SYNERGETIC EFFECT OF LIGHT AND NUTRIENTS ON THE RELEASE OF DISSLOVED ORGANIC CARBON FROM JUVENILES OF SACCHARINA JAPONICA

Macroalgae farming is an essential part of offshore carbon sink. Large-scale macroalgae farming promotes the absorption of atmospheric CO₂ by the ocean. As the primary producer of marine ecosystems, macroalgae can convert Dissolved Inorganic Carbon (DIC) into Organic Carbon (OC) through photosynthesis. Macroalgae can also release 20%—30% of photosynthesis products into seawater in the form of Dissolved Organic Carbon (DOC). According to estimates, the total amount of DOC released by macroalgae farming in China is about 822000 to 915000 tons, which can be transformed into 600000 tons of Recalcitrant Dissolved Organic Carbon (RDOC) every year through the action of Marine Microbial Carbon Pump (MCP). Therefore, seaweed culture plays a vital role in fisheries carbon sink. However, the synergistic effect of light and nutrients on DOC release by algae is still controversial. There are currently two hypotheses. One is the “overflow” hypothesis. Fogg believed that DOC release from algae was positively correlated with light intensity, which promoted the release of high molecular weight storage products. In Cherrier’s laboratory and field studies, the DOC release rate of planktonic algae was positively correlated with light intensity. After studying multiple experimental data on benthic algae, Barrón also found that the net DOC flux of benthic algae was positively correlated with light intensity. These reports show a significant correlation between light intensity and DOC release rate. Another hypothesis is “diffusion”. Bjornsen believes that the release rate of DOC from algae has nothing to do with light, but is closely related to the nutrient concentration. Nutrients promote the release of dissolved low molecules substances in algae. In Marañón’s algae culture experiment, there was no correlation between DOC release and light intensity. In the cross-experiment of light and nutrients carried out by Mueller, the correlation between DOC release of coral symbiotic algae and light disappeared when nutrients were added. The above studies show that light and nutrients are two critical environmental factors that regulate the release of DOC from algae. Saccharina japonica is the most important cluturing spcecies in China. According to statistics from the Food and Agriculture Organization of the United Nations (FAO), China had contributed 18% of the global S. japonica production in 2017.This study used juveniles of S. japonica as the research material and set up indoor crossover experiments based on the two factors of light and nutrient concentration. To explore the synergistic effect of light and nutrients on DOC release from macroalgae, we set up four experimental groups of natural sea water+50% sea surface light intensity, natural sea water+100% sea surface light intensity, and nutrient-enriched sea water+50% sea surface light intensity, nutrient-enriched sea water+100% sea surface light intensity, each group set up five parallel samples. Based on the above experimental conditions, the juveniles of S. japonica were cultured in a light incubator for 8h, and the water temperature was kept at (14±0.5)℃ during culture. The study found that under natural seawater conditions, the DOC rate of juveniles of S. japonica under 50% and 100% sea surface light intensity was (11.67±3.07) μmol/(g·h) and (22.65±4.58) μmol/(g·h), respectively. With the increase of light intensity, the DOC release rate was significantly improved (P＜0.05), a substantial increased in the net oxygen release rate (P＜0.05), and the difference of light spectrum slope was significant (P＜0.05). Under the condition of nutrient-enriched seawater, the DOC release rate of juveniles of S. japonica under 50% and 100% sea surface light intensity was (30.88±7.96) μmol/(g·h) and (39.03±14.78) μmol/(g·h). With the increase of light intensity, the DOC release rate was not significant improved (P＞0.05), the net oxygen release rate was significantly increased by 79.24% (P＜0.05), and there was no significant difference in light spectrum slope (P＞0.05).The results showed that under oligotrophic conditions (natural seawater), the DOC release rate of juveniles of S. japonica was positively correlated with the light intensity, indicating a “spillover” mechanism. Under eutrophic conditions (enriched seawater), the DOC release rate of juveniles of S. japonica was not related to the light intensity, which showed a “Diffusion” mechanism. The release of DOC from juveniles of S. japonica is regulated by two mechanisms, “spillover” and “diffusion”, which mechanism was dominant depends on the nutrient concentration.