| The interactions between different microbes generally exist in most natural environments.Microbial interactions existing among cultures have been exploited in several areas such as treating wastewater and degrading halogens and hydrocarbons.Artificial assemblies of microalgae and yeast have been reported to shown dynamic mutually beneficial relationship.Closer investigation of molecular mechanisms underlying the interaction between microalgae and yeast might possibly lead to novel and unexpected findings,however,the relevant information is largely lacking.In the current study,combinations of oleaginous microalgae and yeast at different ratios by the co-culture mode were evaluated to select out the optimal combination.The underlying mechanisms of the interactions were elucidated by non-targeted metabolomic and transcriptomic analysis.Moreover,the enhancement of single cell oil?SCO?production by co-culture of the optimal combination with non-detoxified cassava bagasse hydrolysate as substrate was investigated.The main results can be summarized as follows:?1?Co-culture combinations of Chlorella species and oleaginous yeast strains were evaluated for the improvement of biomass and lipid production.Compared to other three studied consortia,the co-culture of C.pyrenoidosa and R.glutinis with a ratio of 3:1 achieved significant higher biomass concentration and lipid productivity?p<0.05?.Higher glucose consumption rate and unsaturated fatty acids proportion were achieved compared to the mono-culture.Additionally,dissolved oxygen and pH were found to be adjusted synergistically in the co-culture group.?2?Non-contact co-culture of the previously selected optimal microalgae-yeast combination.Based on GC/TOF-MS analysis,non-targeted metabolomics study was carried out to elucidate the metabolic profile changes of microbe in response to co-culture.The co-cultivation with C.pyrenoidosa promoted the growth and lipophilic fluorescent of R.glutinis dramatically.Compared to the mono-culture,long chain fatty acids biosynthesis pathway of R.glutinis in co-culture was up-regulated.Moreover,stress response related intracellular metabolites,such as inositol,trehalose,arabinol were found to be up-regulated.Chlorophyll fluorescence was significantly higher of C.pyrenoidosa in co-culture with R.glutinis.As one of the intermediate products in Chlorophyll biosynthesis,the abundance of phytol in co-culture was significantly higher than that of mono-culture.Metabolites related to glutathione metabolism in algal cells were significantly up-regulated in response to environmental stress.?3?Extracellular metabolites changes of co-culture were investigated by GC/TOF-MS based non-targeted metabolomics.Multivariate statistical model of Biplot and SUS-plot combined with univariate analysis were employed to select out the significantly different metabolites between co-culture and mono-culture.Four extracellular compounds were induced de novo in co-culture samples including glycerol,pyruvic acid,succinic acid and salicin.The abundance of 10 secretions in mono-culture was significantly increased in co-culture including tryptophan,isoleucine,alanine,pantothenic acid,oxalic acid and indoleacetic acid?IAA?.Growth promoting effect of IAA on R.glutinis was proved by adding it into the culture medium.?4?Differential gene expression of C.pyrenoidosa and R.glutinis in response to co-culture were investigated by transcriptome analysis.Genes involved in pathways such as“Synthesis of tryptophan and indoleacetic acid”,“Porphyrin and chlorophyll metabolism”,“Biosynthesis of ubiquinone and other terpenoid quinones”were significantly up-regulated in co-cultivated C.pyrenoidosa.In response to co-culture,R.glutinis up-regulated the expression of genes involved in“Glycolysis”,“Pyruvate metabolism”and“Tricarboxylic acid cycle”,to provide more metabolic energy and intermediates for fatty acids biosynthesis and elongation.As a result,differential expression genes involved in the key enzymes in fatty acid synthesis and elongation pathways were observed.Moreover,expression of genes related to stearoyl-CoA desaturase were significantly up-regulated?FC>20?,which indicated that more unsaturated fatty acids were synthesized in response to the interaction with microalgae.?5?Non-detoxified cassava bagasse hydrolysate?CBH?was utilized as substrate for the enhancement of single cell oil?SCO?production by co-culture of Chlorella pyrenoidosa and R.glutinis.Compared to the mono-culture,significantly higher biomass(20.37±0.38 g L-1)and lipid yield(10.42±1.21 g L-1)were achieved in the co-culture?p<0.05?.The fed-batch culture further raised the biomass lipid yield to 31.45±4.93 g L-1 and 18.47±3.25 g L-1,respectively.The two strains in the current study were able to tolerate and even degrade some toxic byproducts exist in the CBH,and the co-culture system enhanced the degradation of certain byproducts.This study provided new insights for the integration of the economical SCO production with agroindustrial waste disposal. |
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