Study On Microbial Community And Bioavailability Of Heavy Metals During Bioturbation In Mangrove Sediment

Mangrove wetland is an important buffer zone for terrigenous heavy metals pollutants into the oceans in the estuary ecotone.Mangrove root disturbance,Spartina alterniflora invasion and benthic crab disturbance are dominant bioturbation in coastal mangrove ecosystems in China.The physical and geochemical properties of the sediment can be altered by bioturbation thus affecting ecosystem function.However,the links between bioturbation,physical and chemical characteristics of sediment and metal behavior have been previously poorly studied in mangrove ecosystem.To gain a better insight into metal's behaviors in mangrove ecosystem,here,the aims of this study were to investigate the role of both root and benthic crab bioturbation on bioavailability of heavy metals,integrated with an analysis of the effects on the biogeochemical properties of the sediment.The roots perturbance of three mangrove species(Avicennia marina,Kandelia obovata and Aegiceras corniculatum)and S.alterniflora as well asbenthic crab burrowing and their interactions with heavy metals(Cd,Cu and Pb)bioavailability were investigate in situ We show that the mechanistic basis of how bioturbation mediate dissolved organic matters(DOM)microbial communities diversity and the enzyme activities in wetland sediment thus modified the availability of heavy metals.The main results obtained were summarized as follows:1.Bioturbation significantly affects heavy metals speciation in mangrove sediments.The fraction distribution of Cd and Pb in rhizosphere sediment disturbed by mangrove roots followed the trends that residual>oxidizable>reducible>exchangeable.However,fraction distribution of Cu was in the following order:residual>oxidizable>reducible>exchangeable.The bioturbation of benthic crab and S.alterniflora root increase the bioavailability of Cd and Cu.Compared with undisturbed sediment,the proportion of exchangeable and reducible Cd,Cu in sediment suffering from benthic crabs and S.alterniflora root disturbance were increased by 184.9%,106.5%and 32.5%,33.9%,respectively.2.Bioturbation changes the characteristics of DOM in sediment.The fluorescent characteristics of DOM in sediments were analyzed by Excitation-emission Matrix Spectroscopy coupled with Parallel Factor Analysis,indicated that DOM in wetland sediment sampling contains four fluorescence components:UVC humic-like substance(C1,C3,C4),UVA marine humic-like substance(C2),and the order of its fluorescence intensity was following:C1>C2>C4>C3.With disturbance by mangrove roots,results from the DOM fluorescence index(FI),index of recent autochthonous contribution(BIX),the humification index(HIX)with showed that:the FI index and BIX index increased 6.9%and 51.2%,respectively.Compare to undisturbed sediment,however HIX index decreased 49.8%.The data indicated that microbial activity is the main driving force for the transformation of organic matter in sediments,increasing the newly produced humic components and reducing the humus degree of DOM.The crab disturbance behavior increased the humus degree of the DOM,while thedistubance of S.alterniflora root reduced the humus degree of DOM in sediments.Analysis of common UV-vis absorption spectral characteristics(A260,A250/A365,A240/A420,A465/A665,SUVA254 ? SUVA280)indicated that mangrove plantation increased the proportion of DOM hydrophobic components,as well as the degree of humification and polarity enhancement in sediment.The ratio of DOM hydrophobic components,the ability of ultraviolet light and visible light absorption,the degree of humification,the degree of polarity and the degree of aromatization in crab cave and S.alterniflora gap sediment were reduced by 83.0%,27.95%,79.95,75.0%,75.1%and 60.2%,64.3%,59.7%,55.6%,62.9%,respectively.3.The disturbance of the mangrove root and the crabs increased the species diversity of microorganism in rhizosphere sediments.But there was no significantiy changed obsered in S.alterniflora.Bioturbation significantly increased the content of Phospholipid-derived fatty acids(PLFA)in Gram-positive bacteria,Gram-negative bacteria,fungi,bacteria,AM fungi and actinomycetes PLFA(p<0.05).The sediment in the crab cave,mangrove and S.alterniflora forest transformed from nutrient deficient state to nutrient rich state,and G+/G-decreased by 16.5%,23.4%and 14.2%,respectively.These indicated that bioturbation favors the nutrient cycling in mangrove sediment.The mangrove rhizosphere sediment was confronted with strong environmental stress,and the ratio of C/N was increased by 34.1%,while the environmental stresses in the crab cave and S.alterniflora rhizosphere sediments are low,and nutrient availability was increased by 22.8%and 44%,respectively.Bioturbation increased urease,sucrase and fluorescein diacetate hydrolytic activities,while the accumulation of heavy metals in mangrove rhizosphere sediment inhibited the activity of catalase.4.Bioturbation changed the physicochemical and biological properties of mangrove sediments,and played an important role in the accumulation and transformation of heavy metals.Microbial community structure and enzyme activities in disturbed sediments were significantly related to heavy metals concentration and soil physicochemical properties(p<0.05).The content of DOC was significantly correlated with the microbial populations(p<0.05),and thus synergistically affect the bioavailability of heavy metals.Significantly correlation between FI value and microbial community(p<0.01),indicating that microbial activity was the main driving force for the transformation of organic matter in sediments,while the aromatic compounds contributed little.The availability of heavy metals in sediments was significantly correlated with the composition and fluorescence characteristics of DOM(p<0.05).The present study indicated that bioturbation could change the available state of heavy metal Cu,Cd and Pb in the mangrove sediments.Bioturbation also changed the spectral characteristics of DOM,therefore,affected the combination of heavy metals in DOM as well as changing the bioavailability of metals.Microbial community diversity made a positive response to bioturbation with DOM,and synergistically affect the bioavailability of heavy metals.The results of this study provide the knowledge on linking bioturbation to transformation and bioavailability of heavy metals in mangrove sediments,therefore,allowed them to get further insight in understanding of typical heavy metal source-sink transformation and regional pollution remediation in mangrove ecosystem.