The Interactions Between Synechococcus And Associated Heterotrophic Bacteria

Marine phytoplankton contribute to approximately half of the global primary production.Interactions between photoautotrophs and heterotrophs are central to marine microbial ecosystems,shaping the community structure and diversity of the upper marine ecosystem.Synechococcus are dominant marine phototrophs,which are widespread in global ocean,and their cultures frequently contain associated heterotrophic bacteria.Thus,biological interactions between Synechococcus and associated heterotrophs are critical for understanding biogeochemical cycling in the ocean.In this study,we conducted investigation on the co-culture systems of Synechococcus and associated diverse heterotrophic bacteria in oligotrophic species Synechococcus sp.YX04-3 and eutrophic species Synechococcus sp.XM24 respectively.This study investigated the dynamic abundance relationships,lifestyle strategy choices,and genomic and proteomic level metabolic characteristics of Synechococcus and associated heterotrophic bacteria.The main conclusions of this study are as follows(1)During the 91-day growth period,the abundance of Synechococcus sp.YX04-3 and heterotrophic bacteria ranged from 1.43×107 to 1.60 ×108 cells mL-1 and 7.78 x 106 to 6.39 × 108 cells mL-1,respectively,over the entire incubation period.The associated heterotrophic bacterial assembly was mostly from different five classes:Flavobacteria,Bacteroidetes,Phycisphaerae,Gamma-and Alpha-proteobacteria,which displayed lifestyle preferences(free-living versus attached)and unique responses to the different Synechococcus growth phases.The detected metabolic processes of Synechococcus,the only primary producer in the co-culture system,were mainly involved in inorganic nutrient uptake,photosynthesis,and organic matter biosynthesis and release.Two flavobacteria,including Muricauda and Winogradskyella,as well as one member of the SM1 A02 group,displayed preferences for the initial degradation of complex compounds and biopolymers.The Alphaproteobacteria Oricola sp.(Bin5)mainly captured low molecular weight DOM,including byproducts from flavobacteria,using the ABC,TRAP and TTT transport systems.These heterotrophic bacteria displayed a complementary mechanism in the degradation of Synechococcus-derived organic matter and nutrient re-cycling.In addition to nutrient exchange,removal of reactive oxygen species and vitamin trafficking together contributed to maintain the Synechococcus/heterotroph co-culture system.(2)Bacteria within the Roseobacter clade and Flavobacteria are two of the main bacterial lineages that exhibit intimate associations with Synechococcus sp.XM24 populations.Marivita sp.XM-24 is an ecological generalist of the Roseobacter clade and displays diverse metabolic capacities for the acquisition of nutrients,energy,and carbon,which has the potential for oxidation of carbon monoxide,inorganic sulfur oxidation,DMSP demethylation,and PHA metabolism through investigation on genomic characteristics.The genome of the Flavobacteria representative,Fluviicola sp.XM-24,contained numerous genes for degrading complex organic matter polymer produced by Synechococcus,importing bicarbonate,metabolizing cyanophycin,gliding motility and attachment,and glycan synthesis.Meanwhile,both Marivita sp.XM-24 and Fluviicola sp.XM-24 have the potential for superoxide dismutase synthesis which can remove ROS from the co-culture system.