These compounds are proteins covalently linked to various oligosaccharide chains (glycans). Two major types of sugar chains are found in GPs, those bounded by N-glycosyl linkages or by O-glycosyl linkages ^[1][32]. GPs are located on the cell wall, on the surface of the cell, or free after secretion, and their roles include intercellular interactions and recognition ^[2][33]. Proportions of proteins and sugars of GPs differ from different algae species. For example, Ulva sp. GPs-rich fractions showed a protein proportion up to 33.4% ^[3][34], while in Codium decorticatum, which has GPs of molecular weight (MW) around 48 kDa, the protein proportion reached 60% ^[4][35]. Regarding the composition of the prosthetic fraction, seaweed GPs seem to mainly contain mannose ^[2][33].

Lectins are GPs that bind with high specificity to certain mono- or oligosaccharides. Seaweeds are good sources of novel lectins of low MW, especially red seaweeds ^[5][36]. The functions of lectins include gamete recognition and reproductive cell fusion, as well as defense against pathogens ^[5][6][36,37]. Lectins can be classified into four main groups: chitin-binding lectins, legume lectins, type-2 ribosome-inactivating proteins, and mannose-binding lectins ^[7][38]. In the case of seaweeds, the great majority of these proteins are mannose-binding, which are the major type of glycans found in them ^[2][33]. Likewise, relevant seaweed lectins are mannose-specific lectins, which show high binding affinity with these residues ^[8][39]. This property allows them to “agglutinate” particles containing these residues, as is the case of bacterial, viral, or eukaryote cell surface GPs ^[9][40]. One example is griffithsin, obtained from red Griffithsia sp. seaweeds, which has been described with diverse biological properties ^[10][41]. Due to these pharmacological properties, their characterization and isolation are a focus of research in medicine, molecular biology, or biochemistry. However, despite advances in the chemical characterization of seaweed lectins, additional information is still needed for a deeper understanding of their molecular structures, binding affinities, and possible biological functions for further applications ^[11][42].

PBPs are hydrosoluble chromophore proteins mainly present in cyanobacteria but also in red seaweeds. Their primary metabolic role is to act as light absorbents during photosynthesis ^[12][43]. PBPs include four classes of pigments: blue colored phycocyanin with absorption maxima (λ_max) in the range between 610 and 620 nm, magenta-colored phycoerythrocyanin (λ_max: 575 nm), bluish-green-colored allophycocyanin (λ_max: 652 nm), and deep red- or pink-colored phycoerythrin (PE) with λ_max between 540 and 570 nm ^[12][43]. The major PBP in red seaweeds is PE, of which phycoerythrobilin is its main prosthetic group (Figure 1) ^[13][44].

PBPs are used as natural pigments in food, replacing synthetic dyes, or as fluorescent probes in research ^[14][45]. Despite this, they are being studied for the development of nutraceuticals due to their bioactivities such as anti-viral, anti-cancer, antioxidant, and anti-inflammatory ^[15][46]. For this reason, current research on these molecules lies in function and biosynthesis mechanisms, structure elucidations, and potential applications. Isolation of PBPs has been reported in many species, for instance, Neoporphyra haitanensis ^[14][45], Kappaphycus alvarezii ^[16][47], Centroceras clavulatum ^[12][43], or N. yezoensis ^[16][47].

MAAs are small aminic secondary metabolites of MW below 400 Da (Figure 1) with strong absorption of ultraviolet (UV) radiation, typically between 320 and 360 nm. Thus, MAAs are useful molecules to protect cells from oxidative processes and solar UV-induced damage ^[17][48]. Up to now, more than 20 MAAs have been characterized in seaweeds and microalgae. From these, palaythine, asterina, shinorine, porphyra-334, mycosporine-glycine, usujirene, or palythene are most common in seaweeds ^[17][48]. They are naturally synthesized by various marine organisms, but seaweeds and microalgae are rich sources of them. However, they are much more abundant in red seaweeds, and only a small fraction of brown or green seaweeds presenting them ^[18][49]. For instance, porphyra-334 or shinorine are the most common in green or brown seaweeds such as Ulva intestinalis or Alaria esculenta, but red species like Pyropia columbina or C. crispus may additionally contain palythene much higher levels of mycosporine-glycine ^[18][49].

Peptides are protein fragments containing 2 to 40 AA in length generated from proteins by gastrointestinal digestion or from other hydrolyzation processes ^[19][50]. The importance of these molecules lies with the fact that some AA sequences, which are not active in SPs, show different biological properties after being released from the protein structure ^[20][51]. The development of algal peptides concentrates a blooming sector due to the diverse physiological activities of these molecules. Briefly, after the extraction and isolation of SPs, the main strategy to obtain BAPs from them involves their hydrolyzation (Figure 2). This may be performed by thermal, chemical, or enzymatic degradation, being the latter more extensively used since it allows more homogeneous and consistent patterns of hydrolyzation ^[21][23]. Thus, SP hydrolyzation using proteases can be used to maximize the yield of desired BAPs ^[21][23]. The process to describe the potential bioactive peptides requires testing different proteases in a single seaweed species ^[22][23][24][52,53,54], characterizing the BAP yield ^[25][26][27][55,56,57], optimizing the hydrolysis procedure ^[28][58], and exploring the effect of sequential hydrolysis using different proteases with defined conditions ^[29][59]. As hydrolyzation patterns and BAPs yielded are described, these approaches to define generated peptides allow covering a greater extent of potential bioactivities, as well as efficiency and effectiveness of extraction. Successful production of BPAs from hydrolyzed proteins of Porphyra spp, Palmaria palmata, Ulva spp, among others, have been reported ^[30][31][32][60,61,62].