Unlocking the potential of earthworm compost leachate for microalgae production using sequenced-batch and semi-continuous culture strategies

To reduce freshwater consumption and the use of chemicals in microalgae cultivation, increasing attention has been directed toward the use of wastewater-based culture media. Vermicompost leachate (VL) represents a promising substrate, as it contains assimilable forms of nitrogen and phosphorus as well as oligo-elements. The main objective of this study was to evaluate the optimal cultivation strategy capable of achieving a stable and long production of algal biomass using VL. First, four microalgae and cyanobacteria strains, along with a naturally occurring microalgal consortium, were screened in microplate to evaluate their growth performance in the medium. The selected consortium, dominated by Scenedesmus , Tetradesmus , and Desmodesmus species, was subsequently cultivated in photobioreactors operated under sequenced-batch (SB) and semi-continuous (SC) modes. Growth response, nutrient removal and biomass composition obtained with VL were systematically analyzed and compared to those achieved using a commercial synthetic medium (MBG11). Using VL, both SB and SC cultivation strategies led to a stable biomass productivity of 73 ± 6 mg·L -1 ·d -1 (n=4) and 67 ± 5 mg·L -1 ·d -1 (n=5), respectively, without the need for freshwater or nutrient supplementation. Interestingly, cultivation strategies affected the consumption of residual nutrients and the biochemical composition of the harvested biomass. In SC mode, 100% and 68% of N and P were removed, corresponding to removal rates of 4.3 mgN·L⁻¹·d⁻¹ and 2.0 mgP·L⁻¹·d⁻¹, respectively. The harvested biomass exhibited a high protein content (32.1%), with a substantial proportion of essential amino acids (19.7%) relative to non-essential amino acids (11.0%), highlighting its potential nutritional value. This study highlights the potential of VL as an efficient medium for algal biomass production under controlled laboratory conditions, supporting its prospective use within a circular economy approach.