**1. Introduction**

Marine algae are a potential renewable resource in the marine environment that are used as a culinary item in East Asia and they have been reported to confer several beneficial effects in human health. They are an excellent source of nutritional and bioactive compounds, such as carotenoids, dietary fibers, amino acids, essential fatty acids, vitamins, and minerals [1,2]. Algal carotenoids have received much attention, as they are structurally different from those found in terrestrial plants. It is important to characterize the novel bio-functional activities of carotenoids from marine algae. 

One of the major carotenoids in marine algae is fucoxanthin, and it is found mainly in brown macroalgae and in some classes of microalgae [3–5]. The chemical structure of fucoxanthin includes an allenic bond and oxygenic functional groups, such as hydroxyl, epoxy, carbonyl, and acetyl groups, in addition to its polyene chain (Figure 1A). Recently, fucoxanthin has been reported to possess several beneficial effects related to its anti-cancerous, anti-oxidative, and anti-obesity properties [6–12]. Moreover, fucoxanthin induces uncoupling protein 1 in the mitochondria of abdominal white adipose tissue, leading to the oxidation of fatty acids and heat production [13]. On the other hand, we found that fucoxanthin and its deacetylated product, fucoxanthinol, effectively suppress angiogenesis [14]. This implies that fucoxanthin, with its anti-angiogenic activity, would be useful in preventing angiogenesis-related diseases such as cancer and diabetic retinopathy. Consequently, brown algal fucoxanthin seems to be a useful bioactive and nutraceutical compound for human health. 

**Figure 1.** Structures of fucoxanthin ( **A**); siphonaxanthin ( **B**) and siphonein ( **C**). 

Scientific evidence on the bio-functional properties of algal carotenoids is still very limited. Siphonaxanthin (Figure 1B) is a specific keto-carotenoid of green algae, and its bio-functional properties are yet to be clarified. We focus on the potential of siphonaxanthin as a bioactive compound and outline the evidence associated with functionality functionality. The objective of this review is to summarize the novel bio-functional activities of green algal siphonaxanthin that have been highlighted in our studies. 

## **2. Siphonaxanthin in Green Algae**

Siphonaxanthin is a specific keto-carotenoid of siphonaceous green algae, which helps in absorbing available green and blue green light under water [15,16]. Brown algal fucoxanthin has epoxide and an allenic bond in its structure, whereas siphonaxanthin does not contain either of those functional groups. However, it possesses an additional hydroxyl group on the 19th carbon atom (Figure 1B). In edible green algae such as *Codium fragile*, *Caulerpa lentillifera*, and *Umbraulva japonica*, siphonaxanthin content is approximately 0.03%–0.1% of the dry weight. Siphonein, identified as siphonaxanthin 

19-(*trans*-NJ2-dodecenoate), is present at the same level as siphonaxanthin in these algae. *C. fragile* is consumed as a part of the staple diet in ancient Japanese food culture. 

Generally, carotenoids in plants play important roles in the photosynthetic process, such as 

light-absorption and quenching of excess energy [17]. Ά-Carotene is present in most of the divisions of the reaction-center complexes (RC) and the light-harvesting complexes (LHC) of photosystem I (PSI). On the other hand, in the peripheral LHC of photosystem II (PSII), the bound carotenoids are heterogenous, depending on the classes [5]. In green algae, the major carotenoid in the peripheral LHC of PSII is siphonaxanthin, which exhibited an additional absorption band (the 535 nm band) in the blue to green region. Ά-Carotene in both RC and LHC-PSI might have protective functions, and carotenoids in the peripheral LHC of PSII might have  primarily light-harvesting functions. Employing femtosecond time-resolved fluorescence spectroscopy to analyze purified carotenoids in organic solvents and the LHC in solution, this keto-carotenoid was found to facilitate highly efficient energy transfer of carotenoids to chlorophylls [18]. Siphonaxanthin might have a largely light-harvesting function in the green light-rich underwater habitat [19]. 
