Effect Of Symbiotic Relationship Between Bacteria And Algae On The Aggregation Behavior Of Single-celled Microalgae In Aquatic Environment

Single-cell microalgae are gradually becoming a new generation of biofuels due to their high photosynthetic efficiency,faster growth rate and higher productivity.However,the collection of microalgae has been a bottleneck in the development of algae biofuel applications.Studies have shown that bacteria can cooperate with microalgae to control sewage wastewater and aggregate microalgae efficiently and economically owing to its good flocculation performance.However,the mechanistic understanding of such symbiotic relationship on microalgae aggregation remains unclear yet.In this paper,the single-cell motile Chlamydomonas microsphaera was selected to manifest the role of sludge bacteria in the aggregation process,and to unveil the mechanism of algae aggregation.It would be a useful insight on microalgae collection.In this paper,we constructed micro-microalgae reactor system to investigate the effects of sludge bacteria,extracellular polymeric substance(EPS)and sludge bacterial signal molecules on the microalgae self-aggregation.The main conclusions are listed as follows:First,the emergence of sludge bacteria can significantly promote the aggregation of C.microsphaera.The aggregation rate of pure cultured C.microsphaera was only about 12.94%.The aggregation rate increased with increasing sludge bacteria in the mixed culture.When the ratio of algal bacteria was 1:10,the aggregation rate reached 62.92%,and the speed of the microalgae increased rapidly from initially 22.08 ?m/s to 48.81 ?m/s,resulting in high probability of cell-to-cell collision and aggregation.At the same time,the amounts of EPS secretion of algae aggregates increased by about 163.38 mg/L compared to the pure cultured microalgae.Further analysis found that the appearance of sludge bacteria changed the zeta potential of the system,from the initial-25.00 mV to-14.60 mV,indicating that the intercellular repulsion decreased and tends to aggregate.Analysis of sludge population dynamics showed that Pseudomonas and Rhizobium played an important role in promoting the aggregation of C.microsphaera.Secondly,in order to distinguish the degree of influence of sludge bacteria and EPS on microalgae accumulation,the effect of exogenous sludge EPS on the self-aggregation behavior of microalgae was studied.It showed that with the increase of EPS concentration of exogenous sludge,the aggregation rate of C.microsphaera increased significantlyto a maximum of 81.82% at 110 mg/L EPS.The microalgae cell movement rate also increased from 26.82 ?m/s to 49.36 ?m/spromoting the aggregation of microalgae.However,the zeta potential change of system was negatively correlated with microalgae aggregation.At the same time,it was found that the level of exogenous EPS was positively correlated with the amount of extracellular tryptophan and aromatic protein secreted by microalgae.Thirdly,in order to verify whether bacterial quorum sensing signal molecules(N?Acylhomoserine Lactones,AHLs)from sludge EPS can affect the aggregation of C.microsphaera,AHLs derived from aerobic flocs,aerobic granules,anaerobic flocs,and anaerobic granules were added to the algae culture.It was found that the all AHLs contributed to the aggregation C.microsphaera at some extent.Among them,granular sludge AHLs induced highest level of microalgae aggregation with rate of 43.44%,while aerobic floc sludge bacteria AHLs showed limited effect with rate of 19.90%.After the addition of AHLs,the secretion of tryptophan and aromatic protein increased,which positively correlated with microalgae aggregation rate.The algae cell movement rate decreased to 24.88-30.83 ?m/s,while the zeta potential increased by about 7.44% compared with the control group.