The Biodiversity of the Genus Dictyota: Phytochemical and Pharmacological Natural Products Prospectives

Although a broad variety of classes of bioactive compounds have already been isolated from seaweeds of the genus Dictyota, most different species are still chemically and biologically unexplored. Dictyota species are well-known brown seaweeds belonging to the Dictyotaceae (Phaeophyta). The phytochemical composition within the genus Dictyota has recently received considerable interest, and a vast array of components, including diterpenes, sesquiterepenes, sterols, amino acids, as well as saturated and polyunsaturated fatty acids, have been characterized. The contribution of these valued metabolites to the biological potential, which includes anti-proliferative, anti-microbial, antiviral, antioxidant, anti-inflammatory, and anti-hyperpigmentation activities, of the genus Dictyota has also been explored. Therefore, this is the most comprehensive review, focusing on the published literature relevant to the chemically and pharmacologically diverse biopharmaceuticals isolated from different species of the genus Dictyota during the period from 1976 to now.


Introduction
Marine natural products are characterized by the diversity of their chemical structures, displaying a wide spectrum of biological activities. The Red Sea is one of the most important marine hotspots comprising high seaweed biodiversity [1]. Several marine algae, particularly seaweeds, are biogeographically native to the Red Sea, but only a few species have been chemically examined during the past decades. Seaweeds have a growing number of successful applications in the food, medicine, and cosmetic industries, which increases the importance of evaluating their chemical composition [2]. Species of the genus Dictyota J.V. Lamouroux, as representatives of these seaweeds, are characterized by the following taxonomic features: flattened thalli, ribbon-like, erect or prostrate, with smooth dentate, crenulate, and ciliate margins; thallus attachment is fulfilled by means of uniseriate, multicellular branching, and hyaline rhizoids that may be divided terminally into fixing disk stoloniferous holdfasts, which may be present or absent; dichotomous orrarely falcate branching; obtuse, rounded, apiculate, or acute apices; Phaeophyceae hairs and superficial proliferations may be present or absent; the thallus differentiates into a cortex

The Pharmacological and Phytochemical Potential of the Genus Dictyota
The current review concentrates on studies on the chemical composition of either isolated compounds or extracts and their bioactivities, whenever possible, afforded from different species of genus Dictyota [8,9].

