Environmental and bioenergetic constraints on the synthesis of biomass

Abstract:

Aside from nutrients and water, the main factor determining whether a given microbial population increases, decreases or stays the same size is the amount of energy that is available to the community and the rate at which it is delivered, the power supply. The power supply characterizing an environment - the Gibbs energy of catabolic reactions multiplied by the rate of reactants supply - is relatively straightforward to determine. However, the power required to synthesize biomolecules needed for growth is not only a function of the temperature, pressure and composition of the environment where it is happening, but also the actual amount of biomass needed to make a new cell. The purpose of this presentation is to quantify the cost of synthesizing biomass under different environmental conditions for differently sized microbial cells.

In particular, we will show that the amount of energy required for anabolism is strongly dependent on the average nominal oxidation state of carbon, NOSC, in the compounds that are used for anabolism and the oxidation state of the environment where growth is occurring. For instance, the energy required to synthesize glycine (NOSC = +1) from CO₂ (NOSC = +4 under relatively oxic conditions, log a_H2 = -9, is +87 kJ (mol glycine)^-1, whereas under more reducing conditions, log a_H2 = -4, it’s only + 1.8 kJ (mol glycine)^-1. If instead acetate (NOSC = 0) is used as the carbon source for glycine anabolism, it’s exergonic to synthesize glycine throughout a very broad range of redox states (from O₂ saturated water to log a_O2 = -45).

Furthermore, and unlike earlier efforts, the amount of energy required to make microbial cells is corrected for their variable sizes. Based on a literature survey, the mass of carbon in Archaeal and Bacterial cells ranges by at least a factor of 17. Similarly, of the ~3100 complete Bacterial genomes that have been sequenced and reported on the Nation Center for Biotechnology Information, NCBI, website, the number of base pairs per genome varies by a factor of 138.

Taken together, the number of cells that can be synthesized by a joule of energy can vary by several orders of magnitude depending on the environmental conditions and the size of the cells being made.