Screening For Phosphate-solubilizing Microorganisms To Promote Growth Of Aegiceras Corniculatum(L.) Blanco Seedlings And To Immobilize Lead Contaminant Combined With Phosphate Rock

In recent decades,mangrove wetland faces multiple threats,including those from urbanization,coastal landfills,agriculture,aquaculture ponds,and global warming.This ecosystem falls into a series of dilemma,such as vegetation degradation,habitat fragmentation,biodiversity decrease,ecological function decline,and so on.Moreover,nutrient availability is generally low and restricted,especially for phosphorus.Therefore,it is essential to fully utilize beneficial partnerships between phosphate-solubilizing microorganisms and mangrove plants.In the meantime,heavy metal pollution is a worldwide environmental problems,which is urgent to be solved in the future.As the potential donor of soluble phosphate,the phosphate-solubilizing microorganisms not only play important roles in nutrient uptake and plant growth,but also have great potential in lead(Pb)immobilization combined with phosphate rock.However,previous studies only considered Ca3(PO4)2 as the sole metal-P compounds for isolating and screening of phosphate-solubilizing microorganisms in mangrove ecosystem.Unfortunately,limited information is available on the potential functions of phosphate-solubilizing microorganisms in Pb immobilization.Hence,the main contents and results of this paper are as following:1)Based on the main speciation of phosphorus in manrove sediments,several phosphate compounds[namely FePO4,AIPO4,Ca3(PO4)2,and lecithin]were employed to isolate and screen phosphate-solubilizing bacteria from Zhangjiangkou Mangrove National Nature Reserve.Results showed that the clear halos were only found in agar plates with lecithin or Ca3(PO4)2.Most of the soluble phosphate released by organic phosphate solubilizing bacteria was used for their own growth.These strains displayed weak or no P-solubilizing efficiencies towards FePO4 or AIPO4.The tested strains were effective in dissolving Ca3(PO4)2,and medium acidification and organic acids release were considered as the two principal P-solubilization mechanisms with Ca3(PO4)2.These tested strains with efficient Ca3(PO4)2-solubilization were identified as genera Bacillus,Gallaecimonas and Mangrovibacter.Among them,the Bacillus sp.was the most common phosphate-solubilizing bacteria,and the genera Gallaecimonas and Mangrovibacter were isolated from the mangrove rhizosphere and are reported as phosphate-solubilizing bacteria for the first time.2)In order to evaluate the plant growth effects on Aegiceras corniculatum(L.)Blanco seedlings,five efficient phosphate-solubilizing bacteria(including Gallaecimonas sp.,Mangrovibacter sp.and three strains of Bacillus sp.)were employed in a pot experiment.Results showed that several phosphate-solubilizing bacteria(especially Mangrovibacter sp.IPSB28 strain)positively promoted A.corniculatum growth,but the plant growth-promoting effect was not necessarily related to P-uptake,the production of indole-3-acetic acid(IAA)and siderophore also regulated plant growth,photosynthetic capacity,root vitality,and so on.Mangrovibacter sp.IPSB28 strain deserves to be further exploited for reforestation in coastal and estuarine areas.3)We further isolated and screened culturable phosphate-solubilizing fungi from mangrove rhizospheric sediments,and evaluated their potential P-solubilizing capacity with multiple insoluble phosphates.Results showed that all tested strains were able to solubilize various insoluble phosphates in the order of Ca3(PO4)2>AlPO4>FePO4.Efficient phosphate-solubilizing fungi were identified as genera Penicillium and Aspergillus.Additionally,inoculation with Penicillium sp.PSF-FJ1 strain,which have IAA and siderophore synthetic abilities,positively improved plant growth of A.corniculatum seedlings.4)Soluble phosphate released by phosphate-solubilizing fungi was also the pivotal factor in metal Pb immobilization.Four phosphate-solubilizing fungi(namely Penicillium sp.PSF-FJ1,Penicillium sp.PSF-TW,Aspergillus sp.PSF-FJ3 and Aspergillus sp.PSF-HN),which had the phosphate rock-solubilizing capacity and metal Pb tolerance ability,were isolated from Fujian,Hainan and Taiwan provinces.Compared to total Pb content in culture medium,fungal strains were more sensitive to soluble Pb concentration.Moreover,high level of Pb concentration would cause a loss of P-solubilizing capacity by several fungal strains.5)We further explored the potential functions of four Pb-tolerance phosphate-solubilizing fungi combined with phosphate rock in Pb immobilization in aqueous solution.Results indicated that all tested fungal strains,with or without P-solubilizing ability,could effectively immobilize Pb at 200 mg·L-1 of Pb concentration.This could be due to the biosorption of fungal strains or organic acids released by metabolic activity.At the level of 800 mg·L-1 of Pb addition,fungal strains Penicillium sp.PSF-FJ1 and Penicillium sp.PSF-TW,which exhibited effective phosphate rock-solubilizing capacity and enhanced soluble phosphate concentration in solution,subsequently achieved the immobilization of Pb.6)Additionally,two phosphate-solubilizing fungi(namely Penicillium sp.PSF-FJ1 and Penicillium sp.PSF-TW)were first employed for Pb immobilization in Pb contaminated soil combined with phosphate rock.Results found that fungal strains Penicillium sp.PSF-FJ1 and Penicillium sp.PSF-TW could significantly reduce the concentrations of water-extractable and NH4NO3-extractable Pb in soil solution,thereby increasing the Pb immobilization.Furthermore,the addition of bamboo shoot shell extraction as biostimulation of fungal strains growth could further improve the immobilization of metal Pb.