Effects Of UV-B Radiation On Growth And Physiological-biochemical Characteristics Of Kareoia Mikimotoi Before And After Axenation

In this paper, Kareoia mikimotoi, which is red tide algae of frequent outbreaks in recent yearsin China, was selected to study the effects of enhanced UV-B irradiation on its growth andphysiological-biochemical characteristics with presence and absence of symbiotic heterotrophicbacteria. Molecular identification and functional analysis of four strains of heterotrophic bacteriaisolated from K. mikimotoi were carried out to provide research base in exploring the relationshipbetween algae and heterotrophic bacteria and its response to enhanced ultraviolet radiation. Themain contents are as follows:Axenation of K. mikimotoi was carried out by using several common antibiotics (includingpenicillin, gentamycin, kanamycin and streptomycin). The growth characteristics of K.mikimotoi between axenic and nonaxenic cultures were investigated. The results showed: the growthof microalgae would be stimulated if the concentrations of penicillin, gentamycin andkanamycin were lower than200μg/ml, while the growth would be inhibited if the concentrationsof streptomycin were higher than50μg/ml. The axenic culture of K. mikimotoi was obtained bycombinedly using400μg/ml of penicillin and gentamycin,600μg/ml of kanamycin. Thegrowth characteristics of K. mikimotoi were changed and algal growth cycle significantly prolongedafter axenation of microalgae.The effects of UV-B radiation on growth and physiological-biochemical characteristics ofaxenic and nonaxenic K. mikimotoi: The algal cell densities of low-dose treatment groups (0.2、0.4、0.8J/m2) were higher than control after axenation of microalgae and high-dose groups hadno evident exponential growth stage, prompting stimulation effect of low dose of UV-B radiation onalgal cell growth. However, high-dose treatment groups of nonaxenic K. mikimotoi could maintain acertain population growth. Chla contents of low-dose groups were higher than control afteraxenation of microalgae and those of nonaxenic K. mikimotoi were higher than the control exceptfor3.2J/m2treatment group. Car contents of sterilizing algae (0.8,1.6and3.2J/m2treatmentgroups) increased slowly and is always lower than those of the control group. Those of nonaxenic K.mikimotoi first increased and then decreased. Soluble protein contents of axenic K. mikimotoi were higher than control and decreased to below the level of control at9d. The change of soluble proteincontent of nonaxenic K. mikimotoi was similar to that of axenic microalgae. SOD activities in eachtreatment group of axenic K. mikimotoi were higher than the control and showed a dose effect. SODactivities of0.8,1.6and3.2J/m2treatment groups decreased to below the level of control at9d.Change of SOD activities of nonaxenic K. mikimotoi was similar to that of axenic microalgae. SODactivities of treatment groups were still higher than those of control at9d. The change of PODactivities of nonaxenic K. mikimotoi was similar to that of axenic microalgae and both increased toabove the level of control at3d and6d. MDA contents of axenic K. mikimotoi were higher thancontrol and increased with increasing dose of UV-B radiation. MDA contents of nonaxenic K.mikimotoi were higher than control at3d, and decreased to below the level of control in low-dosetreatment groups at6d and9d.Four major cultivable heterotrophic bacteria isolated from K. mikimotoi is belonging toFormosa, Erythrobacter, Shewanella and Marinobacter after16SrRNA fragments amplified.Homology analysis of P1strains is identified to genus level, the homology analysis of P2, P3and P4is higher than99%, which can be determined to species level. Phylogenetic tree was built betweenfour heterotrophic bacteria and its homology24bacterial, and the results showed that28bacteriacould be divided into four branches in the phylogenetic tree, which is consistent with homologyanalysis.