| Freshwater cyanobacteria can produce numerous potent toxins and represent anincreasing environmental hazard. Microcystis aeruginosa (M. aeruginosa) is acosmopolitan and toxicogenic planktonic cyanobacteria that produces and exudes copiousamounts of dissolved organic materials. In this research, the effect of citral, limonene andmenthol on the growth of and microcystins (MCs) release by the M. aeruginosa weretested by evaluating the results obtained from the batch culture experiments with M.aeruginosa FACHB-905. The time series of cell, intracellular photosynthetic pigments,esterase activity (EA), dehydrogenase activity (DHA), antioxidase enzyme activity aswell as intracellular and extracellular MCs concentrations were evaluated during5d ofthe incubation.In the controls, the cell density and dry weight were almost constant when the cellswere in stationary phase, and increased rapidly when the cells were in logarithmicgrowing phase, the intracellular photosynthetic pigments content exhibited similar trendswith the cell density. The EA, DHA and antioxidants enzymes activity did not exhibit anobvious difference in the control samples (p>0.05). The intracellular MCs contentincreased rapidly (p <0.05) in logarithmic growing samples and the extracellular MCscontent increased slowly (p>0.05). When the cells were in stationary phase, theintracellular MCs content did not showed a distinct difference (p>0.05) during the120hof incubation and the extracellular MCs content grew constantly (p <0.05).After exposure to citral, the number of cells gradually diminished; the net log cellreduction after5d of the exposure was3.1,4.4,5.0log when the initial cell densitieswere set at1.3×107,1.1×106, and2.9×105cell·mL1, respectively. The cell dry weightdecreased to0.07,0.002, and0.001mg·L1after120h of exposure, which indicated thatcitral can effectively inhibit the growth of M. aeruginosa. The intracellular chl-a, PC andAPC content exhibited a gradual decrease in the120h of exposure(p <0.05). After36hof exposure to citral, the intracellular chl-a content decreased to0.8,0.06and0.008mg·mL1when the initial cell densities were set at1.3×107,1.1×106, and2.9×105cell·mL1, respectively. The intracellular PC content exhibited a similar trend, after24hof exposure to citral, the intracellular PC content decreased to0.5,0.03and0.009μg·mL1 when the initial cell densities were set at1.3×107,1.1×106, and2.9×105cell·mL1,respectively. The intracellular APC content also showed a distinct decrease after36h ofexposure, it decreased to0.03,0.001and0.0006μg·mL1when the initial cell densitieswere set at1.3×107,1.1×106, and2.9×105cell·mL1, respectively. This phenomenonindicated that the photosynthetic activity declined under the stress of citral. The cellviability was evaluated by determining the EA and DHA. The EA and DHA decreasednotably (p <0.05) after citral addition. After120h of exposure, the EA was less than3%in contrast to controls and the DHA was less than2%in contrast to controls, whichindicated that the viability of algal cells declined obviously. The antioxidase activityexhibited a gradually decrease (p <0.05) during the120h of cultivation, the activity ofantioxidase kept on decreasing. After72h of exposure, the activity of SOD was5.4U·mgprot1(p <0.05), the activity of POD was and the activity of CAT decreased to2.0and1.2U·mgprot1(p <0.05), while the average amount of MDA content increased to27fmol·cell1, which indicated that the algal antioxidant system was vulnerable to citralexposure. And the oxidative damage was induced, which might be a possibility of themechanisms involved in the inhibit effect to M. aeruginosa.Citral was found to significantly influence the release of MCs. As the citral exposurecould inhibit the increase in the number of cells, the increase in the total MCsconcentration in the medium was also inhibited. In the presence of limonene, theintracellular MCs was gradually released into the medium through a gradual reduction inthe number of cells. After120h of exposure, the intracellular MCs concentrationdecreased to0.24,0.01and0.001μg·L1(p <0.05). The extracellular MCs concentrationin the medium was significantly higher in the citral-exposed samples than in the controlsamples, after120h of exposure to citral, the extracellular MCs concentration increasedto277,28and12μg·L1(p <0.05). This phenomenon confirmed that MCs wasbiosynthesized in algal cells and could be released after cell disruption. The averageamount of intracellular MCs content was25and23fg·cell1before and after120h ofexposure (p>0.05), which illustrated that MCs can not pass the cell membrane whenalgal cells are alive. Besides, the