Ecological Effects Between Chlorella Vulgaris And Dominant Associated Heterotrophic Bacteria And Response To Quorum Sensing Signal Molecules

Marine microalgae and numerous heterotrophic bacteria in its phycosphereconstitute a specific microcosm and have complicated ecological interactions witheach other, either mutualism or mutual antagonism. Bacteria can synthesize andrelease quorum sensing signal molecules into surrounding environment, and detect thecell density by sensing the concentration of quorum sensing signal molecules.Bacteria can control gene expression collectively and synchronize group behaviour toadapt to stress the changing environment and coordinate the relationship with the hostwhen the signal molecule concentration reaching to a threshold value.In present study, a common diet microalgae-Chlorella vulgaris was selected toserve as experimental material, and the mutual ecological effects of C. vulgaris anddominant associated heterotrophic bacteria and the response of C. vulgaris to quorumsensing signal molecules were studied under controlled laboratory conditions. Theresults could further clearly defined the mutual action rule of marine microalgae andheterotrophic bacteria, and then supply scientific method to reveal the mechanism ofred tide and solve the problem of genetic degradation of marine economic microalgae,and finally provide scientific and theoretical base for recoving rational structure oceanecological system and improving ecological condition of aquaculture pond. Resultsshowed:Seven associated heterotrophic bacteria, named Z-TG01, Z-TG02, Z-TG03,Z-TG04, Z-TG05, Z-TG06and Z-TG07, were isolated and purified from theco-cultural system using the method of plate coating, and Z-TG01, Z-TG02andZ-TG03were dominant associated heterotrophic bacteria, which the dominance indexbeing48.8%,14.0%and18.6%respectively. Using GN cards, three dominantassociated heterotrophic bacteria were identified Sphingomonas paucimobilis, Pantoea spp. and Myroides spp. respectively by the VITEK2Compact AutomaticBacteria Identification System (BioMérieux, France). Using Agrobacteriumtumefaciens KYC55, we found3dominant associated heterotrophic bacteria haveAHLs bioactivity, and increased gradually in the growth process of dominantassociated heterotrophic bacteria by measuring the activities of β-D-galactosidase.The supernatant of C. vulgaris culture in the exponential phases inhibited AHLsbioactivity of Z-TG01and Z-TG03significantly (p<0.01or p<0.05), and thesupernatant of C. vulgaris culture in the exponential phases inhibited AHLsbioactivity of Z-TG02significantly (p<0.01or p<0.05). C. vulgaris, which we used inpresent study, might secret analogs or quorum-quenching enzymes to inhibit AHLsbioactivity to interfere the release of toxins mediated by quorum sensing, and makeitself growthed in a suitable environment.The mutual action rule of C. vulgaris and dominant associated heterotrophicbacteria and the change rule of AHLs bioactivity of coculture system were studied inpresent study. The results showed:①Bacteria Z-TG01within a certain cell densityand C. vulgaris could promote mutual growth in co-culture system, and C. vulgaris ofthe exponential phases could obviously inhibited AHLs bioactivity of Z-TG01;②Bacteria Z-TG02of different cell density could inhibited the growth of C. vulgaris invarying degrees, and the AHLs bioactivity were not obviously inhibited, and the lowdensity of C. vulgaris or the instability of AHLs in the co-culture system of C.vulgaris and bacteria Z-TG02;③Bacteria Z-TG03within a certain cell density and C.vulgaris could promote mutual growth in co-culture system, and C. vulgaris of theexponential phases could obviously inhibited AHLs bioactivity of Z-TG03.Effects of AHLs (N-hexanoyl-DL-homoserine lactone, C6-HSL) on growth andantioxidant defense system of Chlorella vulgaris were studied in present study. Theresults showed that that low concentrations of C6-HSL (50,100and200nM) couldincrease the relative growth rate of C. vulgaris obviously at8h (p<0.05), higherconcentrations of C6-HSL (≥400nM) could decrease the relative growth rate of C.vulgaris obviously (p<0.05), and exhibited high concentration-dependent inhibition effects.50nM C6-HSL activated defense response of C. vulgaris in a short time (4h),increased activities of SOD, POD, CAT and GPX of C. vulgaris obviously (p<0.05),and decreased MDA content significantly (p<0.05). Compared to low concentrations,high concentration of C6-HSL (400nM) increased MDA content of C. vulgarissignificantly (p<0.05). The antioxidative enzymes of C. vulgaris increased to a peakthen decreased, and the MDA content of C. vulgaris increased significantly0to24hexposure to400nM C6-HSL (p<0.05).The mechanisms of C. vulgaris responding to AHLs were studied by iTRAQ(isobaric tags for relative and absolute quantification) at the protein expression level.The results showed that281proteins were identified,10proteins were differentiallyexpressed in C. vulgaris treated by400nmol/L C6-HSL,5were induced and5wererepressed. The exprsssion of photosystem II reaction center protein D1and heat shockprotein90was repressed, indicating the algal photosystem and other physiologicalfunctions had damaged in varying degrees. Meanwhile, the exprsssion of enolase,sedoheptulose-bisphosphatase, ATP synthase beta subunit and vacuolar ATP synthase,subunit A were significantly induced, indicating enhance anaerobic respiration andefficiency of ATP synthesis of the algal cell to provide energy for respairing thedamaged photosystem, the transmembrane transport of toxin, and normal growth.Moreover, the exprsssion of sulfolipid biosynthesis protein was significantly induced,indicating the algal cells were actively respairing the damaged cytomembrane.