The Ecological Distribution Pattern Of Synechococcus In The Yellow Sea And Its Interaction With Heterotrophic Bacteria

Synechococcus is a widely distributed photosynthetic pico-phytoplankton,which contributes mainly to carbon fixation and maintains the stability of the marine ecosystem.The Yellow Sea is a typical semi-enclosed marginal sea with complex seasonal variations.The Yellow Sea Cold Water Mass（YSCWM）is an important and prominent hydrological phenomenon,which has an important impact on the distribution of planktonic microorganisms.In the past,it has been found that Synechococcus contributes primarily to total phytoplankton biomass in the Yellow Sea.However,recent studies on Synechococcus in the Yellow Sea have mainly focused on its abundance distribution,but rarely on its phylogenetic composition and its environmental drivers.The symbiotic relationship is created between Synechococcus and its phycosphere bacteria through reciprocal and complementary functions.The interaction of the two can promote the production of recalcitrant dissolved organic carbon and eventually form recalcitrant material storage,which plays an important role in marine carbon sequestration.Nevertheless,the composition of the stable symbiotic bacterial community of Synechococcus and metabolic complementarity is still unknown.Therefore,this study collected samples from the Yellow Sea in September 2018,analyzed the abundance distribution of Synechococcus using flow cytometry,and explored the phylogenetic diversity of Synechococcus in the Yellow Sea based on the high-throughput sequencing technology of rpo C1 specific primer amplification.In addition,the correspondence between the phylogenetic branches of Synechococcus and environmental factors was studied through redundancy analysis（RDA）.The culture system of Synechococcus and its stable symbiotic bacteria was established by antibiotic treatment.In order to study the characteristics of the symbiotic bacterial community of Synechococcus,and the influence of the change of the composition of the symbiotic bacterial community on the physiological characteristics of Synechococcus and the metabolic process in the culture system.The main results are as follows:（1）Synechococcus abundance ranged from 6.36×10² to 4.51×10⁴ cells·m L^-1,and two Synechococcus subclusters,S5.1 and S5.2,were found to coexist in the studied area.Several clades of S5.1,including Clades I,II,and III,were the dominant components,accounting for 6.63%,26.11%,and 45.5%of the total genus,respectively.Redundancy analysis（RDA）showed that nitrite was the main environmental factor that explained the genus composition among samples.Furthermore,the high-throughput sequencing technology based on 16S r RNA gene universal primer amplification was used to study the correlation between Synechococcus and other bacteria as well as the involved element cycling process.Co-occurrence network revealed that the main phyla that coexisted with Synechococcus were Proteobacteria,Bacteroidetes,Actinobacteria,Planctomycetes,and Verrucomicrobia,which were involved in the carbon（C）,nitrogen（N）,sulfur（S）,and manganese（Mn）cycles.（2）Synechococcus was mainly belonged to S5.1＿VIII subgroup in culture systems,meanwhile the dominant classes of symbiotic bacteria wereα-Proteobacteria,γ-Proteobacteria,and Bacteroides.After bacterial removal,the recombinant bacteria community tended into consistent with Ruegeria（36.40%～66.04%）and Marianobacter（17.75%～24.47%）as the dominant populations.The functions annotation revealed that the bacterial groups participating in C and N cycles accounted for 88.47%and 11.08%respectively.For physiological aspect of Synechococcus,phycobiliprotein contents and maximum photosynthetic capacity changed significantly（p<0.05）,and the light conversion efficiency increased.The symbiotic bacteria could rapid chemotaxis and dynamical respond to dissolved organic matter produced by Synechococcus.Furthermore,Synechococcus released large amounts of silicon during cultivation,which may provide ballasting mineral for the aggregation and sinking of pico-phytoplankton and promote marine carbon cycle.To sum up,this work thoroughly studied the distribution pattern of abundance and phylogenetic diversity of Synechococcus in the Yellow and revealed the great significance of functional complementation built during the interaction between Synechococcus and symbiotic bacteria for promoting carbon cycle in the sea.