Exploring the biostimulant potential of Scenedesmus sp. grown in wastewater: impacts on plant growth and photosynthetic activity of lettuce.

Microalgae can produce different biostimulant compounds, which might increase plant growth and make them tolerant to abiotic stress. The aim of this study is to assess the biostimulant capacity of microalgal biomass cultivated in wastewater. Microalgae biomass was obtained in a high rate algae pond treating urban wastewater and then harvested and lyophilized. Heavy metals and phytohormones were quantified. Two different extraction methods were tested: a physical extraction (PE) with a sonication probe and a chemical extraction (CE) with sulfuric acid and thermal bath. Then, an agronomic assay was performed in a greenhouse cultivating lettuce seedlings. After 35 days, a first foliar application of both extracts was performed. A negative control with tap water was also included. After the first application, drought stress was induced to half of the plants (stressed plants) by withholding water for five days. The rest of the plants were daily watered (unstressed plants). Then, a second foliar application was provided and all plants were watered again for another week. The microalgal biomass presented a lower content of heavy metals than the European regulation threshold and presented high concentration of phytohormones, in particular cytokinins (TZ, 21 mg/gDW) and auxins (IAA, 7 mg/gDW). After the first foliar application, unstressed plants with CE presented chlorophyll content higher than the control (10 %) whereas foliar fresh weight was higher than the control for both extracts (9-13 %). Stressed plants presented higher chlorophyll content for both extracts (9-11 %) whereas fresh weight was only higher with PE (12 %). At the end of the experiment, unstressed plants presented an increase in shoot fresh weight of 12-13 % with the PE and CE compared to the control. In conclusion, in this study, biomass dominated by Scenedesmus sp., presented high concentration of phytohormones and improved the growth of lettuce.