Search

Diatoms are one of the most widespread groups of microalgae on Earth. They possess extraordinary metabolic capabilities, including a great 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 suggest 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 great interest for their biological activities in human model systems.

Diatoms are one of the most widespread groups of microalgae on Earth. They possess extraordinary metabolic capabilities, including a great 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 suggest 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 great interest for their biological activities in human model systems.

Photosynthesis is known to produce reactive oxygen species together with the transformation of light into biochemical energy. To fill the gap of the knowledge on the protective antioxidant network of microalgae, a series of experiments to explore the role of spectral composition and intensity of light in the modulation of the photodefence mechanisms developed by the coastal diatom Skeletonema marinoi were performed. The modulation of the total phenolic content, ascorbic acid and the enzymes glutathione reductase, catalase, ascorbate peroxidase and superoxide dismutase together with xanthophyll cycle and non-photochemical quenching in response to variations in the light environment were analysed. Most of the enzymes' activity was promptly affected by the red light. Yet, the monochromatic high intensity blue light enhanced the synthesis of total phenolic content and ascorbic acid in parallel to the xanthophyll cycle activity. This study reveals the dual effects of spectral composition and intensity of light on the modulation of photoprotective mechanisms. Diatoms developed a complementary and/or alternative tuning processes to cope with the variable light environment they experience in the water column. They also provided valuable insights into light manipulation regimes for diatom cultivation that will help to maximize production of bioactive molecules.

The ecological success of diatoms is mostly attributed to their ability to adjust biological performances to the variable environmental conditions they experience in the water column. While the efficient photoacclimative and photoprotective mechanisms in diatoms are already reported, the involvement of the antioxidant network in lowering or repairing light stress or damage is poorly understood. My thesis consists of the concomitant investigation of the photoprotective and antioxidant network functioning in the coastal diatom Skeletonema marinoi, and during an experiment at sea in the Gulf of Naples. The aims of my work are to explore the antioxidant network in diatoms to better understand its activation in coping with light variation, and the functional link between antioxidant molecules synthesis/activity and photoprotection. Results showed the spectral light dependent activity of antioxidant enzymes such as ascorbate peroxidase, catalase, and superoxide dismutase. They act in complementarity of the antioxidant molecules synthesis. A high concentration of ascorbic acid, phenolics compounds and among them flavonoids were found in diatoms. These molecules respond to light variations in terms of spectral composition, photon flux density, daily light dose, light shape distribution and photoperiod duration. Also, in a natural microalgal community, these molecules were found at high concentration following a dynamics that relied on light, nutrient stress and photosynthetic regulation. The photoprotective xanthophyll cycle, involving the pigments diatoxanthin and diadinoxanthin, is activated by light and modulated in concordance with antioxidant molecules synthesis. Indeed, under different light climates, a link between these two defense processes was found. I also show that the xanthophyll cycle pigments have a high antioxidant activity. This feature explains why they increase together with antioxidant molec

The ecological success of diatoms is mostly attributed to their ability to adjust biological performances to the variable environmental conditions they experience in the water column. While the efficient photoacclimative and photoprotective mechanisms in diatoms are already reported, the involvement of the antioxidant network in lowering or repairing light stress or damage is poorly understood. My thesis consists of the concomitant investigation of the photoprotective and antioxidant network functioning in the coastal diatom Skeletonema marinoi, and during an experiment at sea in the Gulf of Naples. The aims of my work are to explore the antioxidant network in diatoms to better understand its activation in coping with light variation, and the functional link between antioxidant molecules synthesis/activity and photoprotection. Results showed the spectral light dependent activity of antioxidant enzymes such as ascorbate peroxidase, catalase, and superoxide dismutase. They act in complementarity of the antioxidant molecules synthesis. A high concentration of ascorbic acid, phenolics compounds and among them flavonoids were found in diatoms. These molecules respond to light variations in terms of spectral composition, photon flux density, daily light dose, light shape distribution and photoperiod duration. Also, in a natural microalgal community, these molecules were found at high concentration following a dynamics that relied on light, nutrient stress and photosynthetic regulation. The photoprotective xanthophyll cycle, involving the pigments diatoxanthin and diadinoxanthin, is activated by light and modulated in concordance with antioxidant molecules synthesis. Indeed, under different light climates, a link between these two defense processes was found. I also show that the xanthophyll cycle pigments have a high antioxidant activity. This feature explains why they increase together with antioxidant molec