## 1 Introduction  
In recent decades, macroalgae have attracted significant biotechnological interest due to their wide biochemical composition (e.g., polysaccharides, fatty acids, carotenoids, phycobiliproteins, or mycosporine-like amino acids) with diverse potential bioactivities (e.g., antioxidant, anti-inflammatory, or photoprotective activities) [1]. Algae biomass can be used for different applications: (1) human consumption or feed due to its high nutritional value; (2) food additives due to its phycocolloid content; (3) biostimulants for plants due to algal growth-stimulating compounds; (4) cosmeceutical, nutraceutical, or pharmaceutical products due to its antioxidant, anti-inflammatory, or photoprotective properties [2, 3]. This review is focused on the cosmeceutical applications of marine red macroalgae (Rhodophyta), and particularly on their use as natural ultraviolet (UV) filters due to their antioxidant and photoprotective capacity. The cosmetic industry is growing at a global scale and it is seeking innovations such as natural active principles. In recent decades, consumers have also tended to prefer natural products rather than synthetic ones, because the former look better, are safer, and are more beneficial for health and the environment. UV radiation (UVR) that reaches the Earth's surface (<10% of the total received) is composed mainly of UV-A (320-400 nm) and a small portion of UV-B (290-320 nm). UV-B is the most dangerous, causing DNA mutations such as cyclobutane pyrimidine dimers (CPDs) or 6-4 photoproducts (6-4 PP), which can be associated with cancer cell development. UV-A is considered less harmful, but can produce reactive oxygen species (ROS), altering the expression of certain genes related to premature skin aging [4; 5; 6]. Sunscreens are composed by different UV filters that protect the skin from exposure to UVR. UV filters are normally classified as: (1) synthetic or organic, based on the capacity to absorb UVR (e.g., octocrylene, butyl methoxydibenzoylmethane, oxybenzene); and (2) physical or inorganic, which can reflect the UVR (e.g., titanium dioxide). These filters have been found in different aquatic environments [7; 8; 9] associated with marine organisms such as mussels, fish or mammals [10; 11]. Their presence can provoke negative effects such as inhibition of growth in phytoplankton [12], endocrine disruptions in fish [13; 14] or stimulus of coral bleaching [15; 16; 17]. Different studies have also reported their negative effects in humans, such as in photoallergies or hormone disorders [18; 19]. Thus, it is necessary to develop natural ataxic and biodegradable photoprotectors. Mycosporine-like amino acids (MAAs) are secondary metabolites, mainly implicated in photoprotection, and produced by various organisms (algae or corals), of which red macroalgae are one of the main producers [20; 21]. MAAs are a diverse group of nitrogenous compounds with low molecular weight (<400 Da), and are colorless, uncharged, and water soluble. More than 30 types have been identified. All MAAs share the same central structure, a cyclohexenome or cyclohexenimine ring, responsible for the UV absorption. The different types of MAAs vary according to their nitrogen substituent (amino acid or its imino alcohol) in the chromophore, which determines their specific absorption spectra [22; 23] (Figure 1). Different pathways have been proposed for its synthesis: (1) Firstly, the shikimate pathway was suggested [24; 25; 26]. The production of MAAs was inhibited in the coral _Stylophora pistillata_ using glyphosate, an inhibitor of the shikimate pathway [25]. (2) More recently, the pentose-phosphate pathway was also demonstrated to be implicated in MAA biosynthesis [27; 28; 29; 30]. Balskus and Walsh [27] found that a four-gene cluster linked to the pentose-phosphate pathway in _Trichormus variabilis_ (previously known as _Anabena variabilis_) was able to produce the MAA shinorine when inserted into _Escherichia coli_. However, Spence et al. [31] showed that this is not the major route of MAA synthesis. _T.