Dictyota caribaea Hörnig and Schnetter 1992
The cytotoxic and anti-leishmanial activities of crude extracts of D. caribaea collected in the Gulf of Mexico and the Caribbean coast have been reported. The study revealed the presence of fucoidan, a humofucan with fucose as the neutral sugar. The sulfated polysaccharide-treated mice with 25 and 50 mg/kg/day depicted Sarcoma 180 tumor growth inhibition of 40% and 51%, respectively, compared to negative control group. Sulfated polysaccharides treatment induced spleen weight increasing compared to the negative control group of 40% and 70% on mice treated with 25 and 50 mg/kg/day, respectively, along with intense white pulp disorganization. These results combined support an immunostimulant effect related to the in vivo antitumor effect of D. caribaea sulfated polysaccharides [19].
The sulfates were found to occur at C2, C4, and C6 of the glucose residues. The anticancer effects of DdF, DdL, and DdLs (200 µg/mL) were measured in vitro on colon cancer cells (HCT-116, HT-29, and DLD-1). The effect of polysaccharides (40 µg/mL) on colony formation of DLD-1 cancer cells after irradiation was investigated. All polysaccharides showed a synergistic effect with X-ray irradiation against cancer cells, decreasing the amount and size of cancer cell colonies [36].
The laminarans are neutral, water-soluble β-D-glucans possessing potent immunomodulating, radioprotective, and anticancer activities. The in vitro anticancer, radioprotective, and radiosensitizing activities of laminaran from D. dichotoma and its sulfated derivative were investigated. The native and sulfated laminarans by themselves at non-toxic doses possessed significant anticancer activity against melanoma cells. Both polysaccharides protected normal epidermal cells, while only sulfated laminaran was able to sensitize melanoma cells to X-ray irradiation, resulting in significant inhibition of cell proliferation, colony formation, and migration of cancer cells [37].
The molecular mechanism of this action was found to be related to the inhibition of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9) proteinases activity and to the down-regulation of kinases phosphorylation of the ERK1/2 signaling cascade [37].
The galactofucan-rich fractions, obtained from D. dichotoma thalli exhibited moderate inhibitory effects against Herpes simplex virus (HSV-1) and Coxsackie virus (CVB3) [38]. The ethanolic extract of D. dichotoma possessed active compounds for the development of larvi-cidal activity against 4th instars larvae of the yellow-fever mosquito, Aedes aegypti. The extract of D. dichotoma revealed that the minimum level of an LC 50 value was 0.0683 ± 0.0084 µg/mL.
For D. dichotoma, collected from the Western seacoast of Yemen, anticancer and antioxidant activities were detected. The chloroform fraction of the D. dichotoma displayed the highest cytotoxic activity and was most effective against MCF-7, human prostate cancer cells (PC-3), and colorectal adenocarcinoma cells (CACO), (IC 50 = 1.93 ± 0.25, 2.2 ± 0.18, and 2.71 ± 0.53 µg/mL, respectively). The petroleum ether fraction was also effective, particularly against MCF-7 and PC-3 (IC 50 = 4.77 ± 0.51 and 3.93 ± 0.51 µg/mL, respectively), whereas the activity of the ethyl acetate fraction was more pronounced against HepG2 and CACO (IC 50 = 5.06 ± 0.21 and 5.06 ± 0.23 µg/mL, respectively). Of all the extracts tested, the crude methanolic extract of the algae exhibited the best antioxidant potential (IC 50 = 204.6 ± 8.3 µg/mL). Doses as high as 5000 mg/kg body weight of D. dichotoma methanolic extracts were safe and well tolerated by rats [42].
Headspace  (Figure 7) are tricyclic diterpenoids of the dolastane ring system detected in D. dichotoma var. divaricata, collected from the Caribbean Sea [70].
Fucoxanthin (198) was extracted from D. dichotoma var. indica, collected from Qeshm Island, Persian Gulf. In a 24 h treatment at a concentration of 50 µg/mL, it displayed effective anticancer activity against the breast cancer cell line, without toxic effects to the normal cells [78,79].
Palladium oxide nanoparticles using the extract of D. dichotoma var. indica collected from the Oman Sea coasts in Chabahar, Iran, have a spherical shape with an average particle diameter of 19 nm [80]. Polyshaped gold nanoparticles derived from an extract of leaves from D. dichotoma of size 8 ± 21 nm were synthesized. Antibacterial activities were observed by the agar well diffusion method for the action of streptomycin and gentamycin and their formulation with biocapped gold nanoparticles. The profound efficacies were supported by the increase in fold area of inhibition against the tested bacteria [81].
Isopachydictyolal (208) and 4α-acetyldictyodial (209) were detected in the methanolic extract of D. dichotoma var. linearis. The antiviral activity of these metabolites isolated in adequate amounts was evaluated in laboratory assays against Herpes simplex virus I (HSV I) and poliomyelitis virus I, using Vero cells as the hosts [85].  (Figure 8) [87].
The influence of temperature (20, 40, and 60 • C) and of the extraction solvents (water, ethanol) on the ultrasound-assisted extraction of phenolics from the Adriatic D. dichotoma was recorded. The extracts were analyzed for major phenolic sub-groups (total phenolics, flavonoids, and tannins), while the individual phenolics were detected by HPLC. The antioxidant activities were evaluated using three methods: ferric reducing/antioxidant power (FRAP), scavenging of the stabile 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical, and the oxygen radical antioxidant capacity (ORAC). The total phenolics, flavonoids, and tannins were obtained using an alcoholic solvent, while a general conclusion about the applied temperature was not established. These extracts also showed good antioxidant activity, with high reducing capacity (690-792 mM TE) and ORAC values (38.7-40.8 mM TE in 400-fold diluted extracts). The PCA pointed out the significant influence of flavonoids and tannins on the investigated properties. The results of this investigation could be interesting for future studies dealing with the application of D. dichotoma in foods, cosmetics, and pharmaceuticals [88]. The aqueous and ethanolic extracts of the D. dichotoma, collected from Hurghada, the Red Sea coast of Egypt, exert an alleviation of salt stress on the germination of rice seeds at concentrations of 0, 5, 10, 20, and 50 g/L. The % germination of rice increased from 84% in non-treated seeds to 100% when treated with 20 g/L. The lower recovery of salt-treated seeds compared with the control seed germination suggests that rice suffered from the toxic ion effect of salinity on embryo rather than from the osmotic effect. Extracts of D. dichotoma can enhance and alleviate salinity stress on rice seed germination [89].

