Study On The Diversity Of Nanoflagellates And Its Impact Factors In The Northern South China Sea

Marine nanoflagellate(NF)is a significant part of microbial food loop,with a wide range of distribution in the ocean.In this research,we studied the biodiversity and dynamic changes of NF's bio-community structure in four seasons in the northern South China Sea.We investigated the temporal and spatial distribution of dinoflagellates in the northern South China Sea by using denaturing gradient gel electrophoresis(DGGE)and constructed clone libraries of nanoflagellates in SEATS station(South East Asia Timeseries Station).Meanwhile,the relationships between community structure and environmental factors were analyzed.The main results were as follows:(1)In total,we cut 96 bands from DGGE gel that had been successfully sequenced,including 51 bands of Gymnodiniales,18 bands of Peridiniales,11 bands of Syndiniales 4 bands of Gonyaulacales and 9 bands of unclassified dinophyceae.(2)The change of quantity of bands in DGGE profiles represented the dynamic changes of community composition.The quantities of bands of dinoflagellates in northern South China Sea had spatial and temporal difference with the highest number in winter(19 bands in average)and lowest in spring(9 bands in average).The results showed that there were more species of dinoflagellates in the northern South China Sea in winter,but fewer species in spring.Horizontally,the number of bands from spring samples increased gradually with the increase in distance from the shore.In summer,the highest number of bands were in LE section(17 bands in average)while the less number were in A section(5 bands in average).The samples in autumn were few,mainly in surface,and there was no obvious difference of bands quantity among different samples.The relatively homogeneous distributions of the bands quantity were found in different samples in winter.Also,the trend was similar with that in spring(A?C?E6?S4 section).In the other hand,the changes of the quantity of bands were more complicated from the vertical distribution.(3)In this study,the variation ranges of Shannon index of dinoflagellates in the four seasons in northern South China Sea were 0.67?2.68,1.93?2.98,1.43?1.80,2.34?2.97 respectively.The trend of Shannon average index:winter>summer>spring>autumn.Different sections have different trends in Shannon index in different seasons.Specifically,in spring,Shannon index in Section A decreased firstly and increased subsequently with the increase in distance from the shore,while it increased continually in section S4.In summer,there were no obvious changes of Shannon index among different stations in section A(from 1.93 to 2.17).Higher Shannon index were observed in the two stations in section E4 without big difference.S101a station was affected by Pearl River Diluted Water and runoff,so the Shannon index was relatively low(2.08).On the contrary,two stations in section LE showed higher Shannon index due to farther distance from the shore.The samples in autumn were few and there was no obvious difference of Shannon index among different samples.In winter,the trend was accordant with that of band quantities in the third point(A?E6?S4 station except C station).Shannon index in station C was lower in station C3(2.73)and higher in station C7(2.90),with highest in station QD02(2.97),which showed an opposite trend compared with other three sections.As the changes of band quantity,the changes of Shannon index were complicated from the vertical distribution.(4)The biodiversity of nano-plankton's 18S rDNA clone library varied largely among different seasons in SEATS station in the South China Sea.The highest Shannon diversity index was found in autumn,then summer and winter,while the lowest was in spring.The dominant species were different among seasons.Generally,Dinophyceae(24%of total abundance)and Chlorophyta(14%of total abundance)were primary compositions in the community,with Dinophyceae dominating in the spring and summer,Chlorophyta dominating in the autumn and winter.We obtained eighty effective sequences of NF using BLAST in the NCBI database and found that Dinophyceae,Prasinophyceae and Haptophyceae were three main flagella groups in the clone library during the study period.Dinophyceae accounted for 68%of NF community.In spring,heterotrophic NF(HNF)was more diverse than autotrophic NF.In the other three seasons there were more autotrophic species than HNF.The diversity of autotrophic species gradually increased with seasons while HNF decreased.(5)After the correlation analysis,we found that there was no significant correlation between Shannon diversity index of clone libraries and environmental factors,but the correlations between Shannon index and the abundance of cyanobacteria and pico-phytoplankton were obvious,which showed a high effect of food concentration on diversity.The Shannon index of eukaryotic microplankton increased with time from spring to autumn,coming to a maximum in autumn and then decreased in winter.The completely opposite trend was observed in the Shannon index of bacteria,decreasing from spring to autumn and then increasing in winter.For dinoflagellates in our study area,there was no significant correlation between the diversity of dinoflagellate and most environmental factores.In spring,there was highly negative correlation between Shannon index of dinoflagellates and abundance of cyanobacteria.In summer,the significant negative correlation was found between Shannon index of dinoflagellates and abundance of heterotrophic bacteria.No significant correlation was found in autumn and in winter,illustrating that the dynamic changes of dinoflagellate's community structure may mainly affected by biotic factors.