| Macroalgae can release part of photosynthesis products into water in the form of Dissolved Organic Carbon（DOC）.This part of DOC can be converted into Recalcitrant DOC（RDOC）by microbiological carbon pump,which is an important part of algal carbon sink.The effects and mechanisms of environmental factors such as light intensity,nutrients and temperature on the release of DOC from macroalgae are still less studied.Therefore,this paper investigates the effect of algae on the release of DOC by studying the effects of different environmental factors on the release of DOC from juveniles of S.japonica.By measuring different light intensities(15,45,75 and 105μmol photons·m-2·s-1),different nutrient conditions（nitrogen enrichment,phosphorus enrichment,co-enrichment,and natural seawater）and different temperatures（5,10,15and 20°C）,the release rate of DOC from juveniles of S.japonica was investigated,and the effects of light,temperature and nutrients on the release of DOC from juveniles of S.japonica were explored.By comparing the juveniles of S.japonica under four conditions:natural seawater+50%sea surface light intensity,natural sea water+100%sea surface light intensity,nutrient-enriched seawater+50%sea surface light intensity,and nutrient-enriched seawater+100%sea surface light intensity The release rate of DOC was investigated to explore the effect of the synergistic effect of light and nitrogen and phosphorus nutrients on the release of DOC from juveniles of S.japonica,in order to reveal the mechanism of the synergistic effect of light and nutrients on the release of DOC from macroalgae.At the same time,through the metabolomic study from juveniles of S.japonica in different breeding environments,the influence of environmental factors on the metabolites of S.japonica was observed.1)Effects of light intensity and nutrients on Dissolved Organic Carbon released from juveniles of S.japonica.The results showed that the DOC release rate from juveniles of S.japonica was positively correlated with light under natural seawater conditions.The highest value is24.31±5.84μmol·g-1·h-1in the 105μmol photons·m-2·s-1light 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 juveniles of S.japonica,with releasing rate of 23.04±4.24μmol·g-1·h-1and 18.18±4.59μmol·g-1·h-1,respectively.The co-enrichment of nitrogen and phosphorus significantly increased the DOC releasing rate of young S.japonica seedlings to 37.15±6.77μmol·g-1·h-1,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 juveniles of S.japonica.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 juveniles of S.japonica and provide a scientific basis for kelp cultivation.2)Synergetic effect of light and nutrients on the release of dissloved organic carbon from juveniles of S.japonica.In the study,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 8 hours,and the water temperature was kept at 14±0.5℃during culture.The study found that under natural seawater conditions,the DOC rate from juveniles of S.japonica under 50%and 100%sea surface light intensity was 11.67±3.07μmol·g-1·h-1and 22.65±4.58μmol·g-1·h-1,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 from juveniles of S.japonica under 50%and 100%sea surface light intensity was 30.88±7.96μmol·g-1·h-1and 39.03±14.78μmol·g-1·h-1.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 from juveniles of S.japonica was positively correlated with the light intensity,indicating a“spillover”mechanism.Under eutrophic conditions（enriched seawater）,the DOC release rate from 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.3)Effects of temperature on the release of dissolved organic carbon from juveniles of S.japonica.The results showed that within the optimum growth temperature range,the DOC release rate from juveniles of S.japonica was positively correlated with temperature（P<0.05）,and the DOC release rate from juveniles of S.japonica reached the highest when the temperature was 15℃,which was 26.18±2.43μmol·g-1·h-1,at the same time,with the increase of temperature,the oxygen release rate from juveniles of S.japonica gradually increased（P<0.05）,and the PER value also changed significantly（P<0.05）,indicating that temperature changes would promote the release of DOC from juveniles of S.japonica.However,when the temperature exceeded the optimum growth temperature range,the DOC release rate decreased significantly,reaching a minimum value of 10.14±3.63μmol·g-1·h-1,and the oxygen release rate also decreased significantly,indicating that temperature stress will inhibit the photosynthesis of algae effect,thereby affecting the release of algal DOC.4)Omic analysis of the effects of different environmental factors on the metabolites from juveniles of S.japonica.Studies have shown that the metabolites produced by juveniles of S.japonica under different conditions are the same,and most of the differential metabolites are lipids and lipid molecules,amino acids and benzene rings and their derivatives.There are 10,11,22 and 2 differential metabolites in juveniles of S.japonica in light intensity experiment,nutrient experiment,synergistic effect of light intensity and nutrient and temperature experiment,respectively.Among them,lipids such as terpineol and tridecanedioic acid Substances are up-regulated to varying degrees with the increase of light intensity and temperature,but down-regulated when the concentration of nutrient salts increases.In addition,amino acids and benzene ring substances are up-regulated when the concentration of nutrient salts increases.Among them,under the N-enriched condition,the up-regulation of amino acids was the most obvious. |
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