Pyroligneous acid as a multifunctional biostimulant enhances microalgal growth and soil beneficial metabolites for sustainable agriculture.

Microalgae are recognized as eco-functional inoculants that improve soil quality and stimulate plant productivity. This study explored the potential of pyroligneous acid (PA), a low-cost byproduct of biomass pyrolysis, as a biostimulant enhancing microalgal growth and metabolite production. We investigated the effects of PA on growth, and synthesis of indole-3-acetic acid (IAA) and extracellular polymeric substances (EPS) in two microalgal strains, Desmodesmus sp. MAS1 and Chlorella sp., when cultivated with PA in presence or absence of light. Preliminary studies showed that 0.01% (v/v) of PA enhanced growth of the microalgae, and its increased concentrations were inhibitory. Growth rate of Chlorella sp. with PA was 0.444 d^‒1 in a photobioreactor compared to 0.465 d^‒1 without PA, while the corresponding values for strain MAS1 were 0.372 d^‒1 and 0.334 d^‒1, respectively. Biomass production for Chlorella sp. and strain MAS1 when grown in presence of PA was 0.80 and 1.54 g L^‒1, respectively, while the biomass of cultures grown in the absence of PA was 1.13 and 1.20 g L^‒1, respectively. Chlorophyll content increased by 27% (P < 0.05) in Chlorella sp. and 17% (P < 0.05) in strain MAS1 in presence of PA. For Chlorella sp., IAA production in presence and absence of PA was 4.40 and 2.40 µg g^‒1, respectively, while it was 6.0 and 2.0 µg g^‒1, respectively, for strain MAS1. EPS yield increased by 2.0 mg g^‒1 with PA in Chlorella sp., and the increase was 8.0 mg g^‒1 in case of strain MAS1. Chlorella sp. exhibited higher chlorophyll content, enhanced IAA and EPS production but there was no increase in biomass. This suggests that PA can improve specific metabolic activities in both the microalgal strains, possibly contributing to soil health by promoting soil structure through increased EPS and IAA yield. Subsequent soil-based microcosm experiments confirmed that microalgae together with PA significantly increased soil EPS, IAA, dehydrogenase activity (DHA), and chlorophyll a content. The increase in DHA was 2.4-fold in soils that received PA and strain MAS1, while EPS production was 1.6 times higher. Chlorophyll a content was maximum in PA-amended soils inoculated with Chlorella sp., reaching a significant 6.5-fold increase that remained thereafter. These results highlight the dual role of PA in stimulating microalgal metabolic activity and enhancing soil microbial function, positioning it as a multifunctional biostimulant for sustainable agriculture.