Improving the nutritional value of edible Caulerpa lentillifera (Chlorophyta) using high light intensities. A realistic tool for sea grape farmers

Edible sea grapes (Caulerpa lentillifera) are produced in shaded ponds (~ 50 μmol photons m⁻² s⁻¹) and are of increasing demand for direct human consumption. The controlled exposure to light-stress after harvest could increase algae's content of nutritionally valuable antioxidants, including phenols. In order to define a light-stress tool for farmers to use, we investigated the effect of five irradiances (50–600 μmol photons m⁻² s⁻¹) for an exposure time of up to 14 days. Antioxidant activity (AOA, initial of 150.93 ± 25.50 mmol Trolox equivalents 100 g⁻¹ DW) and total phenolic content (TPC, initial of 124.45 ± 10.07 mg Gallic acid equivalents 100 g⁻¹ DW) significantly increased to values of up to 228.8 ± 12.4 and 222.2 ± 22.7 % of initial, respectively. However, targeted increases in antioxidant parameters correlated with decreased values of Fᵥ/Fₘ (p < 0.001) and the chlorophyll (Chl) a content in the edible frond was significantly lower at irradiances ≥400 μmol photons m⁻² s⁻¹ compared to the control (50 μmol photons m⁻² s⁻¹), causing bleaching. As the physiological response of the alga (Fᵥ/Fₘ, AOA, TPC, Chl a) depended on exposure time and irradiance treatment, both parameters could be adjusted to define treatments according to the consumer's needs. Light irradiances measured at a sea grape farm revealed that farmers could integrate such treatments labour- and cost-effectively by removing layers of plastic-meshes off their culture or post-harvest units. The pattern of Chl a loss and red (R) colour channel values in sea grapes thallus parts implied, that a combination of degradation and chloroplast migration was responsible for bleaching at light-stress. Additionally, R channel values extracted from pictures could be used for Chl a estimation, due to strong correlation (rS = −0.786, p < 0.001). In conclusion farmers can use high light as post-harvest treatment in order to increase valuable antioxidants.