The use of natural biostimulants is becoming an attractive option in order to reduce the use of fertilizer and increase the yield of crops. In particular, algal extracts are suitable candidates as they positively affect plant physiology. Among crops, lettuce often requires the use of biostimulants to improve both the quality and quantity of production. The aim of this work is to investigate the potential use of a Scenedesmus quadricauda extract as a biostimulant in order to obtain sustainable cultivation and a reduction in the cost of chemical fertilizers in lettuce cultivation. Therefore, the effect of S. quadricauda extract on lettuce seedlings was explored by evaluating the physiological parameters, chlorophyll, carotenoid, and total protein contents as well as several plant enzymatic activities involved in primary and secondary metabolisms. The experiment was performed by growing plants on inert substrate (pumice) with a 16-h photoperiod, by carrying out two consecutive radical treatments, one week apart, using a concentration of the extract corresponding to 1 mg Corg L−1. Lettuce plants were sampled at 1, 4, and 7 days from the first treatment and 7 days from the second treatment. The results showed that the S. quadricauda extract positively affected the growth of lettuce seedlings, mainly acting at the shoot level, determining an increase in dry matter, chlorophylls, carotenoids, proteins, and influencing the activities of several enzymes involved in the primary metabolism.

Lettuce seedlings often require the use of fertilizers for their cultivation management to achieve appropriate yield. However, for eco-sustainable chemical-fertilizers-free agronomy, the implementation of totally organic farming often cannot support lettuce productivity, therefore new natural biostimulants able to increase lettuce yield could be considered of great interest. In this preliminary work, the foliar spray application of a Chlorella vulgaris extract in lettuce seedlings was investigated in order to achieve better yield performance. Its biostimulant effect was evaluated by monitoring the morphobiometric parameters, chlorophylls, carotenoids, total protein contents, and several enzymatic activities involved in primary and secondary metabolisms of the plant. The experimental trials were carried out by growing lettuce seedlings on inert substrate (pumice) with a 16 h photoperiod for 21 days. The treatment consisted of three consecutive applications by foliar spraying using a concentration of the C. vulgaris extract, corresponding to 1 mg Corg L−1, which were performed one week apart. The results showed that the C. vulgaris extract positively influenced the growth of lettuce seedlings, by increasing the fresh and dry weights, chlorophylls, carotenoids, protein content, and ashes at shoot level. From a biochemical point of view, primary and secondary metabolisms of shoots, in particular nitrogen metabolism, were positively influenced. At the root level, the extract increased dry matter, proteins, and ash content.

The pollution of water caused by the excessive presence of organic and inorganic compounds, such as nitrates, phosphates, heavy metals, antibiotics, agrochemicals, etc., is one of the major environmental problems in many countries. Various approaches to remediate wastewater are available, and this review mainly provides the state of the art about the possible adoption of microalgae-based treatments (phycoremediation), which may represent a good alternative to conventional purification methods. Because of its composition, wastewater can provide several nutritional compounds (e.g., carbon, nitrogen, and phosphorus), which represent the essential nutrients for microalgae growth. Microalgae are also attracting the interest of worldwide researchers due to their multipurpose applications; in particular, microalgae cells can represent a useful feedstock for various sectors, among these, the agricultural sector. This review proposes a detailed description of the possible application of microalgae in the process of remediation of wastewaters of different sources, highlighting their possible advantages. Moreover, the review aims to report the application of the microalgae biomasses and their extracts in agriculture, as microalgae-based products can represent a valid alternative to traditional agrochemicals, offering sustainable solutions to improve agricultural technologies. Therefore, since the recently developed wastewater depuration technology based on phycoremediation may directly provide valuable microalgae biomasses, it can be used as a powerful starting means to produce agricultural products able to improve yield and quality of crops (biostimulants, biofertilizers), as well as induce pest and disease resistance (biopesticides).

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The cultivation of microalgae using urban wastewater as a nutrient substrate represents a promising bio-refinery concept that can serve multiple purposes; indeed, it allows for the generation of biomass, which can be used for various applications while meanwhile removing nutrients from wastewater. In this study, the potential of urban wastewater collected at two different time periods in a farmhouse as a nutrient substrate for microalgal growth was assessed. Wastewater samples were treated on a laboratory scale, inoculating reactors with two common species, Chlorella vulgaris (CV) and Scenedesmus quadricauda (SQ), and with an autochthonous strain of Klebsormidium sp. K39 (Kleb), directly isolated from effluents of the same system. The main aim of the study was to compare the microalgae’s performances in terms of wastewater re-mediation and biomass productivity. In the first case study, which involved an effluent with a lower pollutant level, microalgal cultivation showed removal efficiencies in the range of 57–63% for total nitrogen, 65–92% for total phosphorous, 94–95% for COD, and 100% for E. coli. In the second case study, involving an effluent with a higher pollutant level, the remediation performances of the three microalgae strains ranged from 93 to 96% for total nitrogen, from 62 to 74% for total phosphorous, from 96 to 97% for COD, and 100% for E. coli. At the end of the experimental trials, treated waters showed values of pollutants suitable for irrigation use, in accordance with environmental and national legislation, which established specific thresholds for irrigation purposes.