Transcriptomic Responses of the Interactions between Clostridium cellulovorans 743B and Rhodopseudomonas palustris CGA009 in a Cellulose-Grown Coculture for Enhanced Hydrogen Production

Abstract

Coculturing dark-and photofermentative bacteria is a promising strategy for enhanced hydrogen (H 2 ) production. In this study, next-generation sequencing was used to query the global transcriptomic responses of an artificial coculture of Clostridium cellulovorans 743B and Rhodopseudomonas palustris CGA009. By analyzing differentially regulated gene expression, we showed that, consistent with the physiological observations of enhanced H 2 production and cellulose degradation, the nitrogen fixation genes in R. palustris and the cellulosomal genes in C. cellulovorans were upregulated in cocultures. Unexpectedly, genes related to H 2 production in C. cellulovorans were downregulated, suggesting that the enhanced H 2 yield was contributed mainly by R. palustris. A number of genes related to biosynthesis of volatile fatty acids (VFAs) in C. cellulovorans were upregulated, and correspondingly, a gene that mediates organic compound catabolism in R. palustris was also upregulated. Interestingly, a number of genes responsible for chemotaxis in R. palustris were upregulated, which might be elicited by the VFA concentration gradient created by C. cellulovorans. In addition, genes responsible for sulfur and thiamine metabolism in C. cellulovorans were downregulated in cocultures, and this could be due to a response to pH changes. A conceptual model illustrating the interactions between the two organisms was constructed based on the transcriptomic results. IMPORTANCEThe findings of this study have important biotechnology applications for biohydrogen production using renewable cellulose, which is an industrially and economically important bioenergy process. Since the molecular characteristics of the interactions of a coculture when cellulose is the substrate are still unclear, this work will be of interest to microbiologists seeking to better understand and optimize hydrogen-producing coculture systems. Microorganisms do not live in isolation in nature; instead, they interact with each other in complex ecological networks within a microbial community in order to function and resist stresses in various environments (1). In general, interspecies interactions define the characteristics and robustness of microbial communities. The concept of microbe-microbe interactions has been harnessed and applied in the bioremediation (2, 3), food and beverage (4-7), and biofuel (8-12) industries. The application of multispecies systems could be a promising alternative bioprocess strategy to the use of single species, which requires extensive genetic engineering before multiple desirable traits are incorporated, whereas these functions are distributed among different organisms in a multispecies system (13). However, multispecies systems can be challenging to control (14); therefore, a detailed understanding of the physiological and molecular mechanisms of microbial interactions is important in engineering robust complex microbial communities.Studying the interactions that occur in a complex microbial community involving hundreds of sp...

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