## 3 Metabolomics Applied to Marine Organisms in Cancer Studies
### Structure Elucidation of Bioactive Molecules from Algae  
Algae are a complex and heterogeneous group of photosynthetic organisms, showing an extraordinary biological diversity represented by more than 166,000 species [109]. It is convenient to divide them into micro- (unicellular) and macro- (multicellular) algae according to their structure, evolution, ecological properties and sizes. These organisms produce and store a huge variety of metabolites, which include biologically active compounds (e.g., pigments, proteins and polysaccharides, antioxidants and polyunsaturated fatty acids), and several secondary metabolites produced in response to the pressures received in a wide range of environments, characterized by different conditions of temperature, light and salinity [110], among others. Bioactive molecules extracted both from microalgae and macroalgae show cytotoxic, antiviral and anti-inflammatory effects, with high potential for use in various medical fields. Some of those compounds are effective as therapeutic agents against cancer, showing high specificity for target molecules [111; 112]. Moreover, seaweeds represent an excellent source of bioactive compounds because they are easy to cultivate, allowing for the production of larger biomasses that can be used for industrial purposes. Among the edible species, having an historical importance as source of food for human consumption, there are the seaweeds belonging to the genus Ulva, which have a range of health-promoting bioactive components. In particular, ulvan, a polysaccharide contained in its cell walls, is mainly composed of sulfated rhamnose, uronic acids (glucuron acid and iduronic acid) and xylose. It showed cytotoxic activity against cancer cells [113; 114]. In fact, according to [PERSON] et al. [114], ulvan extracted from _Ulva lactuca_ had strong effect at various concentrations (0.8, 4, 20 and 100 \(\upmu\)g/mL) against HepG2, MCF7 and Hela cancer cell lines. They also assessed the value of IC\({}_{50}\) on the above three cell lines, being 29.7 \(\upmu\)g/mL, 25.1 \(\upmu\)g/mL and 36.3 \(\upmu\)g/mL, respectively. In conclusion, ulvan showed a significant cytotoxic activity in a dose-dependent manner and it can be developed as a promising cancer-fighting compound. In the same work, the researchers determined the fine structure of the ulvan, using \({}^{1}\)H-NMR and MS methods. Similarly, [PERSON] et al. [115] tested an organic extract of _U. lactuca_ and _Ulva fascita_ on MCF7 and HTC-116 (colorectal carcinoma) cell lines at different concentrations (12.5, 25, 50 and 100 \(\upmu\)g/mL), highlighting a significant dose-dependent response after 48 h of exposure. They also tested the extracts of three additional species of seaweeds, which, similarly, exhibited cytotoxic activity: two red algae, namely _Amphiro ances_ and _Corallina mediterranean_, and the fucales _Sargassum filiperndula_. The extracts were analysed by means of GC-MS. Moreover, various researches have been carried out on seaweed belonging to order Fucales, such as _Fucus vesiculos_. [PERSON] et al. [116] reported the inhibition of the cellular cycle in several cancer cell lines of pancreatic ductal adenocarcinoma (Panc1, PancTU1 and Panc89) and pancreatic adenosquamous carcinoma (Colo 357), testing fractions from a hydrophilic extract, after separation through HPLC. This effect seems related to the up-regulation of cell cycle inhibitors, showing an alteration of the expression levels of proteins and mRNA. Additionally, in the case of _F. vesiculosus_, [PERSON] et al. [117] analysed the effect of six crude extracts, each of which was analysed by \({}^{1}\)H-NMR spectroscopy techniques, revealing a characteristic fingerprint that was significantly correlated with the activity. In particular, the acetonic crude extract (FvT_A) showed the strongest activity against Panc89 and PancTu1, with an inhibitory rate of 80.3% and 82.6%, respectively.