Studies On The Growth And Aggregation Trend Of Microalgae In Bacteria-algal Interaction System

Microalgae have been widely used in biodiesel production due to their highly comprehensive value,where the efficicency of microalgae growth and harvesting severely limit their large-scale application.Bacterial-algal symbiotic system can promote microalgal flocculation,however,microalgal growth pattern and aggregation process in the coculture system remain unclear yet.In this study,the effects of carbon source and growth factors on microalgal growth and aggregation were investigated in a coculture system with Chlamydomonas microsphaera and activated sludge bacteria as the model algae and mixed bacterial strains.The effects of carbon source,feeding method,and initial ratios of bacteria-algae were analyzed,and the main conclusions are listed as follows:Firstly,although exogenous inorganic carbon sources did not affect the growth of pure-cultured C.microsphaera,but the growth of microalgae were promoted remarkbly in the mixed bacterial-algal system.At low inorganic carbon concentration(38mg-C/L),the biomass of microalgae reached 1.22×10⁶cells/m L after 4 days incubation with an initial ratio of 15/1(bacteria/algae),which was 30%higher than that of the pure cultured algae.As carbon source concentration increased to 188 mg-C/L,microalgae biomass increased to 1.64×10⁶cells/m L,which was 68%higher than that from the pure cultured algae system.Meanwhile,the cocultured bacteria also contributed to the aggregation of microalgae,with obvious aggregation observed at increasing concentration of inorganic carbon.For example,at the initial ratio of 15:1,59%?67%microalgae aggregated in the experimental group after 6 days incubation,which was24%?27%higher than those from pure culture system.Secondly,organic carbon source could promote the growth of C.microsphaera in pure culture system,and after 6 days of culture,the biomass of C.microsphaera reached(1.26?1.38)×10⁶cells/m L,which was higher than the biomass of 1.16×10⁶cells/m L in pure culture.However,in the mixed system,the growth of C.microsphaera was related to either the concentration of organic carbon source or initial ratio of bacteria and microalgae.At an initial bacteria-algae ratio of 15:1,the high concentration of glucose organic carbon source((Chemical oxygen demand,COD)500 mg/L)significantly promoted the growth of microalgae,and biomass rose up to 1.93×10⁶cells/m L after 6 days of culture.In comparison,the microalgae biomass did not change significantly from the pure culture at the bacteria-algae ratio of 3:1.Interesetingly,decreasing the concentration of organic carbon source reduced the biomass of microalgae in the mixed system of bacteria and algae.For instance,at the bacteria-algae ratio of 3:1,the biomass was only 1.06×10⁶cells/m L at 6 days with low concentration(COD 100 mg/L)of glucose.When glucose was replaced by sodium acetate,microalgae growth was inhibited in different extent in the mixed system.The inhibition effect was gradually reliefed with increaseing the initial sodium acetate concentration.On the other hand,organic carbon source noticeably enhanced the aggregation of C.microsphaera in both pure and mixed culture systems,especially in the experimental groups with high bacterial to algal ratio,with the aggregation rate of 61%?85%.Thirdly,indole-3-acetic acid(IAA)and vitamin B₁₂slightly inhibited the growth of microalgae in either pure culture or mixed bacteria-algal system.At 1.0?g/L IAA,microalgae biomass was 0.96×10⁶cells/m L and 0.92×10⁶cells/m L in pure and mixed system(bacteria-algae ratio of 3:1)after 6 days,respectively,which were lower than that of microalgae in the system without IAA.At the same time,IAA and vitamin B₁₂were found to promote the aggregation of microalgae in pure culture or mixed bacteria-algal systems.For example,when IAA or vitamin B₁₂was added,after 6 days of culture,the average aggregation rates of small algae in the mixed system with pure algae and bacteria-algae ratio of 3:1 were 17%and 37%,respectively,which were higher than the microalgae aggregation rates in the culture system without IAA.