## 1 Introduction  
The use of marine organisms for the exploitation of bioactive compounds has recently increased due to their biotechnological potential in different areas as pharmaceutical, cosmetic/cosmeceutical, food/feed, and many others [1; 2]. Within this group, macroalgae are known to be great producers of bioactive compounds [2], with growth rates that ensure biomass availability from harvesting and aquaculture. _Gratelopia turuturu_ is an edible red macroalga native from Korea and Japan that has been classified as invasive species in the Atlantic Ocean, and was first reported in Portugal in 1997 [3]. This seaweed is characterized as rich in carbohydrates and proteins, as well as having a low lipid content [4]. It is present in \"mid-intertidal\" areas and is easily found in intertidal pools [5], where the exposure to solar radiation and oxidative stress levels are high [5]. The ability to protect themselves against harmful radiation make these organisms potential sources for the discovery of compounds with antioxidants and UV shielding activities [4; 6]. Moreover, it has product development potential in the cosme industry. For a successful screening of such compounds and to ensure the industrial feasibility of their application, an adequate method of extraction should be developed [7]. Although more recent and effective methods have been developed (the novel extraction methodologies, characterized by more technological approaches resulting in better performance and lower environmental impact), the utilization of solid-liquid extraction (SLE) is still one of the best solutions for the production of extracts with the intent of industrialization, given the low technical and facility requirements for its up-scale implementation. However, SLE can be fine-tuned to perform better (be more efficient, selective, eco-friendly, and profitable) by optimizing several operational parameters: physicochemical properties of the raw material, the selected solvents/respective concentrations, pH, temperature, extraction time, and others [8]. A balance between the time of extraction and the temperature is extremely important to avoid thermal degradation. Room temperature can be an advantage, as it preserves compounds and reduces the associated economic costs of the procedure [7]. Regarding the type of solvent, water-based extractions are privileged and ethanol and acetone are the most accepted organic solvents considering their security and cost [9]. Solvent selection depends not only on their inherent safety and cost but also on the cost associated to their evaporation, and ability to extract target compounds. For all these reasons, binary mixtures of water and ethanol are popular in the natural extract production for industry. Specifically, in the case of \(G\). _turuturu_, hydroethanolic extraction is the most biotechnology relevant since this is the best solvent to recover this species' main bioactivities: sulphated galactans (known antioxidant, anticoagulant, and antimicrobial activities [10]), phycobiliproteins (PBPs), and mycosporine-like amino acids (MAAs) (with antioxidant activity and very high UV-shielding activity [4; 10]). The optimization of the extraction parameters for the obtention of compounds groups described above using a one-dimensional method, where only one factor is modified at a time, is not only time-consuming but more expensive [11]. Using the response surface methodology (RSM), it is possible to avoid these difficulties, since this statistical method allows for the simultaneous evaluation of different variables (enabling the detection of potential interactions between the tested conditions), reducing the high number of experiments, and, consequently, the quantity of reagents used and the time associated to the process [12]. Thus, the main goal of this study was to optimize the extraction process of bioactivities as measured by antioxidants and UV-shielding from the red macrocalga _Grateloupia turuturu_, with the potential for use in the cosmetic industry.