Dictyota dumosa Børgesen 1935
Several phytoconstituents were detected in the extracts of D. dumosa, D. dichotoma var. intricata, and D. dichotoma var. indica, collected from the Gulf of Mannar (India), where phenols, terpenoids, and cardiac glycosides were the major constituents. Antibacterial activity was tested against K. pneumoniae, S. aureus, P. aeruginosa, and B. subtilis, while antifungal activity was tested against A. niger, A. fumigatus, A. terreus, F. oxysporum, and C. albicans. Antibacterial activity was more promising in all the extracts of seaweed than antifungal activity, with the highest zone of inhibition of 17 mm shown by a methanolic extract of D. dumosa against K. pneumonia followed by 15 mm exerted by a chloroform extract of D. dichotoma var. indica against B. subtilis [90]. Fatty-acid analyses of D. dichotoma, D. dumosa, D. hauckiana, D. dichotoma var. indica, and D. maxima, found in Karachi coastal waters, evidenced the occurrence of ten saturated and four unsaturated fatty acids. Only palmitic acid occurred in all Dictyota species, while eicosatrienoic acid displayed a more selective distribution in D. indica [91].
Methanolic extracts of D. ciliolata (MEDC) and D. menstrualis (MED), collected from the Northeast of Brazil, were assessed as apoptosis-inducing agents on human cervical adenocarcinoma (HeLa) cells. All extracts showed different levels of cytotoxicity against the cells. In addition, MEDC and MEDM also inhibited SiHa (cervix carcinoma) cell proliferation. Studies with these two extracts showed that HeLa cells exposed to MEDM and MEDC exhibited morphological and biochemical changes that characterize apoptosis as shown by loss of cell viability, chromatin condensation, phosphatidylserine externalization, and sub-G1 cell cycle phase accumulation, and MEDC also induces cell cycle arrest in cell cycle phase S [107].
Extracted sulfated polysaccharides from Brazilian D. menstrualis, followed by separation into five fractions by sequential acetone precipitation, demonstrated that all fractions were composed mainly of fucose, xylose, galactose, uronic acid, and inorganic sulfate. The anticoagulant activity of these heterofucans was determined by activated partial thromboplastin time (APTT) using citrate normal human plasma. Only the fucans F1.0v and F1.5v showed anticoagulant activity. To prolong the coagulation time to double the baseline value in the APTT, the required concentration of fucan F1.0v (20 µg/mL) was only 4.88-fold higher than that of the low-molecular-weight heparin Clexane ® (4.1 µg/mL), whereas 80 µg/mL fucan 1.5 was needed to obtain the same effect. For both fucans, the effect was abolished by desulfation [108].
A heterofucan (F2.0v) from D. menstrualis, collected from Búzios Beach, Rio Grande do Norte State (Brazil), was evaluated as an antinociceptive and anti-inflammatory agent. F2.0v (20.0 mg/kg) inhibited 100% of leukocyte migration into the peritoneal cavity after chemical stimulation. However, F2.0v did not alter the expression of interleukin-1 beta (IL-1β) and interleukin-6 (IL-6), as well as tumor necrosis factor-alpha (TNF-α). F2.0v (20.0 mg/kg) had peripheral antinociceptive activity with potency like dipyrone. On the other hand, it did not affect pain response on the hot-plate test. Confocal microscopy analysis and flow cytometry showed that F2.0v binds to the surface of leucocytes, which leads us to suggest that the mechanisms of action of anti-inflammatory and antinociceptive F2.0v are related to its ability to inhibit the migration of leukocytes to the site of tissue injury. In summary, the data show that the F2.0v compound may have a great potential as an antinociceptive and anti-inflammatory agent [109]. The crude extract of Brazilian D. menstrualis and its derived fractions were analyzed for their antiviral potential, alone and in combination with ribavirin at 20 µg/mL. The fractions that were rich in cyclic diterpenes with aldehyde groupings, inhibited Zika virus replication by >74%, with inhibition behaving in a dosedependent manner, with EC 50 values of 2.80 (F-6) and 0.81 (FAc-2) µg/mL. Regarding the mechanism of action, FAc-2 had a strong virucidal potential, and F-6 inhibited viral adsorption. Associating FAc-2 with ribavirin at suboptimal dosages produced a strong synergistic effect that completely inhibited viral replication [110].

