Effects Of Bacterial Quorum Sensing Molecules On Microalgae Lipid Production And The Mechanisms

Coupling microalgae cultivation with wastewater is a promising approach for environmental and energy dilemma.And the relationship between the microalgae and bacteria determines the stability and treatment efficiency of the coupling system.The microalgal-bacterial interaction depends on the behavior of cell-to-cell communication,one of which is quorum sensing with quorum sensing molecules?QSMs?as a language.However,the effect and mechanisms of bacterial QSMs on microalgae lipid production remains unknown to us.Based on the above,this study investigatedthe effect of bacterial QSMs on the lipid production and response mechanisms of microalgae in monoculture and co-culture from the perspective of cell-to-cell communication.Firstly,the effect of QSMsextracted from activated sludgeon monoculture of Chlorella sp.lipid production was investigated.The results showed that compared with control group?no QSMs added?,the lipid content of Chlorella sp.significantly increased 95%in logarithmic phase and by 84%in stable phase with QSMs stimulation.The lipid content of QSMs group also reached its highest values 2 days earlier than the control.Meanwhile,under the QSMs stimulation,the biomass and maximum specific growth rate of Q?+++?were 77%and 53%lower than the control group in the logarithmic phase.Transcriptomic analysis showed that the key enzymes of lipid biosynthesis pathway were generally up-regulated after the QSMs stimulation,e.g.the 40 transcripts of 14 enzymes encoding acetyl coenzyme A carboxylase and fatty acid synthase increased 2-8 times.However,the biosynthesis of important precursor,5-phosphate ribosome,was significantly blocked.And 41 transcripts of DNA polymerase and DNA ligase involved in DNA replication were also down-regulated 2-9 time.While this negative effect seemed negligible at the stationary phase.On the 4^th day,the lipid productivity and total lipid production of microalgae were improved by 88%and 84%,respectively under the QSMs stimulation.Next,weinvestigatedtheeffectofatypicalbacterial QSMs,N-hexanoyl-L-homoserinelactone?C₆-HSL?,on co-culture of Chlorella sp.and Scenedesmussp..The growth curve illustrated that under the stimulation of C₆-HSL,the biomass of Chlorella sp.and Scenedesmus sp.in monoculture was reduced by 50%and 35%compared with the control,respectively.However,the growth and mitochondrial activity of Chlorella sp.and Scenedesmus sp.in the co-culture system seemed not to be affected by C₆-HSL.Thus,the lipid production and productivity of co-culture system was improved 209%.Analysis of the antioxidant defense system revealed that antioxidative enzymes of microalgal monoculture increased significantly by 55%-300%in the 4^th hour,while no significant changes were observed in co-culture system.As can be seen from the cell morphology,under the influence of C₆-HSL,Chlorella sp.could only form four mature spores and Scenedesmus sp.were unable to maintain a normal cell shape.In contrast,the two species still maintained their original morphology with the C₆-HSL stress in the co-culture system.The results showed that the microalgal co-cultivation may prove to be an approach to mitigate the stress of QSMs and a highly robust and stable strategy for microalgae cultivation...