Defatted biomass of the green microalga Chlorella sp. as a sustainable biostimulant to enhance barley growth under saline conditions.

Defatted microalgal biomass represents a potentially sustainable resource for biostimulant production, reducing waste from biodiesel processing while enhancing crop resilience to abiotic stress. This study evaluated the potential of defatted Chlorella sp. extract as a biostimulant to alleviate salinity-induced stress in barley (Hordeum vulgare) grown hydroponically under electrical conductivity levels of 2, 8, and 12 mS cm⁻¹. Gas chromatography-mass spectrometry (GC-MS) analysis of the ethanol extract revealed 28 bioactive compounds, mainly fatty acids and esters associated with plant growth promotion and stress tolerance. Among the three barley genotypes tested (Giza 123, Giza 132, and Giza 134), Giza 123 exhibited the greatest resilience to salinity, showing the highest grain yield and photosynthetic activity. Application of algal extract, particularly through the combined seed-soaking and foliar spray treatment, led to marked improvements, increasing grain dry weight by 37%, electron transport rate (ETR) by 22%, and P, K, and N grain contents by 9-14% compared to the control. These enhancements were accompanied by higher protein content (9.6%) and chlorophyll a concentration (0.41 mg g⁻¹ FW). Significant genotype × treatment interactions indicated variable responses among cultivars. Overall, the results demonstrate that defatted Chlorella biomass extract serves as an effective and eco-friendly biostimulant, improving barley growth, nutrient uptake, and salinity tolerance while contributing to the circular valorization of algal residues.