Growth Inhibition Of Algae By Ultraviolet Combined With Ultrasonic Radiation

Both ultraviolet (UV) and ultrasonic (US) radiations have the great potential to beused as effective approaches to control algal growth. The thesis explored theinhibition effects of typical algal growth under single UV, single US, and the USassisted UV radiation. The responses of the typical algae to radiation wereinvestigated based on cell shape, cell density, optical density, and relative fluorescentstrength of chlorophyll a. For Microcystis aeruginosa, degradation of microcystin-LR(MC-LR) after UV radiating were studied as well, in order to provide usefulinformation for the application of UV to control algal blooms under certainconditions.UV radiation at the100mJ/cm2could effectively inhibit the growth ofChlamydomonas microsphaera, Microcystis aeruginosa, Scenedesmus obliqnus,Dunaliella salina and Phaeodactylum tricornutum for10days of incubation, whereasthe inhibition effect could only last for3days for Prorocentrum lima. It was foundthat100mJ/cm2was a critical value beyond which the inhibition effect didn’t showsignificant increase any more. At this certain value, the highest inhibition rate was forProrocentrum lima, followed by Dunaliella salina, Microcystis aeruginosa,Scenedesmus obliqnus, Chlamydomonas microsphaera, and Phaeodactylumtricornutum showed the lowest inhibition rate. UV radiation could effectively reducethe concentration of MC-LR whose degradation rate was affected by UV dose andinitial concentration of MC-LR.The optical density and relative fluorescent strength of chlorophyll a couldn’taccurately reflect the inhibition effect of UV radiation on algal growth, and theirrelation with cell density was depended on the species of algae. As for Microcystisaeruginosa, the relative fluorescent strength of chlorophyll a was in consistent withthe cell intensity, but had no relativity with optical density. Differently, the celldensity of Prorocentrum lima coincided with its relative fluorescent strength ofchlorophyll a and optical density. The effect of US radiation was strongly dependedon the species and growing season. Microcystis aeruginosa, with gas vesicles, was the most sensitive to US radiation, and the Microcystis aeruginosa in the lag phasewas more sensitive the the one in the exponential phase. With the extension of UStime, the inhibition effect on Microcystis aeruginosa, Prorocentrum lima andChlamydomonas microsphaera was enhanced, and the promoting effect on thegrowth of Scenedesmus obliqnus increased as well. The influences of US power andfrequency on algal growth were different with different species, the optimumfrequencies for Microcystis aeruginosa and Chlamydomonas microsphaera weregreater than94kHz, while the value was ranging from20to94kHz for Scenedesmusobliqnus.Compared with single UV and single US radiation, the inhibition effect of USassisted UV radiation on algae could be enhanced remarkably. In view of bothefficiency and costs, the desirable combinations of US assisted UV for Prorocentrumlima and Microcystis aeruginosa were40kHz/100W（5min）+40mJ/cm2,20kHz/300W（5min）+40mJ/cm2, and94kHz/300W（5min）+300mJ/cm2for Chlamydomonasmicrosphaera.