Dictyota pulchella Hörnig and Schnetter 1988
The cardiovascular effects elicited by D. pulchella were investigated using in vitro and in vivo experiments. In normotensive conscious rats, CH 2 Cl 2 /MeOH extract (CME, 5, 10, 20, and 40 mg/kg) from D. pulchella produced dose-dependent hypotension (−4 ± 1; −8 ± 2; −53 ± 8; and −63 ± 3 mmHg) and bradycardia (−8 ± 6; −17 ± 11; −257 ± 36; and −285 ± 27 b.p.m.). In addition, the CME and hexane/EtOAc phases (HEP) (0.01-300 µg/mL) from D. pulchella induced a concentration-dependent relaxation in mesenteric artery rings pre-contracted with phenylephrine (Phe, 1 µM). The vasorelaxant effect was not modified by the removal of the vascular endothelium or by pre-incubation with KCl (20 mM), tetraethylammonium (TEA, 3 mM), or thromboxane A2 agonist U-46619 (100 nM). Furthermore, CME and HEP reversed CaCl 2 -induced vascular contractions. These results suggest that both CME and HEP acted on the voltage-operated calcium channel to produce vasorelaxation. In addition, CME-induced vasodilatation occurred after the vessels had been pre-contracted with an L-type Ca 2+ channel agonist (Bay K 8644, 200 nM). Taken together, these data show that CME induced hypotension and bradycardia in vivo and that both CME and HEP induced endothelium-independent vasodilatation in vitro that seemed to involve the inhibition of the Ca 2+ influx through the blockade of voltage-operated calcium channels [116]. Despite these interesting activities, no pure compounds have so far been isolated from this seaweed.

Dictyota sandvicensis Sonder 1859
D. sandvicensis is an edible Hawaiian seaweed. Its contents (%) of water (relative to the total fresh weight), total ash, total soluble protein, total soluble carbohydrates and crude lipids (relative to the total dry weight) were measured (86.4 ± 0.3, 28.9 ± 0.1, 6.4 ± 0.6, 6.7 ± 0.4, and 20.2 ± 0.1, respectively) [117]. No pure compounds have so far been isolated from this species. Compounds 268-269 and 271-273 were screened for their potential to prevent the adhesion of three bacterial strains isolated from marine biofilms in comparison with four commercial antifoulants. Compounds 272-273 exhibited the strongest anti-adhesion effects, with moderate toxicity [120].

