Distribution Of Biogenic Elements And Germinated Phytoplankton Community Structure In The Surface Sediments From The Beibu Gulf Rim

Forty-eight surface sediment samples were collected from seven sea areas of the Beibu Gulf Rim,including Zhangjiang Port in the west Guangdong coast,Haikou Port,Xinhai Port,Yangpu Port,and Basuo Port in northwest Hainan coast,Fangcheng Port and Beihai Port in Guangxi coast.The distribution of biogenic elements including total organic carbon?TOC?,total nitrogen?TN?,total phosphate?TP?and biogenic silica?BSi?were analyzed.The potential ecological risks of biogenic elements were assessed to understand pollution status of biogenic elements in the Beibu Gulf Rim.Resting cells in surface sediments from these seven sea areas were incubated under different conditions,and germinated phytoplankton were analyzed under microscopic observation.Furthermore genomic analyses of 18S rDNA gene V4 region were undertaken using high-throughput sequencing.The purpose of this study was to understand the contribution of resting cells in sediments to the vegetative population in the upper water column,and to discuss the potential of the occurrence of harmful algal blooms?HABs?in the Beibu Gulf Rim coastal waters.The results of this study provide a scientific basis for the prevention and controlling of HABs in the Beibu Gulf Rim and also for the protection of marine environments.Main results are as followings:1)The concentrations of biogenic elements were 0.27%-1.64%for TOC,0.09%-0.25%for TN,0.02%-0.05%for TP,and 0.55%-4.56%for BSi,respectively.Biogenic elements were higher in Zhanjiang Port,and followed by Beihai Port and Fangcheng Port.Concentrations of biogenic elements were comparable in four ports of Hainan Coast.The spatial distribution of biogenic elements varied in different ports,for example,biogenic elements were higher in the inshore areas than the offshore areas in Haikou Port.While the distribution of biogenic elements were affected by shipping,and were higher in the shipping channel in Xinhai Port.However,concentrations were higher in the outer bay areas than in the central bay in Fangcheng Port.2)TOC in the Beibu Gulf Rim was mainly originated from marine phytoplankton,and some stations were influenced by the terrestrial organic carbon based on the molar ratios of TOC/TN.Significant coefficients among biogenic elements indicated their similar origins.The pollution of TN in all stations belonged to Class II pollution standard,and the TOC contents in most stations belonged to Class II as well,which might cause certain potential harm to benthic organisms.TP belonged to the uncontaminated level.The pollution of biogenic elments were more severe in Zhangjiang Port,Beihai Port and Basuo Port.3)A total of 74 taxa belonging to 52 genera of 7 phyla were identified in germinated phytoplankton based on the microscopic observation.Within them,there were 33 species of diatoms accounting for 45%of the total species richness,30 species of dinoflagellates accounting for 40%,3 species of chrysophytes,3 species of chlorophytas and 5 species of other groups including haptophyta,dictyochophyceae and cyanobacteria.The species richness showed no significant differences among the five different treatments.Generally species richness was higher under the treatment of25°C+Si,and diatoms and dinoflagellates were the predominant groups.However,species richness decreased under the treatment of 25°C+GeO₂ as the decreasing of diatoms.On the contrary,the species numbers of dinoflagellates and other groups increased at this treatment.Some species,which have not been reported in study areas in previous surveys,were observed in the germinated phytoplankton.4)Cell densities of germinated phytoplankton in the Beibu Gulf Rim ranged between 3.69×10⁴ and 3.31×10⁶ cells/cm³.In general,cell densities were higher at 25°C treatment,and higher in Zhangjiang Port and Xinhai Port as well.Diatoms were higher in cultures adding Si.The growth of diatoms were inhibited in cultures with GeO₂,however the growth of thoses in dinoflagellates,chrysophyta,and chlorophyta were accelerated.Speices diversities of germintated phytoplankton were generally low in this study,with the values of Shannon-Wiener species diversity index of 0.02-2.38.The low species diversity and high predominant index indicated unstability of germinated phytoplankton communuty.5)Genomic sequencing was undertaken at treatments of 25°C+Si and25°C+GeO₂ based on 18S rDNA V4 region.After annotation of sequences of all OTUs,a total of 46 eukaryotic algal taxa belonging to 36 genera of 7 phylum were identified,including bacillariophyta,dinoflagellates,chrysophyta,and chlorophyta etc.Eighteen species were recorded in all of the three sea areas,the western Guangdong coast,the northwest Hainan coast,and the Guangxi coast.Chrysophytes and diatoms were the predominant groups in the treatments with Si.Haptophytes predominated in the treatment adding GeO₂,followed by dinoflagellates and chlorophytes.The results suggested that GeO₂ had the ability to inhibit the growth of diatoms and chrysophytes,however enhanced the growth of non-diatom species.6)Diatoms and dinoflagellates were the predominant groups of phytoplankton based on microscopic observation,and most taxa could be identified only at genus level.However,most taxa could be annotated at species level based on genome sequencing,and proportions of haptophytes and dinoflagellates increased.A lot of pico-to nano-sized phytoplankton were recorded to occur abundantly in germinated phytoplankton based on genome sequencing,such as Picochlorum eukaryotum in Trebouxiophyceae,Micromonas pusilla in Mamiellophyceae,and Paraphysomonas butcheri and Pedospumella encystans in Chrysophyceae.The results suggested that more genetic and species information could be obtained from genome sequencing,and thus enrich phytoplankton information especially those of nanno-and pico-sized phytoplankton based on genome sequencing.