Feast–Famine in Cyclic Autotrophy/Heterotrophy Doubles Microalgal Productivity while Controlling Bacterial Contamination

Conventional outdoor microalgal cultures are limited by light availability, with the absence of light during the night causing approximately 50% reduction of daily productivity in Europe. In this study, an innovative process was developed to enhance microalgal biomass productivity in photobioreactors by supplementing organic substrate at sunset, thereby implementing a cyclic autotrophic/heterotrophic cultivation (day, autotrophic; night, heterotrophic). When organic substrate was supplied at sunset in quantities sufficient to sustain growth throughout the whole night, significant bacterial contamination occurred, leading to a yield YX/S = 0.18 g g–1. To address this issue, a "feast–famine" feeding strategy was here designed, optimizing substrate dosage and supplementation frequency based on kinetic models predicting both microalgae and bacteria growth. Under 12 h/12 h day/night cycles, this optimized strategy nearly doubled biomass productivity, from 0.68 to 1.28 g L–1 day–1, while keeping bacterial contamination negligible (comparable to autotrophic control) improving YX/S to 0.54 g g–1. Using cheese whey as a source of organic substrate resulted in a modest increase in biomass productivity and lower yield. This study provides general guidelines for designing effective organic substrate feeding strategies to enhance microalgae biomass productivity under cyclic autotrophic/heterotrophic cultivation while keeping bacterial contamination below prescribed threshold levels.