received little attention in the literature. In this study, the effects of pH on growth and photosynthetic activity of Microchloropsis salina were investigated in turbidostatic cultivations using an online monitoring system based on pulse amplitude modulation (PAM) fluorometry and rapid light curves (RLC). Two types of pH control were investigated: pH control via CO 2 and via acid/base at constant CO 2 supply. Parameters of growth and photosynthesis, i.e. optimum specific growth rate μ /optimum dilution rate D , quantum efficiency of photosynthesis α , maximum relative electron transport rate rETR and minimum saturating photosynthetic photon flux density PPFD K , were quantified as functions of pH using the cardinal pH model (CpHM) to determine pH optima and tolerance ranges. While low pH values generally inhibited both growth and photosynthetic activity, the effects of high pH values depended on the type of pH control applied: via CO 2 , photosynthesis remained active even when growth declined, indicating a certain decoupling between photosynthetic activity and growth; via acid/base, stronger inhibition of both processes was observed. This shows that the type of pH control considerably affects microalgal physiological responses. Modeling using the CpHM yielded specific optimum parameter ranges for growth ( pH opt ≈7.8–8.2, μ opt ≈0.40 d −1 ) and for photosynthetic activity ( pH opt ≈7.2–7.6, α opt ≈0.58, ETR max , opt ≈226 μmol e- m −2 s −1 , PPFD K , opt ≈384 μmol γ m −2 s −1 ), with growth showing narrower pH tolerance than photosynthesis. Based on all modeling results, M. salina should be cultivated at pH values of around 7.6 to enable favorable conditions for both growth and photosynthetic activity. • M. salina turbidostat cultivations to model pH-dependent growth and photosynthesis • Novel application of the cardinal pH model to photosynthesis-related parameters • Model-based identification of different pH optima for growth and photosynthesis • Type of pH control affects microalgal growth and photosynthesis in different ways • Strain to be cultivated at pH ≈ 7.6 for favorable growth and photosynthetic activity
Ringleben et al. (Sun,) studied this question.