# Use of _Chlorella vulgaris_ and _Ulva lactuca_ as Biostimulant on Lettuce
## 1 Introduction  
Today, food production must be increased to ensure the wellness of the growing population around the world, but, at the same time, it is important to reduce the use of synthetic chemical fertilizers that are dangerous for human and environmental health. To prevent this issue, it is essential to take the direction towards more sustainable agricultural practices that can assure an increased production with the minimum negative impact on the environment [1, 2]. One of the strategies to face this goal is the use of biostimulants, the definition of which has evolved over time. Biostimulants are defined as a biological formulation that enhance plant health and productivity as a result of an induced action led by the complex mixture's compounds, rather than just the presence of mineral content and/or the presence of plant growth regulators [3]. There are several kinds of biostimulants involved in agriculture, such as humic substances (humic acid, fulvic acids, and humins), fungi, bacteria, and algae extracts [4]. The market for these products is growing steadily for different reasons: (a) unexpected climatic changes that adversely affect crop yields, (b) the need to promote efficient use of organic materials, and (c) the increasing availability of new commercial formulates [5]. Sustainable development is a central issue in XXI century, considering that the United Nation Agenda 2030 plan promotes, among other things, sustainable food production in its goal 12: \"Ensure sustainable consumption and production patterns\" [6]. Therefore, interest in biostimulants used in crop production is increasing, and studies demonstrate that algae contain components used to perform this task on crop plants [4]. _Chorella vulgaris_ and _Ulva lactuca_ are both green algae; the first is a unicellular microalga which can grow in both fresh and brackish sweet water and in varied environmental conditions, while the second is a macroalga (seaweed) which grows in brackish and seawater. Both have several applications, from biofuel production to human nutrition, from animal feed to pharmaceutical use and, not least, in agrochemical applications to increase the growth rate and physiological responses of many crops. When applied to soil or leaves, seaweed extract demonstrates a variety of biostimulating properties in agricultural output. Biostimulants are metabolic enhancers that promote plant production due to the unique or emergent features of a complex of ingredients employed in modest amounts. Plants benefit from biostimulants in a variety of ways, including enhanced stress tolerance to nutrients, water, and salt, germination and early seed establishment, improved crop performance and production, maximal resilience to abiotic and biotic stress, and a longer shelf life of the fruits after harvesting, which are all favorable impacts on plants. To increase agricultural yield, liquid seaweed extracts are administered as a foliar spray to crops. These seaweed extracts include cytokins, auxins, betaines, gibberellins, carbohydrates, vitamins, polysaccharides, alginates, amino acids, and trace elements (Fe, Cu, Zn, Co, Mo, Mn, and Ni) that operate in the control and development of plants. The methods of action of these seaweed chemicals are complicated, and it is unknown why they are effective; nonetheless, it is probable that these substances operate synergistically when combined [7; 8; 9; 10]. _Lactuca sativa_ L. is one of the most valuable vegetable crops around the world used in a human diet for its nutritional properties rich in vitamins, fibers, and polyphenols. Furthermore, studies confirm that lettuce has important effects in the prevention of cardiovascular diseases [11]. The consumption of this species is increasingly becoming one of the leading vegetable crops in the United States [12]. Moreover, lettuce is an important crop in different countries including Italy, Spain, Japan, and Mexico [11].