The maintenance coefficient $m_S$ (g substrate g⁻¹ biomass h⁻¹) represents substrate used for non-growth energy demands: osmotic work, membrane potential maintenance, protein turnover, and DNA repair. It becomes significant at low growth rates. The Pirt equation captures this: $\dfrac{q_S}{\mu} = \dfrac{1}{Y_{max}} + \dfrac{m_S}{\mu}$, where $Y_{max}$ is the true maximum yield.