**6. Research Concepts**

The keyword co-occurrence analysis provides an overview of fucoxanthin research concepts. Keywords serve as the fundamental of bibliographic research of academic literature [45]. A total of 40 relevant keywords were classified into four different clusters using a minimum occurrence of the 15-fold keyword (Figure 4). The first cluster (in red) consists of 18 keywords that revolved around "antioxidant activity", "neuroprotection", and "cancer inhibition". This cluster highlighted the bioactivities of fucoxanthin via in vitro study and in vivo study. Ten keywords such as "photosynthesis", "photosystem", "chlorophyllbinding proteins", and "energy transfer" were seen in the second cluster (in green). This cluster represents the research of photosynthesis that involved fucoxanthin. The third cluster (in blue) encompassed "chemistry", "biosynthesis", "extraction", "purification", and "biotechnology" keywords. Thus, this cluster depicted the optimization of processes in obtaining fucoxanthin as valuable compounds. The fourth cluster (in yellow) revolved around the major keywords such as "phytoplankton", "community structure", "pigment", and "environmental monitoring", which denoted the role of fucoxanthin in the environmental study of phytoplankton.

**Figure 4.** Visualization of keyword co-occurrence network analysis (minimum occurrences: 15).

#### *6.1. Research Trends of Fucoxanthin Research from 1928 to June 2021*

The four clusters (bioactivities, photosynthesis, optimization of process, environment) were utilized to analyze the fucoxanthin research trends from 1928 to June 2021. In the first period (1928–1970), the researchers focused on the optimization of process cluster (88.88%) (Figure 5). An equal percentage of fucoxanthin articles (5.56%) was observed in photosynthesis and environment clusters, respectively. No bioactivities of fucoxanthin publication were produced. In the second period (1971–1980), the percentage of articles in the optimization of process cluster decreased by 22.92% compared to the previous period. Both photosynthesis and environment clusters demonstrated the same percentage of publications, 17.02% (increased by 11.46%). The bioactivities cluster remained 0%. There was a further reduction (2.95%) of publications in the optimization of process cluster in the third period (1981–1990) compared to the previous period. In the same period, the bioactivities, photosynthesis, and environment clusters increased the percentage of publications, 1.37, 2.16, and 1.42%, respectively.

In the fourth period (1991–2000), bioactivities and environment clusters experienced an increase of publications percentage, 4.26, and 12.66%, respectively, compared to the previous period. On the other hand, the optimization of process and photosynthesis clusters exhibited a fall of publications percentage, 4.13, and 12.69%, respectively. In the fifth period (2001–2010), the only optimization of production cluster experienced a reduction of articles percentage to 38.13% compared to the earlier period. The remaining clusters displayed an increase in articles percentage. The biggest upsurge in terms of publications percentage (16.77%) was detected in the bioactivities cluster among the other clusters. The photosynthesis cluster showed 8.8% of publications, while 30.67% of publications were identified in the environment cluster. In the sixth period (2011–June 2021), the bioactivities cluster (39.97%) demonstrated a similar publications percentage with the optimization of process cluster (40.79%). The photosynthesis cluster displayed 8.03% of publications, whereas 11.22% of publications were seen in the environment cluster. Throughout the six periods (1928–June 2021), the interest in fucoxanthin research shifted from the optimization of the processing cluster to the bioactivities cluster. This could be because brown macroalgae and microalgae are considered a potential natural source of bioactive compounds (i.e., fucoxanthin), and these compounds provide various health benefits [46].

**Figure 5.** Research trends of fucoxanthin research from 1928 to June 2021. The asterisk after 2021 denotes that the data were extracted up to June 2021.

#### 6.1.1. Bioactivities of Fucoxanthin

The interest in natural sources of bioactive compounds is growing immensely due to the numerous benefits to humans. According to the Scopus database, the first article on the bioactivities of fucoxanthin was published in 1990. In the first article on the bioactivities of fucoxanthin, Okuzumi et al. (1990) demonstrated the anti-tumor activity of fucoxanthin (isolated from brown algae, *Hijikia fusiforme*) on human neuroblastoma cells, GOTO cells. The authors showed that fucoxanthin caused the arrest in the G0–G1 phase of the cell cycle. In addition, fucoxanthin decreased the expression of the N-myc gene, and the authors suggested this mechanism could prevent the proliferation of cancer cells [47]. In the next several years, Okuzumi et al. (1993) showed that fucoxanthin inhibited duodenal carcinogenesis induced by N-ethyl-N'-nitro-N-nitrosoguanidine in mice [48]. Fucoxanthin was documented as a chemopreventive agen<sup>t</sup> in the first bioactivities of fucoxanthin review based on Scopus search results [49].

