Effects Of Salinity And Ammonia-nitrogen On N-caproate Production From Lactate And The Mixed Microbial Community

As an important platform to recover resources from discarded organic substance,carboxylate platform can transform complex macromolecular organic matter into carboxylate.To summarize,lactate oxidation coupled with chain elongation to produce medium chain fatty acids containing 6～12 carbon atoms(such as n-caproate),which has become an advanced technology of waste recycling in recent years.The unique mixed microbiome involved in n-caproate-producing is taken from the fermentation pit used for Chinese strongflavour liquor(CSFL),Clostridium cluster Ⅳ(the family of Ruminococcaceae)is the dominant group.Metabolic process for the functional flora that is,lactate oxidation provides necessary reducing equivalents(NADH)and acetyl-CoA to maintain the electron flow,coupled with reverse β-oxidation for n-caproate production by chain elongation.As the main product of carboxylate platform,n-caproate has advantages of easy extraction and high added value,which not only has many chemical applications,but also can be further catalyzed into liquid fuel.With the rapid development of social economy,kitchen waste has become a major part of urban organic waste increasingly,and poses a potential threat to the environment,food safety and economic development.Consequently,it is an problem demanding prompt solution for our society,to make more efficient utilization of kitchen waste and improve the economics of waste disposal.Kitchen waste is rich in carbohydrates and protein(60～70%of dry matter),which is an ideal raw material for the synthesis of n-caproate,and n-caproate is a high value-added organic compound,which can greatly improve the economic benefits.To sum up the above discussion,the technology of converting kitchen waste into n-caproate has high research value in the field of organic waste recycling,and contribute to strategy of sustainable development.Special physicochemica properties such as high salinity content and ammonia-nitrogen of kitchen waste will affect the metabolism of microorganisms,so it is different from general organic waste.Up to now,scholars from home and abroad know little about the unique microbiome for n-caproate production,therefore,it is necessary to study the effects of salinity(NaCl)and ammonia-nitrogen on synthesis efficiency of mixed microbiome for n-caproateproducing.The study taked the Clostridium Ⅳ dominating mixed microbiome as the research subject,to explore:(1)optimization of the fermentation reaction conditions(associated with initial lactate concentration,pH and temperature);(2)effects of salinity on n-caproate production from lactate and the mixed microbial community structure;(3)effects of ammonia-nitrogen on n-caproate production from lactate and the mixed microbial community.The test results obtained through experiments are as follows:1.Under the reaction condition of lactate concentration 30 g/L,pH 6 and temperature control within 30 to 40℃,the main production n-caproate could reach the highest synthetic yield,and Clostridium Ⅳ would take up the dominant ecological niche;2.For n-caproate synthesis system,salinity(NaCl)is an unfavourable environmental factor.NaCl is an adverse factor for reactor microbiome producing n-caproate from lactate.When the concentration of NaCl is lower than 6 g/L,the reactor microbiome can still be able to produce n-caproate from lactate as dominating product.Futher increase of NaCl concentration(from 2 to 10 g/L)triggered the decrease of n-caproate production,while the relative abundance of Clostridium Ⅳ for n-caproate product in total reads decreased from 47.78%to 35.06%;when NaCl concentration reached 20 g/L,metabolic pathways for ncaproate production ceased,and was replaced by propionate,n-butyrate and n-valerate production.Proportion of Propionibacterium involved in the synthesis of propionate increased from 0.006%(0 g/L)to 0.09%(30 g/L).3.The production efficiency of n-caproate showed a significant downward trend with increasement of ammonia-concentration from 0 to 3000 mg/L,at the same time production of acetate,propionate and n-valerate got rising gradually;with a further increase of ammonianitrogen concentration to 4500 mg/L,chain elongation for n-caproate showed a stagnation trend,propionate became the main product.While the relative abundance of Clostridium Ⅳin total reads decreased by 13.19%,Propionibacterium increased by 5.62%,n-caproate yield was positively correlated with proportion of Clostridium Ⅳ.Baesd on the above conclusions,concentrations of ammonia-nitrogen in kitchen waste should be lower than 1500 mg/L to ensure normal function for n-caproate product.4.Given that the concentration of NaCl in kitchen waste is generally 2～6 g/L and the concentration of ammonia-nitrogen is 300 mg/L,mixed microbiome for n-caproateproducing with Clostridium Ⅳ as the core can tolerate high salt and ammonia-nitrogen of kitchen waste.Hence,the technology of n-caproate synthesis is very feasible in the disposal of kitchen waste,this research has very important theoretical value and significance for the technical development of n-caproate production from kitchen waste resources.