Modelling optimized allocation of energy by microbes and their physiological state

English Abstract:

We developed a model that simulates microbes, extracellular enzymes, and cellulose as complex growth substrates, and monomers resulting from hydrolysis of cellulose. The microbes can grow, produce extracellular enzymes, go dormant or resuscitate, and require maintenance energy. We propose that the corresponding regulation can be addressed in a bioenergetic framework of allocating the energy gained by the catabolism of the monomers. This energy can be allocated to (1) biomass growth, (2) production of extracellular hydrolytic enzymes, and (3) maintenance of the cell. The maintenance energies (ME) used are different for active and dormant cells and assumed to be constant in the model.

There could be two possible scenarios based on the amount of energy generated from the catabolic reaction. In scenario one, the energy gained from the reaction is smaller than the total ME requirement of the active cells. In this case, there is no growth or enzyme production. The deficiency will be compensated by the energy generated through decay of active cells. Under these conditions, a change from active to dormant state is beneficiary. In scenario two, if the energy gained by the bacteria is larger than ME of the active cells, resuscitation of dormant cells is beneficial. Under these conditions, the active cells use the generated energy minus the ME for growth and enzyme production. We assume that the fraction of excess energy which is spent on the production of extracellular hydrolytic enzymes, versus that spent on biomass growth, is controlled by the number of free enzyme reactive centers transforming the monomers: If the monomer concentration is low, the corresponding Michaelis-Menten term is small, and the excess energy is invested to increase the monomer concentration by enhanced production of extracellular hydrolytic enzymes. In contrast, if the monomer concentration is high, the corresponding Michaelis-Menten term is large, and further production of enzymes would not be beneficial so that the excess energy is used for biomass growth. The dormant cells don’t respire the monomers, thus maintenance energy requires utilizing their own biomass. The model is used to illustrate an anaerobic degradation pathway for cellulose.