Study On Singlet Oxygen Generation By DOM Photosensitized Reaction And Its Inhibitory Effect On The Growth Of Microcystis Aeruginosa

People found very early the phenomenon that barley straw was able to significantly inhibit the growth of filamentous algae in a pond.Previous studies indicated that straw in water environment might undergo slow decomposition with the participation of some bacteria,fungi species,and some special dissolved organic matters(DOM)were released during straw decomposition.Under the solar irradiation,some reactive intermediates were generated from DOM,and then these reactive intermediates probably inhibited the growth of bacteria or phytoplankton cells.Environmental chemistry tells us that reactive oxygen species(ROS)can be produced continuously under the DOM irradiation,and include hydrogen peroxide,singlet oxygen,and hydroxyl radicals,which possess strong oxidation ability and enable to destroy DNA and cell membrane and proteins,and block cellular metabolism.Similarly,DOM irradiation in water environment is able to inhibit bacteria or algae growth via singlet oxygen,and affects microbes community and structures.Therefore,it has an significant meaning to investigate the singlet oxygen generation with DOM irradication and its inhibition performance for freshwater algae.In this study,DOM release processes were studied during the decomposition of wheat straw,barley straw and rice straw;Ultrafiltration fractionation was used to analyse the optical and structure properties for different DOM.Main methods includefluorescence characteristics,UV-vis spectroscopy,functional group titration,3D-EEM and protein detection.DOM from different sources were exposed to the simulated solar irradiation,and steady-state reactive oxygen species concentrations and production rates of reactive oxygen species were calulated.Apparent quantum yields of reactive oxygen species was calulated to assess ROS production efficiencies.Parallel experiments were carried out to investigate the inhibition effect of singlet oxygen from DOM irradication on the growth of Microcyst aeruginosa.Results of ultrafiltration fractionation showed that protein-like fluorescence peaks were found in DOM solutions from decomposition of barley straw and wheat straw,and also found in grading fractions(30-100 kD and ? 100 kD graded components)from decomposition of rice straw.The protein content in barley straw decomposition DOM solution was much higher(49.23?g/mL)than those of rice and wheat straw.Secondary effluent owned the lowest protein content(41.74?g/mL).The content of phenolic hydroxyl groups of barley decomposition DOM solution and its graded fractions were higher than those of wheat and rice decomposition DOM solutions and their graded fractions.Secondary effluent owned the lowest phenolic hydroxyl groups content(0.25?g/mL).E2/E3 and SUVA values of straw decomposition DOM solution were lower than those of natural organic matter.Barley straw DOM had the highest value of E2/E3,while wheat straw DOM had the lowest E2/E3 value.We also found the negative relationship between E2/E3 and SUVA for straw decomposition DOM solutions.Photochemial experiments showed that FA had the the highest apparent quantum yeilds of singlet oxygen(3.04 × 10-3),and the apparent quantum yeilds of singlet oxygen for wheat straw DOM solution(2.76 × 10-3)is 1.3 times higher than that of barley straw DOM solution and 1.2 times higher than that rice straw DOM solution.The secondary effluent had the lowest yeild value(1.13 × 10-3)in all DOM solutions.Analysis indicated that the generation capacity of singlet oxygen was positively correlated with E2/E3.So generally ?1k D fractions had the maximum quantum yeilds of 1O2.Quantum yeilds of ?1kD fractions of wheat,barley and rice straw DOM solutions were 2.92 × 10-3,2.46 × 10-3,and 2.57 × 10-3,respectively.Barley straw DOM soluiton had the the highest apparent quantum yeilds of hydrogen peroxide(13.5 × 10-5),which is 3.3 times higher than that of rice straw DOM solution and 6.7 times higher than that wheat straw DOM solution.Apparent quantum yeilds of hydrogen peroxide for FA and HA solutions were much lower than those of barley straw DOM soluiton,but higher than those of rice and wheat straw DOM soluitons.The yield of secondary effluent is the lowest value in all DOM solutions.Since the process of generating H2O2 requires the charge transfer,there is a negative correlation between H2O2 and E2/E3.The lower E2/E3 value is,the stronger generation ability of H2O2 is,So generally ?100kD fractions had the maximum quantum yeilds of hydrogen peroxide.Quantum yeilds of ?1kD fractions of wheat,barley and rice straw DOM solutions were 2.72 × 10-5,16.1 × 10-3,and 4.08 × 10-3,respectively.The CDOM is the main source of ·OH.H2O2 photolysis also could be an important source of ·OH.Therefore,it probably had similar generation mechanism with H2O2.Singlet oxygen were determined during the irradiation of 25 kinds of DOM,which had a different degree of inhibition for the growth of Microcystis aeruginosa.The molecular weight of organic matter is negatively correlated with inhibition rates.In other words,the greater the molecular weight is,the smaller the inhibition rate is.Singlet oxygen produced by natural organic matter FA had the highest inhibition performance for Microcystis aeruginosa.Among the three kinds of straws,wheat straw DOM is the most effective inhibitor.In all fractions the ?1kD showed the highest inhibition effiency,while the DOM in secondary effluent showed the lowest inhibition effiency for Microcystis aeruginosa.(0.48×10-5?g ml-1 s-1).Our results indicated that the reactive oxygen species(ROS)produced by DOM photosensitization has significant inhibition effect on the growth of Microcystis aeruginosa.Although singlet oxygen production is relatively small,it shows a much higher efficient ihibition performance for the growth of Microcystis aeruginosa.Especilly,when FA or DOM from wheat straw decomposition are the dominant source of DOM in water bodies,the photo-generated singlet oxygen could be an effective inhibitor for phytoplankton growth.This study not only contributes to the understanding of the compilicated relationship between DOM and phytoplankton biomass in water environment,but also provides theoretical foundation and technical guildline for straw utilizaiton in terms of water bloom control via DOM irradiation.