Sulfur metabolism and response to light in Ulva prolifera green tides

The outbreak of Ulva prolifera blooms causes significant changes in the coastal sulfur cycle due to the high production of dimethylsulfoniopropionate (DMSP) and the emission of dimethylsulfide (DMS). However, the sulfur metabolism mechanism of U. prolifera has not been thoroughly investigated. In this study, we examined the levels of intracellular and extracellular sulfate (SO₄^2--S), total sulfur (TS), DMSP, and DMS in fresh U. prolifera under different light intensity conditions (54, 108 and 162 μmol photons m^-2 s^-1) during algal growth. We also conducted transcriptome analyses to investigate sulfur uptake and metabolism. When the light intensity increased by 50% (from 108 to 162 μmol photons m^-2 s^-1), the amount of absorbed SO₄^2--S increased by 3.5 times after 24 h, while the fresh weight of U. prolifera increased by 16%, and the average release rates of DMS and DMSP increased by 136% and 100%, respectively. However, the expression of sulfate transporter and assimilation-related genes did not show significant up- or down-regulation in response to the light intensity changes. Therefore, it is speculated that the key gene responsible for DMSP synthesis in U. prolifera has not yet been identified. The sulfate metabolic pathway of U. prolifera was established, and four Alma genes, including DMSP lyase, were identified. During the bloom period, it is estimated that U. prolifera releases a maximum of approximately 0.4 tons of sulfur and 0.3 tons of carbon in the form of DMS into the atmosphere per day. Additionally, biogenic sulfur dissolved in seawater or within algae could potentially impact the regional climate and environment.