Conclusions and Future Directions
The genus Dictyota is one of the most important brown algae in the coastal zones because of its biomass and wide distribution in tropical to temperate seas. Despite the folk uses of brown algae, particularly of Dictyota, all over the world, the knowledge about its phytopharmacological potentials is still underexplored. Several species, such as D. divaricata, D. implexa, D. indica, D. linearis, and D. volubilis, have recently been considered the holotype of the genus Dictyota. Analysis of the reported data regarding its phytoconstituents has implied some remarkable gaps in the wholeness of our perception of their attribution to the reported biological activities and health outcomes of Dictyota, besides the roles performed by each of these metabolites. Moreover, existing biological research on several Dictyota species is particularly limited to the investigation of their total extracts or fractions, while the isolated components have so far received much less interest. Hence, further studies should comprehensively be done to discover the metabolites and their actual contribution to the medicinal properties of Dictyota, in addition to the implied mechanisms and their possible synergistic interactions, which would permit a sensible application of the genus Dictyota in modern phytotherapy. From a medicinal point of view, the antiproliferative potential of the genus Dictyota represents an indispensable future research theme. Many studies have extensively approached its cytotoxic properties. However, despite its reported cytotoxicity, the mechanism of action should be considered in future research to best appreciate this potential. Several dermal applications of the genus Dictyota were reported, such as hypopigmentation and anti-alopecia, yet so far without an application of these effects in cosmetic production. Species of the genus Dictyota were found to be cardioprotective and neuroprotective; however, further investigations are required for a full investigation of these effects and the effects on other body organs.
The antimicrobial potential is another considerable challenge because the current reported antimicrobial data are mostly restricted to crude extracts and fractions of Dictyota species. Although some of them exhibited high, broad-spectrum antimicrobial activities, a further detailed investigation of the antimicrobial properties of Dictyota following standardized protocols, involving those of the purified metabolites, is still missing and, hence, highly recommended. The investigation of the bioactive metabolites obtained from the genus Dictyota should be combined with more in-depth structure-activity relationship studies, and with the elucidation of their possible mechanisms of action. Elucidating the cellular and molecular features of the biological activities of Dictyota metabolites will be of pronounced value in the discovery and development of new bioactive compounds. Moreover, artificial analogs of bioactive metabolites of the genus Dictyota should be designed, synthesized, and tested, focusing on improving the efficacy and safety, and on increasing its economic production for nutrition with its further investigation as a natural source of new drugs.
Studies of nanotechnology could permit a safe and effective application of Dictyota species in modern phytotherapy. Many industrial applications as a source of biofuels and agricultural applications as a source of growth-promoting agents of this species are imaginable, but have not been sufficiently studied yet. The publication rate regarding Dictyota has been rising vigorously within the past five years (Figure 11), illustrating the rapidly growing interest in this intriguing genus. So far, 25 species names of Dictyota, representing 24.5% of the current species, have been accepted taxonomically and investigated for their chemical and pharmacological activities (Figure 12), of which the most studied species as a possible drug source is D. dichotoma. Several bioactive compounds with most diverse chemical structures have been reported, including diterpenes sesquiterpenes, carotenoids, hydrocarbons, terpenes, sterols, and sulfated polysaccharides (Figures 2-10, Tables S1 and S2). These isolated compounds and/or extracts displayed diverse biological activities such as antiproliferative, antimicrobial, antiviral, antioxidant, anti-inflammatory, and anti-hyperpigmentation activities. In addition, fucoxanthin displayed antiproliferative activities towards different cancer cell lines. Moreover, the high polyphenolic content of some Dictyota sp. mainly reflects their antioxidant and antimicrobial activities. Finally, several Dictyota sp. extracts exhibited antiviral activities against HSV and HIV. The data compiled in the current review collectively highlight the potential of the genus Dictyota as a promising candidate for the development of alternative medicinal agents and, in the same context, the effective utilization of this alga as a functional-food ingredient, especially considering its known nutritional value, is also warranted.

Supplementary Materials:
The following supporting information can be downloaded, Table S1: Pharmacological potential of compounds isolated from the genus Dictyota; Table S2: Biological assays related to therapeutic potential of Dictyota extracts.