| Dissolved organic matter (DOM) in the ocean is the fundamental part of marine carbon biogeochemical cycles. The amount of carbon stored in the DOM is comparable with that stored in atmospheric CO2, and the DOM pool is the largest organic carbon pool in the ocean. Exopolysaccharides (EPS) produced by marine bacteria are high molecular weight carbohydrate polymers, and it constitutes a large fraction of the total DOM in the sea. Meanwhile, EPS plays an important role in the material and energy cycle in the ocean. It can enhance the survival of marine bacteria by influencing the physicochemical environment around the bacterial cell, and also takes part in various marine biogeochemical processes. The bacteria which can produce EPS are abundant in the sea, and hence marine bacterial EPSs are an uninterrupted source of DOM. However, the significance of bacterial EPS in marine environments is not clearly understood, as not much is known about the contribution of bacterial EPS to the DOM pool and its bioavailability. Additionally, the current insights of EPS production are limited to microorganisms isolated from terrestrial environment and only a few studies have reported the EPS-producing bacteria which were retrieved from typical marine environments.This dissertation investigated the diversity of the EPS-producing bacteria in South China Sea and tentatively explored the bioavailability of EPS, the main experiments and resultscan be concluded as the following five parts:1. Investigation of the diversity of EPS-producing bacteria in South China Sea was conducted. A total of169EPS-producing bacterial strains from13stations were isolated and screened (at5depths for each station, namely surface,75m,200m,1000m, bottom). The results showed that:(1) With the increasing depth, the proportion of EPS-producing bacteria screened from culturable bacteria also enhances, and more than half of the strains isolated from the seawater in the ocean bottom can be identified as EPS-producing bacteria.(2) The EPS-producing strains are classified as Gammaproteobacteria, Alphaproteobacteria and Bacilli, and the majority (87%) of them are Gammaproteobacteria; In the genus level, EPS-producing bacteria are belonged to18genus, most of them are Alteromonas (51%) and Pseudoalteromonas (24%), followed by Sulfitobacter, Erythrobacter and Marinobacter. 2. The characteristic of EPS production of representative strains from different species (a total of27isolates) were studied. The results suggest:firstly, based on the cellular morphology with the transmission electron microscopy, we found three different types of EPS-producing bacteria,(1) strains with fibers or net structure surronding its cells surface;(2) strains with thin capsule;(3) strains with no significant structure around its cell surface. Most of the EPS-producing bacteria appear in can be attributed as type (1) and (2) whereas a few strains showed no significant structural features. Secondly, the quantitative analysis revealed that under the same culture conditions, EPS-producing bacteria of different species could produce different types of EPS:there are certain strains mainly produce bound-EPS, while others most likely produce soluble-EPS. There are a few strains which can produce two kinds of EPS with high yeild.3. A novel bacterial strain JLT2010T was isolated from deep seawater of South China Sea, which can produce capsular extracellular polymer. On the basis of phenotypic and phylogenetic features, JLT2010T is identified as a novel genus and species within the family Flavobacteriaceae, for which the name Altuibacter lentus gen. nov., sp. nov. is proposed.4. A high-yield EPS-producing strain, JL2810was selected for further study. We cultivated this strain in a Jar fermentor and monitored the EPS production during the cultivation as well as the contribution of EPS to DOC. The main conclusions are:strain JL2810can produce EPS during its whole growth phase, with the highest production rate present in the exponential growth phase. At the end of the incubation,40%of the dissolved organic carbon (DOC) in the culture system was stored in EPS, which suggested a potential contribution of EPS to DOC.5. Finally, EPS produced by JL2810was extracted and the components of extracted EPS was analysed. After purification of the extracted EPS, the molecular weight and gycosyl composition of the EPS were analyzed. Furthermore, to estimate the biological availability of EPS, the extracted EPS was added to natural seawater and then the bacterial growth and metabolic activity were analyzed. The main results are:the molecular weight of EPS is more than167kDa, and more than40.0%of monosaccharide is galacturonic acid, followed by mannose (25.0%), rhamnose (18.0%) and glucose (14.6%); when compared with glucose, EPS is relatively refractory for both bacteria growth and metabolism. Fluorescence dissolved organic matter may be accumulated during the degration of EPS. |
