In nature microbes rarely live in isolation, but rather exist in highly diverse and complex communities referred to as consortia . These consortia are often capable of tasks that are far too complex for any single organism to complete themselves including some of the most important global biogeochemical cycles . The organisms living in these communities interact in numerous ways ranging from cooperation to direct competition . Microbiologists and engineers have come to appreciate the diversity and capacity of natural microbial communities and large efforts have been undertaken to understand natural consortia and to engineer synthetic consortia for biotechnological purposes [4–7].
Two of the key challenges in society today are to reduce energy dependence on petroleum and reduce greenhouse gas emissions . The transportation sector is a prime candidate for addressing these two challenges because it relies on petroleum for approximately 93% of its energy and releases almost as much carbon dioxide as both the commercial and residential sectors combined . Lignocellulosic biomass has become an increasingly feasible source of carbohydrates for biological production of alternative fuels such as ethanol . Consolidated bioprocessing (the simultaneous biological hydrolysis and fermentation of biomass; CBP) is thought to be one of the most cost effective means of producing ethanol from lignocellulose [11, 12]. However, no single organism has been isolated or genetically engineered to reach high enough ethanol concentrations, yields and productivities from lignocellulose . Natural microbial consortia, on the other hand, are innately capable of high conversion of lignocellulosic biomass [see Table one in reference  but the resultant products (e.g., organic acids, CO2, CH4) are not suitable large-scale liquid transportation fuels.
An elegant example of a naturally occurring, lignocellulose degrading microbial consortium is the symbionts of the termite hindgut. Complex macromolecules are deconstructed in a series of steps that are facilitated by specific microbial species . Protists produce cellulases for cellulose hydrolysis, while nitrogen fixing bacteria (e.g., Clostridia) sequester atmospheric nitrogen to compensate for the low nitrogen content of wood. Fermentative organisms consume soluble sugars released from cellulose hydrolysis to produce organic acids, CO2 and H2 which are subsequently converted by methanogens to methane . This, along with other examples of natural microbial communities, has prompted interest in utilization of microbial consortia for lignocellulosic biofuels production [5, 7]. However, unlike these natural systems, scientists are currently limited in our ability to generate stable, productive microbial communities. In order to successfully implement large scale consolidated bioprocessing of lignocellulosic materials for fuel ethanol production we must develop stable microbial consortia with the necessary functionality, process control and efficiency.
To transition from natural consortia which contain potentially hundreds of organisms to synthetic consortia containing several defined species, one must develop a mechanism for population control. Generating stable, controllable consortia has been a focus of recent work to genetically engineer mechanisms for establishing intra-species consortia [16–19]. These approaches range from growth-controlling genetic circuits based on quorum sensing compounds  to complimentary auxotrophic amino acid exchange . As proof-of-concept, these studies are quite elegant and encouraging but they may suffer on the large scale due to unstable and difficult genetic modifications in industrially relevant organisms. Others have demonstrated syntrophic interactions in a co-culture of Actinotalea fermentans with an engineered S. cerevisiae which produces methyl halides directly from cellulose . S. cerevisiae relieves acetate inhibition by converting it to methyl halides, which are precursors for various fuel compounds. However, synthetic syntrophic interactions may be unstable since at least one organism does not necessarily rely on the other for survival. Without an additional level of control (e.g., spatial structure [21–23]) the community may breakdown. Obligate mutualisms such as those described by You et al. 2004 and Shou et al. 2007 may be a more stable approach for consortia-mediated lignocellulosic ethanol production. With this in mind, symbiotic consortia of a wide range of organisms could be generated using various mechanisms based on both genetic and environmental control factors .
In this work we develop a symbiotic co-culture of the cellulolytic mesophile, Clostridium phytofermentans and a cellodextrin fermenting yeast, Candida molischiana or S. cerevisiae cdt-1 . We establish a symbiosis (obligate mutualism) between C. phytofermentans and the yeast species by controlling the volumetric transport rate of oxygen. Both yeasts are capable of providing respiratory protection to the obligate anaerobe, C. phytofermentans, in return for soluble carbohydrates released from cellulose hydrolysis. The yeast converts these soluble carbohydrates to ethanol. At high substrate loading we noted a decreased conversion of cellulose by C. phytofermentans therefore endoglucanase was added to further evaluate the potential for improvements in the co-culture approach. In this cellulase-assisted format, the co-culture produces over twice as much ethanol and degrades two and three times as much α-cellulose as the S. cerevisiae cdt-1 and C. phytofermentans mono-cultures, respectively. This work represents a significant step in utilizing a scalable environmental control mechanism (i.e. oxygen transport) to induce a stable symbiosis in a consortium of two diverse organisms. This general approach to symbiosis development is applicable to a variety of organisms, substrates and products and can be used to explore diverse consortia-mediated bioprocesses.