*Article* **Insights into the Light Response of** *Skeletonema marinoi***: Involvement of Ovothiol**

### **Alfonsina Milito 1,2,\*, Ida Orefice 3, Arianna Smerilli 3, Immacolata Castellano 1, Alessandra Napolitano 4, Christophe Brunet 3 and Anna Palumbo 1,\***


Received: 24 July 2020; Accepted: 17 September 2020; Published: 20 September 2020

**Abstract:** Diatoms are one of the most widespread groups of microalgae on Earth. They possess extraordinary metabolic capabilities, including a grea<sup>t</sup> ability to adapt to different light conditions. Recently, we have discovered that the diatom *Skeletonema marinoi* produces the natural antioxidant ovothiol B, until then identified only in clams. In this study, we investigated the light-dependent modulation of ovothiol biosynthesis in *S. marinoi*. Diatoms were exposed to different light conditions, ranging from prolonged darkness to low or high light, also differing in the velocity of intensity increase (sinusoidal *versus* square-wave distribution). The expression of the gene encoding the key ovothiol biosynthetic enzyme, *ovoA*, was upregulated by high sinusoidal light mimicking natural conditions. Under this situation higher levels of reactive oxygen species and nitric oxide as well as ovothiol and glutathione increase were detected. No ovoA modulation was observed under prolonged darkness nor low sinusoidal light. Unnatural conditions such as continuous square-wave light induced a very high oxidative stress leading to a drop in cell growth, without enhancing ovoA gene expression. Only one of the inducible forms of nitric oxide synthase, *nos2*, was upregulated by light with consequent production of NO under sinusoidal light and darkness conditions. Our data sugges<sup>t</sup> that ovothiol biosynthesis is triggered by a combined light stress caused by natural distribution and increased photon flux density, with no influence from the daily light dose. These results open new perspectives for the biotechnological production of ovothiols, which are receiving a grea<sup>t</sup> interest for their biological activities in human model systems.

**Keywords:** algae; antioxidant; diatoms; light; nitric oxide; ovothiol; oxidative stress
