Fucales is one of the largest and most diverse orders of the Phaeophyceae class (i.e., brown algae), which includes some of the most common seaweeds in temperate and subtropical coastal regions, from seven distinct families, namely Sargassaceae, Fucaceae, Himanthaliaceae, Durvillaeaceae, Notheiaceae, Hormosiraceae, and Seirococcaceae [1]. The members of this order are multicellular and have the typical structure of large marine macroalgae, with a well-differentiated rhizoid, stipe, and lamina [2]. The blade is usually enlarged and may have gas vesicles to float freely in the water.

Sargassaceae is the major representative family of Fucales, with Sargassum-genus representing about 65% of its members. In Europe, native-Sargassaceae species, such as Cystoseira, Ericaria, and Gongolaria, occupy mostly the intertidal rockpools and/or the subtidal region. Particularly, Cystoseira, accounting for more than 30 species, is one of the most important genera found in the Mediterranean Sea and Atlantic Ocean, essential for biodiversity and ecosystem functioning [3]. It is also claimed as a promising source of bioactive compounds [4]. Because of their location, species from this genus are exposed to fluctuations in temperature, seawater salinity, and quantity/quality of light, leading them to adapt and develop protective strategies, such as increasing the content of some pigments to deal with light-irradiance fluctuation [5]. Sargassum genus, which comprises more than 350 species [6], is mostly pelagic and is distributed worldwide, despite being found mostly in tropical and subtropical environments [7]. Lately, the two halopelagic species (S. fluitans and S. natans) of this genus have been responsible for Sargassum tides on the coast of the Caribbean and Western Africa, negatively impacting the marine ecosystems [7]. Sargassum species have been used for centuries in agriculture as fertilizers, although their applications may go beyond fertilization purposes. Indeed, they have shown particularly interesting properties for the development of plant-biostimulation strategies and/or abiotic-stress mitigation, such as drought [8,9]. Moreover, these seaweeds have been explored as raw materials for biofuel production (e.g., methanol and bioethanol), for pharmaceutical, cosmetic, and textile industries, for bioplastic development, and for bioremediation [10,11,12]. Recently, the species, Polycladia myrica, widespread throughout the Persian Gulf, northern Australia, Mediterranean Sea, Pacific Ocean, and Indian Ocean tropics and subtropics, was also found to contain phlorotannins with promising UVR-protective effects, granting them great interest for cosmetic applications [13].

Fucaceae is another large family of Fucales. This is a broadly distributed family in the intertidal areas of the Northern Hemisphere, with six taxa recognized: Ascophyllum, Fucus, Hesperophycus, Pelvetia, Pelvetiopsis, and Silvetia, of which Ascophyllum, Fucus, and Silvetia are the most prominent genera [14]. These genera have been traditionally used in human nutrition for decades and, more recently, explored as potential ingredients in functional foods due to their richness in bioactive compounds [15,16]. Fucus is vastly distributed across the regions covered by the Fucaceae family and comprises 10 accepted species, of which F. vesiculosus is the most well-known, whereas Ascophyllum is a monotypic genus represented by Ascophyllum nodosum. F. vesiculosus is a good source of iodine, an essential component of thyroid hormones, and therefore, has been used in traditional medicine for the treatment of thyroid disfunction, including goiter [17] and obesity [18]. In turn, A. nodosum is the principal seaweed used as a source of industrial- and commercial-plant biostimulants, since it is shown to effectively improve plant growth, mitigate some abiotic and biotic stresses, and simultaneously enhance plant defenses by the regulation of molecular, physiological, and biochemical processes [19,20]. Moreover, due to their richness in bioactive compounds, Fucus and Ascophyllum extracts have a high potential to be used as nutraceutics and in healthcare [21].

The remaining families of Fucales are less representative and account for less than ten genera: Himanthaliaceae, a monotypic family comprising only one the genus Himanthalia and one species (H. elongata), is native to the northeast Atlantic Ocean, but can also be found in the North Sea and the Baltic Sea; Durvillaeaceae, also monotypic (Durvillaea), is found in the Southern Hemisphere, where some of the species are used in traditional cuisine [22]; Notheiaceae, another monotypic family (Notheia) with only one known species (N. anomala) that is an obligate epiphyte commonly found on Hormosira species along the southern Australia coast [23]. In turn, Hormosira, a single genus that belongs to the monotypic family, Hormosiraceae, and is native to southeastern Australia and New Zealand, being among the most common intertidal seaweeds on rocky shores found in those regions [24]. Finally, Seirococcaceae is a family with five genera, with only six species occurring exclusively in the Southern Hemisphere, and with a relatively restricted distribution [25].

Fucales members, like brown algae in general, have the metabolic capacity to biosynthesize unique phenolic compounds, named phlorotannins (PTs). PTs have been under the spotlight of many investigations since these compounds can display numerous bioactive properties that give them great potential for application in distinct medical-industrial areas. Indeed, because of their antimicrobial and antioxidant properties, PTs find applications in the food packaging industry, particularly as preserving agents in meat wrapping films for improved shelf-life and meat sensorial properties [26]. In the cosmetic industry, phlorotannins are ingredients of growing interest owing to their anti-aging and UV-protecting actions [27]. In fact, these can already be found in the composition of some sunscreen products, such as lotions [28] and sticks [29].

2. Phlorotannins and Main Classes in Fucale

3. Extraction Processes