Abstract:Global warming caused by human CO2 emissions has endangered the sustainable development of human society. To deal with global warming, the United Nations General Assembly formulated the “United Nations Framework Convention on Climate Change” in 1992, integrating the control of CO2 and other greenhouse gas emissions and the adverse effects of global warming on human economy and society into the framework of international law. As a responsible country, China committed to achieving a carbon peak by 2030 and carbon neutrality by 2060 at the 75th UN General Assembly in September 2020. However, it is limited to achieving carbon neutralization only by controlling CO2 emissions; the most effective way to achieve this is to reduce emissions while increasing foreign exchange. As the largest carbon pool in the world, the ocean plays an essential role in regulating global climate change and has significant potential to increase sinks. To increase the ocean sink, scientists put forward the “blue carbon” strategy: the restoration and protection of mangroves, salt marsh wetland, seagrass bed, and other three types of coastal ecosystems, to strengthen the organic carbon burial, and slow down climate warming. As most macroalgae grow in a batholith environment, the carbon burial process is hindered, and they are excluded from the “blue carbon” system. However, macroalgae also have a strong carbon sink. The algae-released dissolved organic carbon (DOC) can be transformed into refractory DOC (RDOC) by the action of a microbial carbon pump (MCP). Owing to the stable chemical properties of RDOC, carbon sequestration can be achieved on a millennium scale, thus effectively alleviating global warming. Aquaculture activity is highly controllable. If we can increase the carbon sink through algae cultivation, we can achieve a win-win situation of economic and ecological benefits. Preliminary estimates show that the annual DOC release of cultured algae in China has reached 82.2×104~91.5×104 t C/a. The DOC released by cultured algae can generate more than 600,000 tons of RDOC per year through MCPs, approximately 1.7 times the carbon storage capacity of “blue carbon” in China´s coastal zone, with significant carbon sink potential. The DOC released by macroalgae is an important part of the offshore DOC pool, accounting for approximately 20% of the offshore DOC pool. However, the environmental regulation mechanism of DOC released by macroalgae remains controversial. To investigate the effects of light and nutrients on the release of DOC from young S. japonica seedlings, the release rate of DOC in young S. japonica seedlings was measured under different light intensities [0, 83, 165, and 250 μmol photons/(m2·s)] and different nutrient conditions (nitrogen enrichment, phosphorus enrichment, co-enrichment, and natural seawater). Two hypotheses of DOC release from algae, the “overflow” and “diffusion” hypotheses, were tested. According to the “overflow” hypothesis, DOC released by algae is positively correlated with light intensity, and its components are mainly high molecular weight substances; according to the “diffusion” hypothesis, the release of DOC from algae is positively correlated with nutrients and mainly low molecular weight substances. The experimental method is as follows: the first part is the illumination experiment. Four light intensities of 0, 83, 165, and 250 μmol photons/(m2·s) were set. Six parallel light intensities were used for each light intensity. Each parall楥獬†晳潡牭⁰歬敥氠灷捳甠汰瑬楡癣慥瑤椠潩湮⸠ 2-L glass bottle, and the S. japonica young seedlings without loss were cultured in a light incubator at different light intensities for 6 to 8 h. At the same time, three parallel samples without S. japonica young seedlings were set in each light treatment as a blank control. Water samples (100 mL) were collected before and after culture; one for the determination of DOC content and absorption spectrum and one for the determination of dissolved oxygen (DO). The second was a nutrient experiment, in which four experimental groups were set up: nitrogen enrichment, phosphorus enrichment, co-enrichment, and natural seawater. Standard solutions of 100 and 1000 μg/L were prepared with potassium monohydrogen phosphate and potassium nitrate, respectively, for enrichment. The light intensity was 250 μmol photons/(m2·s). Other treatments were the same as those described above. The results showed that the DOC release rate of young S. japonica seedlings was positively correlated with light under natural seawater conditions. The highest value is (24.31±5.84) μmol/(g·h) in the 250 μmol photons/(m2·s) light condition, about four times that in the dark. The single nitrogen and phosphorus enrichment condition had no significant effect on the release of DOC in the young S. japonica seedling, with releasing rate of (23.04±4.24) μmol/(g·h) and (18.18±4.59) μmol/(g·h), respectively. The co-enrichment of nitrogen and phosphorus significantly increased the DOC releasing rate of young S. japonica seedlings to (37.15±6.77) μmol/(g·h), about three times that in natural seawater. In conclusion, there are likely two regulatory mechanisms of “overflow” and “diffusion” in the release of DOC from young S. japonica seedlings. Under the oligotrophic condition, the “overflow” mechanism is dominant; under the eutrophication condition, the “diffusion” mechanism is dominant. The significance of this study is to clarify the environmental regulation mechanism of DOC release from young S. japonica seedlings and provide a scientific bas