The bioactivities of fucoxanthin have been extensively studied. The bioactivities of fucoxanthin in humans have been summarized in many review articles [2,4,5,8,50–53]. For instance, Peng et al. (2011) [2] reviewed fucoxanthin's metabolism, safety, and bioactivities. The bioactivities include antioxidant, anti-inflammatory, anti-obese, anti-diabetic, antimalarial, anticancer, and anti-angiogenic activity and hepatoprotective, skin protective, cerebrovascular protective, bone protective, and ocular protective effects [2]. Furthermore, Thiyagarasaiyar et al. (2020) [5] discussed the cosmeceutical potentials of fucoxanthin as skin whitening, anti-aging, anticancer, antioxidant, anti-inflammation, and antimicrobial. In addition, Sathasivam and Ki (2018) [54] summarized the publications on anti-angiogenic, anticancer, antidiabetic, anti-obesity, and antioxidant activity and then on the neuroprotective, cardioprotective, and osteo-protective effect of fucoxanthin. Despite these bioactivities, the mechanism of these bioactivities also has been reviewed. For example, Rengarajan et al. (2013) [55] summarized the mechanisms of anticancer effects of fucoxanthin. These mechanisms were anti-proliferation, induction of apoptosis, cell cycle arrest, and antiangiogenesis. Moreover, the mechanisms of action of fucoxanthin on different types of cancers were also elucidated [56]. Additionally, the modulation of inflammatory and oxidative stress pathways using fucoxanthin was described [57]. Another review article summarized the possible underlying mechanism of fucoxanthin on lipid metabolism, adiposity, and related conditions [58].

The combined effect of fucoxanthin and the other compound(s) were also studied. Maeda et al. (2007) [6] demonstrated that feeding both fucoxanthin and fish oil to KK-*Ay* mice significantly reduced the WAT weight of mice compared with the mice fed fucoxanthin alone. Moreover, the reduction of serum levels of triacylglycerols, glucose, and

leptin in diet-induced obese rats was observed using a combination of fucoxanthin and conjugated linoleic acid [59]. A combination of fucoxanthin and vitamin C has been shown to increase human lymphocytes' antioxidant and anti-inflammatory effects [60]. Another study demonstrated the combined effects of low-molecular-weight fucoidan and fucoxanthin in a mouse model of type II diabetes. The authors reported that the combination effectively decreased the urinary sugar, glucose, and lipid metabolism in the WAT of the mice than fucoidan or fucoxanthin alone [61]. The effects of fucoidan and fucoxanthin in combination were investigated in aging mice and hyperuricemic rats. The combination of these compounds improved the cardiac status of aging mice via decreased cardiac hypertrophy, cardiac fibrosis, reactive oxygen species level, and shortened QT interval in the mice [62]. For hyperuricemic rats, the combination inhibited xanthine oxidase activity in the liver and controlled the expression of uric acid-related transporters [63]. Furthermore, a combination of fucoxanthin and rosmarinic acid could offer photo-protective effects through the downregulation of NRLP3-inflammasome and increasing the Nrf2 signaling pathway in UVB-irradiated HaCaT keratinocytes [64]. Recently, a combination of fucoxanthin, myoinositol, D-chiro-inositol, and hydroxytyrosol has been reported to decrease systolic blood pressure and improve the vascular reactivity in a pregnan<sup>t</sup> mouse model of hypertension [65].

Apart from this, the bioactivities of fucoxanthin were also studied in the other organisms. Fucoxanthin enhanced the phagocytic activities three times and increased the number of ovulated eggs of sea urchins by 3.25 times compared to the control group [66]. The planktonic larvae stage is crucial in the life cycle of coral. The percentage of a larval metamorphosis of coral *Pseudosiderastrea tayamai* was further enhanced by 60.3% in the presence of fucoxanthin compared to control group [67]. The effect of fucoxanthin was also examined in fly and worm. The median and maximum lifespan of *Drosophila melanogaster* (fly) was extended at least 33% and 12%, respectively using fucoxanthin. The decreased flies' fecundity, increased spontaneous locomotor activity, and resistance to oxidative stress were observed when feeding the flies with fucoxanthin. For *Caenorhabditis elegans* (worm), the feeding of fucoxanthin increased the mean and maximum lifespan by 14% and 24%, respectively [68].
