Preliminary Study On Biological Silicon Production In Diatom

In this thesis,two types of experiments were conducted in lab and in situ for biological silicon production in diatoms.Through laboratory culture experiments,we recorded the growth curves,BSi and PDMPO of the dominant coastal diatom(Skeletonema dohrnii)and two dominant oceanic species(Thalassiosira sp.and Pseudo-nitzschia sp.).Meanwhile,the quantitative relationship of BSi:PDMPO of them at different temperatures and silicate(Si)concentrations were obtained.In addition,we determined the BSi and PMDPO values of size-fractionated diatoms in the western Pacific Ocean,and explored their significance in the silicon cycle.In culture experiments,different temperatures and Si concentrations had different effects on the growth of Skeletonema dohrnii,Thalassiosira sp.and Pseudo-nitzschia sp.High temperature would promote the growth rate of them,while low temperature would inhibit the growth rate of these three microalga.At the same temperature,the growth trends of these three species were almost similar under different Si concentrations,however,the abundances of them were not similar at different Si concentrations.In general,the BSi contents of three species showed a rising trend during the whole growth process at different temperatures and Si concentrations.Since low temperature would limit the growth of diatoms,the BSi production in diatoms would be limited.The Si concentrations also affected the process of Si accumulation in diatoms.When the concentration of Si was high,it would promote the process of Si accumulation in diatoms.The production of BSi would be higher than that at low Si concentration.PDMPO was an important index to characterize the production rate of BSi,and its growth trend was similar to that of the BSi curve measured by the thermokalite method.However,the BSi and PDMPO increments of these three diatoms were different Pseudo-nitzschia sp.>Thalassiosira sp.>and Skeletonema dohrnii.It is speculated that the production of BSi in oceanic waters was higher than in near-shore.6 stations were studied in the western Pacific Ocean for in situ experiments.A total of 206 species of phytoplankton species were identified,including 72 species of diatoms,129 species of dinoflagellates,2 species of Chrysophyta,and 3 species of Cyanophyta.Dinoflagellates were the dominant part of species diversity,while diatoms were the second diverse group.Additionally,diatoms have significantly higher cell abundance than dinoflagellates.In order to facilitate the study of cell sizes in different species groups,the size-fractionated diatoms were converted to equivalent spheres,among which 36 diatoms were 2-20 ?m;36 diatoms>20?m.The diatoms with cell diameter<20 ? m were mainly composed of chain-like algae,such as Thalassiosira subtilis,Thalassiosira minima and Thalassiosira rotula,while the diatoms>20 ?m were mostly unicellular algae,such as Coscinodiscus subtilis,Coscinodiscus marginato-lineatus and Coscinodiscus apiculatus.According to BSi production,picophytoplankton(<2 ?m)accounted for an absolute advantage.The BSi contents in DCM layer were significantly higher than that in the surface layer,which might be due to the silicification of picocyanobacteria.The micro/nanophytoplankton(>2 ?m)had similar water layer distributions with the picophytoplankton.The surfac and bottom were relatively low,while the high concentration of BSi was mainly concentrated in the DCM layer.The vertical distributions were mainly affected by temperature,light and predation.In addition,there were some differences in BSi between different stations in of the western Pacific Ocean,and these differences might be influenced by the different Si concentrations among stations.The BSi detected by PDMPO revealed that the concentrations of BSi had different vertical distribution,and this differences were mainly affected by the shifts of abundance of phytoplankton in different water layers.Finally,we could determine the contents of BSi in size-fractionated diatoms.The main contributions of BSi in marine of 2-20 ?m phytoplankton were Thalassiosira subtilis,and the main contribution of BSi in>20 ?m phytoplankton were Coscinodiscus subtilis.