A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass

We present a critical review of the relationships among three microbial products: extracellular polymeric substances (EPS), soluble microbial products (SMP), and inert biomass. Up to now, two different "schools" of researchers have treated these products separately. The "EPS school" has considered active biomass and EPS, while the "SMP school" has considered active biomass, SMP, and inert biomass. Here, we provide a critical review of each of the microbial products. Then, we develop a unified theory that couples them and reconciles apparent contradictions. In our unified theory, cells use electrons from the electron-donor substrate to build active biomass, and they also produce bound EPS and utilization-associated products (UAP) at the same time and in proportion to substrate utilization. Bound EPS are hydrolyzed to biomass-associated products (BAP), while active biomass undergoes endogenous decay to form residual dead cells. Finally, UAP and BAP, being biodegradable, are utilized by active biomass as recycled electron-donors substrates. Our unified theory shows that the apparently distinct products from the SMP and EPS schools overlap each other. Soluble EPS is actually SMP, or the sum of UAP and BAP. Furthermore, active biomass, as defined by the SMP school, includes bound EPS, while inert biomass includes bound EPS and the residual dead cells.