extracellular MCs concentration did not exhibit anotable decrease after the120h of exposure, which indicated that citral cannotdecompose the extracellular MCs. After the limonene-exposure, the number of cells exhibited a gradual decline; the netlog cell reduction after5d of the exposure was3.1,3.6, and3.8log when the initial celldensities were set at1.3×107,1.1×106and2.9×105cell·mL1, respectively. The celldry weight was0.07,0.002, and0.001mg·L1after120h of exposure, which issignificantly lower than the controls. The content of photosynthetic pigments, chl-a, PCand APC, decreased as the diminish of cell numbers during the120h of exposure tolimonene, and the differences were considered as significant (p <0.05). This phenomenonindicated that the photosynthetic pigments are sensitive and vulnerable under the stress oflimonene. The cell viability was evaluated by determining the EA and DHA. The EA andDHA decreased distinctly (p <0.05) after the limonene addition. After48h of exposure,the EA decreased to and71%,30%and44%in contrast to controls, and the DHAdecreased to67%,35%and56%in contrast to controls (p <0.05) when the initial celldensities were set at1.3×107,1.1×106and2.9×105cell·mL1, respectively, whichindicated that the viability of algal cells declined obviously. The antioxidase activityexhibited a gradually decrease (p <0.05) after48h of the limonene exposure, the activityof SOD decreased to11U·mgprot1, the activity of POD was3.9U·mgprot1and theactivity of CAT decreased to1.5U·mgprot1, while the MDA content had a distinctincrease (p <0.05), after96h of exposure, it increased to71fmol·cell1per cell, whichindicated that the algal antioxidant system was vulnerable to limonene exposure. And theoxidative damage was induced, which might be a possibility of the mechanisms involvedin the inhibit effect to M. aeruginosa.Limonene was found to significantly influence the production and release of MCs.As the limonene exposure could inhibit the increase in the number of cells, the increasein the total MCs concentration in the medium was also inhibited. In the presence oflimonene, the intracellular MCs was gradually released into the medium through agradual reduction in the number of cells. The extracellular MCs concentration in themedium was significantly higher in the limonene-exposed samples than in the controlsamples, which confirmed that limonene cannot decompose the extracellular MCs.The number of cells exhibited a gradual decline after the menthol-exposure; the netlog cell reduction after5d of the exposure was2.2,3.0, and3.4log when the initial celldensities were set at1.3×107,1.1×106and2.9×105cell·mL1, respectively. The celldry weight underwent a gradually decrease as well, it was0.07,0.002, and0.001mg·L1 after120h of exposure to menthol. The intracellular chl-a, PC and APC content exhibiteda gradual decrease during the120h of exposure(p <0.05). This phenomenon indicatedthat the photosynthetic pigments are sensitive and vulnerable under the stress of menthol.The cell viability was evaluated by determining the EA and DHA. The EA and DHAdecreased notably (p <0.05) after menthol-addition. After36h of exposure, the relativeEA decreased to67%,55%and60%when the initial cell densities were set at1.3×107,1.1×106and2.9×105cell·mL1, respectively, and the DHA decreased to65%,44%and74%in contrast to controls after60h of exposure to menthol, which indicated that theviability of algal cells was affected by the presence of menthol. The antioxidase activityexhibited a gradually decrease (p <0.05) after48h of the menthol exposure the activity ofSOD decreased to9.3U·mgprot1, the activity of POD was2.1U·mgprot1and theactivity of CAT decreased to1.8U·mgprot1, However, the MDA content did not exhibitany distinct difference (p>0.05),6.0fmol·cell1to8.3fmol·cell1per cell before andafter120h of exposure, which indicated that the algal antioxidant system was not thetraget of the menthol exposure. Although the activity of antioxidants decrease, theoxidative damage was not obvious.Menthol was found to significantly influence the production and release of MCs. Asthe menthol exposure could inhibit the increase in the number of cells, the increase in thetotal MCs concentration in the medium was also inhibited. In the presence of menthol,the intracellular MCs was gradually released into the medium through a gradual reductionin the number of cells. The extracellular MCs concentration in the medium wassignificantly higher in the menthol-exposed samples than in the control samples, whichconfirmed that menthol cannot decompose the extracellular MCs. |
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