Biostimulant potential of two agrochemical tolerant microalgae isolated from subtropical clay soil

Conventional agriculture is highly dependent on chemical inputs and this leads to adverse environmental effects. Microalgae-based biofertilizers and biostimulants offer a promising solution to the challenges facing modern agriculture. In this work, two axenic microalgae strains were isolated from clay soil in the subtropical rainforest of Misiones, Argentina. These strains were identified as Chlorococcum vacuolatum CMI012 and Graesiella emersonii CMI015, and biomass production was characterised. The strains demonstrated differential effects as biostimulants for tomato germination and plant seedling growth under controlled conditions. Seed priming with C. vacuolatum supernatant improved germination and cotyledon emergence by 0.75 and 1.3-fold, respectively. Cellular extracts from the same strain at low concentration promoted cotyledon emergence by 1.3-fold. The effect of live microalgae cultures on tomato seedling growth was tested and G. emersonii CMI015 demonstrated significant biostimulation, enhancing root length and dry weight by nearly 100% and dry weight and height of the shoot by 50% and 20%, respectively. Both strains showed the ability to grow in 100 µg mL⁻¹ of four commonly used agrochemicals, indicating potential application in soil for the prevention of agrochemical runoff. This research emphasises the importance of isolating new microalgae from soil and evaluating their potential as biostimulants for their use in agriculture.