| Diatoms that grow under water and epiphytic to the surface of the substrates(also known as benthic diatoms)play an important role in the biogeochemical cycle and energy flow of the freshwater ecosystem.For their sensitivity to environmental changes,benthic diatoms have been used as indicator microorganisms for biological monitoring or for the enrichment/immobilization of heavy metal ions in water.Although manganese(Mn)is an essential element for biological growth,the excess of soluble Mn(Ⅱ)in the environment has a negative impact on the ecosystem.In order to study the distribution characteristics of freshwater benthic diatoms living in manganese-contaminated environment to Mn(Ⅱ)exposure for a long time,the benthic diatoms in streams of manganese mining area in winter and summer were investigated,respectively,and the community distribution,species abundance,diversity and habitat preference of epiphytic diatoms were analyzed by 18s r DNA sequencing coupled with morphological identification.Two typical diatoms strains,namely DSN01 and DSS01 were randomly isolated from samples to study their individual responses to manganese exposure,and the growth as well as physiological responses of DSN01,which has stronger manganese resistance to Mn(Ⅱ)were further explored.In addition,DSN01,which preferred plant substrate,was co-cultured with three native aquatic plants(Myriophyllum verticillatum,Pistia stratiotes and Hygrophila polysperma)as epiphytic diatoms to explore the potential use of the plant-diatom system in Mn(Ⅱ)immobilization from aqueous solution.The results are as follows:(1)11 families,31 genera and 116 species of benthic diatoms were identified from different 18 substrate types.Navicula,Diatoma,Nitzschia and Gomphonema were found dominant in winter and in summer,among them,Diatoma vulgare,Navicula reinhardtii,Gomphonema parvulum and Nitzschia palea were dominant species.Ulnaria and Neidium were sensitive to seasonal changes,and the former were found to preferring to grow in summer,with a relative abundance of 8%,but Neidium prefers to winter,with a relative abundance of 13.1%.(2)The two motile diatoms,Navicula and Nitzschia,were found to prefer to assemble on the surface of aquatic plants,whilst,the slow-moving species Ulnaria and Cymbella preferred to colonize on rocks and litter biomass.The genus numbers and Shannon index(H)values of diatoms assembled on the stem surface of Paspalum paspaloides did not show significant difference,which has always been recommended as one of the"hard substrates"for collecting surface diatoms for water quality indicators.(3)DSN01 and DSN02 were isolated randomly,which was identified as Nitzschia palea and Sellaphora seminulum respectively.Both of them have good manganese tolerance and can grow normally at the concentration of 50 mg/L Mn(Ⅱ).The growth and physiological response of strains of DSN01 to manganese exposure showed that excessive Mn(Ⅱ)exposure could inhibit the growth of diatom DSN01.However,within the tolerable concentration range(<50 mg/L),the cell density and chlorophyll a of DSN01 were increased.(4)Strain of DSN01 has the ability of manganese oxidation under Mn(Ⅱ)exposure.The Biogenic manganese oxides(BioMnO_x)mediated by DSN01exposed to 200 mg/L Mn(Ⅱ)levels after 336 h cultivation,contained 36.37,21.61and 42.01%of Mn(Ⅱ),Mn(ⅡI)and Mn(Ⅳ),and the average oxidation state(AOS)was 3.06.DSN01 was found to be able to epiphytically colonize on the surface of submerged tissues of the studied aquatic plants.The epiphytic DSN01 on the root of P.stratiotes can assist the host plant to remove Mn(Ⅱ)from the water.The findings of this thesis will help better understand the ecology of diatoms living in manganese exposed environments,provide theoretical reference for developing environmental benign plant-microorganism systems to achieve efficient environmental remediation and subsequently preserve the ecosystem functions the diatoms provide. |
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