**1. Introduction**

Human skin is one of the most fundamental organs which is directly exposed to the external environment. It acts as a barrier to prevent the body from external damage, including ultraviolet (UV) irradiation exposure [1]. Although UV is beneficial to the human body to a certain extent, the excessive exposure to UV will damage the ability of basal keratinocytes, which are responsible for maintaining skin homeostasis to resist UV-induced damage, which can lead to di fferent skin diseases such as accelerated degradation of collagen, inflammatory reaction, epidermal hyperplasia and skin cancers [2,3]. Solar UV irradiation contains wavelengths from approximately 100 to 400 nm, but only UVB (280–315 nm) and UVA (315–400 nm) reach the terrestrial surface [4]. It is now commonly known that the exposure to solar UVB is the major factor causing keratinocyte damage, causing DNA mutations, which can induce photo-aging and skin cancer [5].

The UVB damages skin through stimulating the production of reactive oxygen species (ROS) [6,7]. The expression of apoptosis-related cytokines induced by excessive ROS can lead to skin oxidative damage, and cause apoptosis [8]. Therefore, inhibiting the excessive ROS generation and eliminating potential cancer-causing cells are important strategies that constitute photoprotection and inhibit apoptosis [9].

Currently, researchers are focusing on developing natural compounds to explore their protective effects on UVB-induced photodamage. Seaweed is a plant abundantly growing in the sea and is broadly distributed all over the world. It can endure strong sunlight without any structural damage, which indicates that seaweed can protect itself from photodynamic damage, including UVB irradiation [10]. Thus, many studies have been conducted to find bioactive extracts or active ingredients derived from seaweed, such as polyphenol, polysaccharide, fucoxanthin, by which the generation of UVB-induced ROS can be modulated [11–13].

*Saccharina japonica* (*S. japonica*), a brown seaweed, used to be known as *Laminaria japonica* (*L. japonica*), which is plentifully produced in Asian countries and used as a traditional Chinese medicine and functional food [14,15]. In addition, it contains plenty of biological compounds, including fucoidan. Previous studies have reported that fucoidan isolated from *S. japonica* has a variety of bioactivities, such as neuroprotective, anti-inflammation, antioxidant, antiviral, immunomodulatory, atherosclerosis mitigation and anticoagulant activities [16–21]. Some researchers have investigated the anti-UVB ability of fucoidan from brown seaweed, such as *Costaria costata*, *Fucus evanescens* and *Undaria pinnatifida* [22–24]; however, this is the first study on the anti-UVB activities of fucoidan extracted from *S. japonica*.

In our latest work, the structure and anti-inflammatory activities of a fucoidan fraction of LJSF4 from *S. japonica* were studied [25]. In this study, we further investigated its photo-protective activity. The possible mechanisms against UVB-induced photodamage were examined in vitro using human keratinocyte (HaCaT) cells and in vivo in a zebrafish model in order to provide evidence for its application as an anti-UVB agen<sup>t</sup> in cosmetics.
