## 1 Introduction  
Proteins are one of the three essential macronutrients in the diet and the most important to prevent malnutrition and hunger [1]. According to the United Nations, the global population will increase to 9.5 billion people by 2050 [2]; as a result, the demand for food, and therefore also for protein, will rise [3]. There are two primary protein sources of similar nutritional quality: animal (which includes meat, fish, eggs, and dairy products) and plant (legumes, and to a lesser extent, seeds, and nuts). However, animal protein is known for its high environmental and climate impact, not only for the emissions related to their production but also due to the arable land used for the intensive crop production for feed and a large amount of water required (for both feed and animals) [4]. Thus, it is necessary to find sustainable plant-based sources and production processes to meet future protein requirements without damaging the planet and human health [5]. One of the most promising alternative feedstocks are microalgae, which are photosynthetic organisms mainly composed of proteins, carbohydrates, and lipids [6, 7]. Microalgae use solar energy to store carbon from CO\({}_{2}\) conversion, and they also uptake nutrients from the culture medium for their growth [8]. This type of biomass has several advantages over others, including superior photosynthetic efficiencies and biomass production potentials than terrestrial crops, growth over a wide range of pH and temperature, capacity to uptake nutrients from different types of sources, such as wastewater, and most of them have a short generation cycle [9].  
The content of proteins, carbohydrates, lipids, and other microalgae biocompounds can be utilized as feedstock for various bioproducts applying biorefinery processes [10]. Nevertheless, the current approaches are still not profitable and have some challenges to be addressed [11]. The price of microalgae cultivation is a notable drawback of microalgae-based biorefineries, but the use of low-cost culture medium as wastewater could contribute to achieve economically sustainable processes. As a result, the microalgae biorefinery approach can be integrated within the circular bioeconomy, whose main goals are to reduce, reuse, and recycle raw materials [11]. Additionally, biomass's integral and fractional valorization could maximize the process benefits by taking advantage of waste streams, promoting this circular bioeconomy [7].  
The characteristics and composition of microalgae depend on the species and the environmental and operational conditions during cultivation [12]. However, in most cases, the main constituents of microalgae biomass are proteins, ranging from 30 to 70% mass percentage in dry weight [13], achieving the higher protein concentrations in microalgae grown in stressful environments such as wastewater. Due to the proteins' high added value and liability, it is reasonable to first recover the protein fraction in the process of fractional valorization of this biomass. The major issue with protein recovery is related to where the proteins are found in the cell. Typically, these biomolecules are inside a very recalcitrant cell wall that is difficult to break and whose thickness and chemical composition depend on various factors such as growth stage, cellular form, and environmental conditions [14]. Hence, the first step of a biorefinery process to recover biomolecules from microalgae is the cell wall disruption by physical, chemical, or biological methods to extract intracellular molecules by different solvents [15]. The use of organic solvents is a well-known technique for the extraction and separation of microalgae biomolecules, but some of them are toxic and not aligned with environmental and human health concerns [16]. Thus, alternative solvents have recently begun to be studied, such as ionic liquids (ILs) and deep eutectic solvents (DESs), with lower environmental impact than conventional solvents and promising applications in microalgae processing [17].