###### Abstract  
Microalgae, as some of the oldest life forms on Earth, are of significant interest to industry and in terms of environmental policies, due to their ability to perform photosynthesis and consume atmospheric carbon dioxide. Moreover, they contain a wide variety of value-added compounds such as amino acids and proteins, carbohydrates, and fatty acids, which can be exploited in multiple fields like medicine, cosmetics, nutritional supplements, and for the production of biodiesel. In this article, _Nunochloropsis gadituna_, a type of microalgae that inhabits both fresh and salt water, is studied for fatty acid recovery using deep eutectic solvents (DES). This microalgae species is a natural source of eicosapentaenoic acid (EPA), an omega-3 compound that is commonly used in the nutritional industry. There are numerous extraction techniques and pretreatments to obtain these compounds. In this work, DES are studied as extractive agents due to their advantages as neoteric solvents. Specifically, this work focuses on an assessment of the effect of the composition of DES on the extraction yield of fatty acids from microalgae. Several DES compositions based on choline chloride, ethylene glycol, and fructose are studied to analyze the influence of water content in these phases. The results show that water content significantly influences recovery yields. The DES with higher extractive capacity were those based on choline chloride, ethylene glycol, and water at a molar ratio of 1:2:2. This composition offered 48.7% of the yield obtained with a conventional solvent like methanol for the recovery of EPA (11.2 mg/g microalgae). Furthermore, the choline chloride-fructose-based DES shows the capability of selective extractions of fatty acids with low carbon content--choline chloride:fructose:water (molar ratio 2:1:2) can extract 0.14 mg of decanoic acid/g of microalgae, indicating that this DES composition can recover 35.7% more decanoic acid in comparison to methanol. microalgae; fatty acids; omega-3; deep eutectic solvents; green solvents; biomass valorization Article  
Study of the Effect of Water Content in Deep Eutectic Phases on the Extraction of Fatty Acids from Microalgae Biomass  
[PERSON], P.  
extraction technique, they could be used for the production of biodiesel. Nevertheless, the production of biodiesel that is sourced from microalgae is not currently optimized, as the process consumes so much energy and the production costs are high; hence, this technique is neither economically feasible nor industrially productive at this time [6; 7]. On the other hand, the use of microalgae as a source of nutritional supplements is more extended. Human beings are unable to synthesize all the vitamins necessary for their proper growth [8], requiring other sources from which to obtain them. For instance, vitamin B12 is not produced by the human body. This kind of vitamin can be found in several means, but not in vegetables, which poses a significant challenge for those on vegetarian and vegan diets, as the deficiency of this nutrient can lead to illnesses such as anemia [9]. To solve this problem, individuals following a plant-based diet can incorporate microalgae into their nutritional intake, as microalgae are rich in this vitamin [3; 10]. Some of the most consumed fatty acids in nutrition are omega-3 types, such as eicosapentaenoic acid (EPA) and docosahexaenoicacid (DHA) [11; 12]. These fatty acids are mainly found in fish, seafood, meat, and dairy products. However, overfishing poses a threat to the sustainability of marine assets. Different species of microalgae (_Arthospira platensis_, _Crypthecodimium combinii_, _Isochrysis galbana_, _Nannochloropsis_ sp., _Nitzschia_ sp., _Porphyridium_ sp., and _Spliczochytrium_ sp., among others) are good sources of this type of fatty acid, offering an excellent alternative for humans [13; 14]. The extraction of fatty acids from microalgae typically involves the use of chemical solvents. Conventional methods are based on organic solvents, such as hexane and methanol [15].