TABLE 15.

Questions inherent to understanding microbial cellulose utilization most appropriately pursued by studies at a cellular level

Physiological featureQuestion(s) relative to growth on cellulose
Uptake of cellulose hydrolysis productsWhat is the distribution of oligoglucan chain lengths taken up by cellulolytic organisms?
What is the bioenergetic requirement for substrate transport?
Metabolic controlWhat is the allocation of substrate carbon to cellulase and cell synthesis, how does this vary with growth conditions, and can such observations be reconciled with our understanding of gene expression at a subcellular level?
BioenergeticsWhat is the allocation of ATP to cellulase and cell synthesis, and how does this vary with growth conditions?
What is the relative importance of phosphorolytic and hydrolytic cleavage of cellodextrins and/or cellobiose, does this depend on growth conditions, and, if so, how is it regulated?
Can the rate of ATP production (e.g., due to phosphorolytic cleavage of oligomers, glycolysis, and post-pyruvate metabolism) be quantitatively reconciled with the rate of ATP consumption (e.g., due to substrate transport, cell synthesis, cellulase synthesis, maintenance, energy spilling, and perhaps other factors)?
Considering all factors—including but not necessarily limited to ATP available from cellobextrin phosphorylation, ATP requirements for substrate transport, and the ATP required for cellulase synthesis—is the ATP available for growth of a microorganism growing on cellulose less than or greater than that available for growth of the same organism on soluble mono- or disaccharides?
Cellulose-microbe adhesion and glycocalyx formationWhat are the roles and relative importance of glycocalyces, cellulosomes, and other factors in microbial adhesion to cellulose, and how much does this vary among different cellulolytic microorganisms?
What is the relative prevalence, origin (e.g., accretion or de novo synthesis), and bioenergetic cost associated with synthesis and assembly of glycocalyx components?
What function is served by the “protuberances” observed to form between cellulose and some cellulolytic microorganisms?
Rates of microbial cellulose hydrolysis (pure cultures)What are the comparative rates of cellulose hydrolysis by different microorganisms on both a cell- and cellulase-specific basis, and what factors are responsible for observed differences?
What is the relative effectiveness of cellulase systems in cellulose-enzyme-microbe complexes compared to cellulose-enzyme complexes?
If cellulase-microbe synergy can be demonstrated, what is its mechanistic basis?
Ecology and evolutionWhat are the ecological strategies and niches of various cellulolytic organisms, including interactions with other organisms, spatial relationship to the substrate (e.g., adhesion and penetration of pores by filaments), and the physical environment?
Can ecological strategies and niches be reconciled with cellular and subcellular features of the cellulolytic apparatus, and can the selective pressures leading to the evolution of such features be understood?
In cases where cell-cell synergy can be demonstrated, what is its quantitative significance and mechanistic basis?