Date Published: October 7, 2011
Author(s): Zhi-Wu Wang, Seung-Hwan Lee, James G Elkins, Jennifer L Morrell-Falvey.
Cellulose degradation is one of the major bottlenecks of a consolidated bioprocess that employs cellulolytic bacterial cells as catalysts to produce biofuels from cellulosic biomass. In this study, we investigated the spatial and temporal dynamics of cellulose degradation by Caldicellulosiruptfor obsidiansis, which does not produce cellulosomes, and Clostridium thermocellum, which does produce cellulosomes. Results showed that the degradation of either regenerated or natural cellulose was synchronized with biofilm formation, a process characterized by the formation and fusion of numerous crater-like depressions on the cellulose surface. In addition, the dynamics of biofilm formation were similar in both bacteria, regardless of cellulosome production. Only the areas of cellulose surface colonized by microbes were significantly degraded, highlighting the essential role of the cellulolytic biofilm in cellulose utilization. After initial attachment, the microbial biofilm structure remained thin, uniform and dense throughout the experiment. A cellular automaton model, constructed under the assumption that the attached cells divide and produce daughter cells that contribute to the hydrolysis of the adjacent cellulose, can largely simulate the observed process of biofilm formation and cellulose degradation. This study presents a model, based on direct observation, correlating cellulolytic biofilm formation with cellulose degradation.
Biofuels provide a number of environmental advantages over fossil fuels, especially in greenhouse gas reduction (Hromadko et al. 2010). Cellulosic biomass is often recognized as one of the best resources for biofuel production based on its cost, abundance, and cleanliness (Lynd et al. 2008). The hydrolysis of cellulosic biomass into soluble sugar, however, is regarded as a rate-limiting step in cellulosic biofuel production (Lynd et al. 2002). Consolidated bioprocessing (CPB) which utilizes cellulolytic bacteria to directly convert biomass into biofuel has the potential to cost significantly less compared to methods using enzymes (Lynd et al. 2008). Despite numerous studies showing biofilm involvement in cellulosic biomass hydrolysis (Cheng et al. 1984; Mooney and Goodwin 1991; Weimer et al. 1993; Miron et al. 2001; Burrell et al. 2004; Song et al. 2005, Lynd et al. 2006), few details are known regarding the dynamic interaction between biofilm formation and cellulose degradation. Some cellulolytic bacteria, such as Clostridium, produce cellulosomes which are protein complexes that facilitate cell attachment to cellulose and provide docking sites for extracellular enzymes involved in biomass hydrolysis (Miron et al. 2001). Yet, not all cellulolytic bacteria produce cellulosomes and very little is known regarding the mechanisms by which these non-cellulosome producing microbes attach to and degrade cellulose (Lynd et al. 2006). Caldicellulosiruptor obsidiansis is an anaerobic non-cellulosome producing bacterium isolated from Yellowstone National Park with an optimal temperature for growth at 78°C (Hamilton-Brehm et al. 2009). This organism hydrolyzes both cellulose and hemicellulose while fermenting hexose and pentose sugars to produce hydrogen, organic acids and ethanol. In this study, the temporal and spatial interactions of C. obsidiansis with cellulose were visualized and compared to C. thermocellum. This study was undertaken with the goal of providing insights into the mechanisms of microbial cellulose utilization, especially in high temperature environments.
In this study, the spatial and temporal dynamics of biofilm formation by two different microorganisms on two different cellulose substrates were investigated and correlated to cellulose degradation. Previous studies of bacterial degradation of biomass in sheep rumen using electron microscopy showed the presence of bacteria within cavities on the plant wall, leading to the hypothesis that the cellulolytic bacteria used a tunneling mechanism to degrade the plant (Dinsdale et al. 1978). Similarly, after incubation with the ruminal cellulolytic bacteria Ruminococcus flavefaciens, cell-sized pits were observed on leaf sheaths which were presumed to be due to bacterial degradation (Shinkai and Kobayashi, 2007). In another study, Gehin et al. (1996) observed the attachment of Clostridium cellulolyticum on Whatman No. 1 filter paper after 30 minutes incubation, although colony formation was not observed during this short experiment.
The authors declare that they have no competing interests.