Metatranscriptomics Analysis Of The Carbon And Nitrogen Fixation Abilities Of Plankton In The Northern South China Sea And Their Influencing Factors

Marine plankton is an important participant in the earth’s carbon and nitrogen cycles,as well as an important contributor to global primary productivity.Plankton is a wide variety of species and widely distributed.The spatial and temporal changes of marine plankton community structure and primary productivity have been widely studied.At present,most plankton cannot be isolated and cultivated,and it is still challenging to study the specific functions of a single lineage of plankton in the environment,such as the study on the carbon fixation activities of some marine microorganisms.Meanwhile,the lack of data base of marine microorganisms in some sea areas also needs to be supplemented.In this study,the whole-assemblage metatranscriptomics method(eukaryote+prokaryote)was used to sequence plankton in the northern shelf and slope area of the South China Sea to establish a functional gene set of plankton in the sea area.The data information covered the functional gene information of eukaryotes,prokaryotes,and a small number of viruses in the euphotic layer.The data from this study complemented the functional genomeset database of planktonic microorganisms in the northern South China Sea.Based on the metatranscriptome database,we investigated the functional differences of different microorganisms in the process of carbon and nitrogen fixation in the continental shelf and continental slope of the northern South China Sea and analyzed the influencing factors.The main findings of this research are summarized as follows:1.The functional gene set of plankton in the euphotic layer of the northern South China Sea was established by metatranscriptome sequencing.A total of 881 Gb data was obtained from transformation.After assembly,4,499,414 unigenes were detected,of which about 37%were annotable genes.Among them,the widely distributed and actively expressed genes were mainly involved in microbial metabolism,photosynthesis,amino acid synthesis,oxidative phosphorylation,and two-component system pathways.The establishment of this database complemented the functional gene set of planktonic microorganisms in the northern South China Sea.2.Based on the macrotranscriptome database and the expression level of plankton RuBisCO in the samples,we estimated the photosynthetic carbon sequestration capacity of each plankton in different environments.The results showed that the photosynthetic carbon fixation in the northern shelf and slope areas of the South China Sea was mainly performed by Bacillariophyta,Chlorophyta,Cyanobacteria,Haptophyta,and non-diatom Stramenopiles,and the contribution of each group of plankton to photo synthetic carbon fixation was different due to environmental factors.The expression of photosynthetic carbon fixation genes and endocytosis pathway genes were positively correlated,indicating that endocytosis promotes photosynthetic carbon fixation,and the photosynthetic carbon fixation genes of the Dinophyta were negatively correlated with the expression of endocytosis genes.We speculated that the endocytosis of Dinophyta is complementary to its photoautotrophic activity.3.The data of this study reveals the great potential of non-photosynthetic carbon fixation(NCF)of plankton.Based on gene expression,we estimated the NCF contribution of different microorganisms,and found that the non-photosynthetic carbon fixation in northern South China Sea was mainly contributed by Flavobacteriales,Alteromonadales,Pelagobacteria and Rhodobacterales,about equally by free-living and particle-associated species.In addition,the activity of non-photosynthetic carbon fixation was significantly positively correlated with the expression of proton pump rhodopsin gene in Flavobacteria,Alteromonadales,Pelagobacteria and Rhodopobacteria,suggesting that their non-photosynthetic carbon fixation might be supported by the light energy obtained from rhodopsin.This discovery lays a methodological and informational foundation for further research on marine carbon fixation.4.Based on the expression information of the key genes in the samples,the phytoplankton involved in the nitrogen fixation process in the continental shelf and slope region of the South China Sea were identified,and their contribution and influencing factors in the process were estimated.In the continental shelf area,the nitrogen fixation was mainly performed by Archea and Bacteroides,while in the continental slope area,the proteobacteria had stronger nitrogen fixation ability,and the cyanobacteria dominated the nitrogen fixation at the bottom of the euphotic layer,which may be related to the distribution of plankton in the sea area.In terms of species information,there were 17 species of nitrogen-fixing microorganisms,of which 2 species only fixed nitrogen in the continental shelf area and 9 species only fixed nitrogen in the continental slope area.In addition,it was found that the nitrogen-fixing activities of Rhodovulum Ph10 and other species were affected by different environmental factors.