Difference In Carbon Metabolism Between Gametophyte And Sporophyte Of Pyropia Haitanensis And Their Responses To Ocean Acidification

Despite life-cycle of the red seaweed Pyropia/Porphyra has been extensively studied,the photosynthesis and CO₂ concentrating mechanism（CCM）have not been well comparatively studied between gametophyte phase（leafy thallus）and sporophyte phase（filamentous conchocelis）to date.In addition,the inorganic carbon from seawater was used as sources for Pyropia to produce organic carbon.Part of the organic carbon become the main component of the algal tissue,and the left was released into the seawater as particulate organic matter.The large scale of seaweed farming is an important part of the coastal ecosystem.Therefore,the release of particulate organic matter from the Pyropia may have important ecological significance,but relative research is still blank.In addition,the seawater acidification is one of the major threats to marine ecosystems.It is necessary to assess the effects of seawater acidification on the carbon budget of Pyropia,including the absorption and utilization of inorganic carbon,and the release of particulate organic carbon,etc.（1）This study analyzes the generational differences in Pyropia carbon acquisition,found that the net photosynthetic rates of thalli were about 2 times of that of conchocelis of Pyropia,and the maximal quantum yield of PSII（Fv/Fm）and respiration of thalli were significantly higher and lower than conchocelis,respectively.Additionally,two phases did not shown significant difference on phycoerythrin（PE）,but thalli exhibited significant higher contents on phycocyanin（PC）,allophycocyanin（APC）and chlorophyllαcompared with conchocelis.The activity of external carbonic anhydrase（eCA）of thalli were also high than that of conchocelis,but no difference was detected on internal CA（iCA）between two phases.As for the carbon acquisition for photosynthesis,the major carbon source of thalli were seawater HCO₃^–which were absorbed via eCA-catalysed conversion to CO₂;but conchocelis applied HCO₃^–transporter to directly absorb seawater HCO₃^–which contributed equally to photosynthesis compared with the eCA or iCA mediated pathway.（2）Through short-term training experiments,this study also analyzes the generational differences of Pyropia carbon budgets under different pH conditions,found that as the pH increased from 7.3 to 8.1,the relative growth rate of the thalli gradually decreased,while the relative growth rate of the conchocelis remained stable.At the same time,the absorption rate of carbon,nitrogen and phosphorus in the thalli algae tissue gradually decreased with the increase of pH.The particulate organic matter content released by the thalli gradually increases with increasing pH.At pH 7.3,the C/N of the thalli algae tissue is 33%higher than that of the conchocelis,and the C/N of the thalli algae tissue is 39%higher than the conchocelis at pH 7.7,and at pH 8.1 the C/N of thalli algae tissue is 42%higher than the conchocelis.But the difference between the two generation in the C/P ratio is small.At pH 7.3,the conchocelis algae tissue N/P is 55%higher than the thalli tissue.At pH 7.7,the conchocelis algae tissue N/P is 56%higher than the thalli.At pH pH 8.1 the conchocelis algae tissue N/P is 45%higher than the thalli.Therefore,there is a significant difference in the demand for elements in Pyropia in different generational,and more carbon and phosphorus are enriched in the thalli,and the demand for nitrogen is small.The ratio of C/N in algae tissue is much higher than that of particulate organic matter,which tends to excrete nitrogen during growth.The N/P ratio and C/P ratio of algal tissues are much higher than those of particulate organic matter,which tends to enrich carbon during the growth process,releasing nitrogen and phosphorus.In this study,from the perspective of carbon acquisition,the differences between different generations of Pyropia were analyzed.The results showed that there were significant differences in photosynthetic carbon fixation rate and inorganic carbon absorption and utilization between thalli and conchocelis.In addition,there were significant differences in the element contents and rations between the thalli and conchocelis.The C/N,N/P and C/P ratios of the algae tissues of both generations were higher than those of the particulate organic matter.Thus,the ecological and biogeochemical effects of two phases will be significantly different.