Ecology Study On Carbon Fixation Of Main Chemoautotrophic Microorganisms In Summer Seawater In Northern South China Sea

Ocean is an important place for the global carbon cycle and plays a crucial role in regulating the global carbon cycle.Ocean is a sink of CO2 and the Net Primary Productivity(NPP)is a major determinant of ocean carbon sinks.The NPP of oceans is mainly contributed by phytoplankton in the oceanic euphotic layer,while people ignore the contribution of chemolithoautotiphic microorganisms to the Primary Productivity of ocean.In this paper,we used the carbon-14 labeled bicarbonate to measure the rate of Dark Carbon Sequestration in the northern South China Sea(nSCS)and the range was from 12.61±1.82?120.81 ±9.81 ?mol C m-3 d-1.Spearman rank correlation analysis showed that the rate of Dark Carbon Sequestration was positively correlated with nitride,nitride/ammonium ratio and chlorophyll a.In the aerobic seawater environment,the major autotrophic carbon pathway are the Calvin-Benson-Bassham(CBB)cycle and 3-hydroxypropionate/4-hydroxybutyrate(3?HP/4-HB)cycle.The waters we studied in nSCS is also an oxidizing environment.The CBB cycle mainly exists in the chemolithoautotrophic bacteria of the Proteobacteria.Ribulose-1,5-bisphosphate carboxylase/oxygenase(RuBisCO)is the key enzyme of the CBB cycle,which is mainly divided into Form I and Form II in bacteria.The cbbL and cbbM gene were respectively taken as the target genes to research the community spatial distribution of Form I-type and Form II-type chemolithoautotrophic bacteria in nSCS.Our results showed that cbbL and cbbM gene were not effectively amplified in the surface water in nSCS at summer.From the perspective of community structure and distribution,Form I-type chemolithoautotrophic bacteria community structure is similar in the bottom and same layers of nSCS.Temperature might be a significant environmental factor on affecting the structure and distribution of Form I-type chemolithoautotrophic bacteria;There was no obvious distribution characteristic of Form II-type chemolithoautotrophic bacteria in the nSCS.However,the structure difference of the total community and the corresponding active community were more obvious.Dissolved oxygen might be a significant environmental factor on affecting the structure and distribution of Form II-type chemolithoautotrophic bacteria.From the perspective of phylogenetic analysis,Form I-type chemolithoautotrophic bacteria were strictly divided into two categories:Form IA group and Form IC group.The former belongs to obligate chemolithoautotrophic bacteria,the latter belongs to facultative chemolithoautotrophic bacteria.Both of them are mainly belonging to sulfur-oxidizing bacteria,hydrogen-oxidizing bacteria,ammonia-oxidizing bacteria,nitrite-oxidizing bacteria and ferric-oxidizing bacteria.Form II-type chemolithoautotrophic bacteria were more sulfur-oxidizing related.At the DNA level and cDNA level,most Form I-type chemolithoautotrophic bacteria in nSCS do not express the cbbL gene;Form II-type chemolithoautotrophic bacteria with cbbM gene expression activity only exists in one clade,and only distributed in the bottom of N1,N2 and N3 stations in the nearshore of nSCS.The 3-HP/4-HB cycle mainly exists in the chemolithoautotrophic Thaumarchaeota(ammonia.oxidizing archaea,AOA),Acetyl-CoA carboxylase is one of the key enzymes in this cycle.The accA gene was taken as the target gene to research the community spatial distribution of chemolithoautotrophic Thaumarchaeota in nSCS,we found that none of the layers were effectively amplified in the surface water in nSCS except surface water of N4 station.From the perspective of community structure and distribution,the chemolithoautotrophic Thaumarchaeota in nSCS can be clearly divided into Shallow group(<200 m)and Deep group(?200 m),and the in situ community and the corresponding active community from the same water layer is similar.Water depth,nitrite and pH might be significant environmental factors on affecting the structure and distribution of chemolithoautotrophic Thaumarchaeota in nSCS.According to the phylogenetic analysis,the chemolithoautotrophic Thaumarchaeota in nSCS were divided into three evolutionary clades:"Deep cluster","Shallow cluster" and "SCM1-like cluster".From the DNA level and cDNA level,the majority of chemolithoautotrophic Thaumarchaeota in nSCS can express accA gene,indicating that the majority of chemolithoautotrophic Thaumarchaeota in nSCS are directly involved in the assimilation of CO2.In conclusion,there are a various chemolithoautotrophic bacteria and a large number of chemolithoautotrophic Thaumarchaeota in nSCS,their contributions to the primary productivity of the nSCS can not be ignored.