Marine Microbial Utilization And Transformation Of Synechococcus-derived Organic Matter

Microorganisms play significant roles in the productivity and biogeochemical cycling of oceans.Phytoplankton-derived organic matter is the dominant source of labile organic matter in the upper ocean.Heterotrophic bacteria utilize these labile compounds for growth and subsequently produce dissolved organic matter(DOM)that is resistant to microbial degradation and contributes to the large recalcitrant DOM(RDOM)pool in the ocean.The interactions between heterotrophs and phytoplankton organic matter are hot topics of scientist's concern currently.Marine picocyanobacteria that primarily consist of Synechococcus and Prochlorococcus are the most abundant photosynthetic organisms on Earth and contribute to-40%of the oceanic primary production.In particular,Synechococcus are widely distributed in oceans globally and are especially abundant in estuarine and coastal waters.In this study,we explored the interactions between Synechococcus sp.XM-24 and its associated heterotrophic bacteria in the laboratory culture;then,the amended-experiments with organic matter derived from Synechococcus sp.XM-24 were carried out in the oligotrophic and eutrophic environmental seawaters,respectively.The main conclusions of this study are as follows:1.In the Synechococcus sp.XM-24 co-culture system,the growth of Synechococcus could be divided into four periods:the lag,exponential,stationary and decline phases.The variations of POC concentrations was consistent with the Synechococcus aboundance,indicating that Synechococcus was the main source of POC.DOC concentrations increased rapidly and exceeded the POC in decline phase.The dominant bacterial groups were Flavobacteriales,Roseobacter clade,Gammaproteobacteria and Actinobacteria over the entire incubation.In this Synechococcus culture,flavobacteria,which was the most abundant group throughout the culture period,tended to be aggregated or attached to the Synechococcus cells,whereas the actinobacteria demonstrated a free-living lifestyle,and roseobacters displayed different patterns depending on the culture growth phase.Factors contributing to these succession patterns for the heterotrophs likely include interactions among the culture community members,their relative abilities to utilize different compounds produced by Synechococcus cells and changes in the compounds released as culture growth proceeds,and their responses to other changes in the environmental conditions through the culture period.2.We investigated the microbial responses to the addition of Synechococcus-derived organic matter(SOM)in the oligotrophic environment of the South China Sea.During the short-term incubations,60-73%of SOM was metabolized by microbes wtih an obvious increase of phosphate concentrations.Gammaproteobacteria,Alphaproteobacteria and Bacteroidetes were the dominant active microbial populations,and exhibited a clear succession pattern during the incubations.Gammaproteobacteria was the most dominant microbial group,accounting for half or even more of the active microbial community,indicating that they responded quickly to fresh organic matter with a competitive advantage.SOM was not completely degraded in the short-term incubaitons,suggesting the different biological availabilities of SOM,and the left organic matter needs more time for further degradation and utilization by microorganisms.3.We investigated the bioavailability of Synechococcus-derived organic matter(SOM)by estuarine and coastal microbes during 180-day incubations.Variations in organic carbon,inorganic nutrients,fluorescent dissolved organic matter(FDOM),and total/active microbial communities were monitored.The incubations were partitioned into three phases(labeled I,II,and III)based on the total organic carbon consumption rates of 6.38-7.01,0.53-0.64,and 0.10-0.13 ?mol C L-1 day-1,respectively.This corresponded with accumulation processes of NH4+,NO2-,and NO3-,respectively.One tryptophan-like(C1)and three humic-like(C2,C3,and C4)FDOM components were identified.The intensity variation of C1 followed bacterial growth activities,showing a labile DOM characteristic.C2,C3,and C4 displayed labile,semi-labile,and refractory DOM characteristics,respectively.Alphaproteobacteria,Gammaproteobacteria,Bacteroidetes,and Actinobacteria dominated the quickly-consumed process of SOM(phase I)coupled with a substantial amount of NH4+ generation.Thaumarchaeota became an abundant population with the highest activities in phase ?,especially in the free-living size-fraction,and these organisms could perform chemoautotroph processes through the ammonia oxidation.Microbial populations frequently found in the dark ocean,even the deep sea,became abundant during phase ?,in which Nitrospinae/Nitrospirae obtained energy through nitrite oxidation.Our results provide greater insight into the carbon and nitrogen coupling processes mediated by coastal microbial metabolizing phytoplankton-derived organic matter.4.We tracked the changes of SOM(including POM and DOM)compositons with our 180-day incubations using gas chromatography-mass spectrometer(GC-MS)and fourier transform ion cyclotron resonance mass spectrometer(FT-ICR MS).The composition of POM mainly comprised fatty acids,aromatics,alkanes and alcohols.Fatty acids dominated in POM in the early period of incubation,and then decreased in the later period.The detected POM compostion was much more diverse and with relatively high aromatics compounds in the later period of incubation.The results of FT-ICR MS showed that the DOM molecular composition obviously changed during the incubations with the relative intensities of molecules decreasing or increaseing over times.Furthermore,bacteria tended to utilize and transform DOM molecules with high oxidation state,and produced molecules with lower H/C and O/C ratios.