The core concept¶

NutMEG is built on the idea of energetic habitability. Energy is the universal currency for life as we understand it. While metabolic processes vary, they will have a net energetic cost or yield which is quantifiable via standard chemical thermodynamics. By identifying energy sources and processes corresponding to maintenance (e.g. survival in adverse conditions) and growth we can assess the habitability of various environments.

If the energetic availability in an environment outweighs the energetic cost of surviving there, we say it is energetically habitable. NutMEG estimates microbial growth via an approach where efficiencies are applied to the energetic input from metabolism (\(P_{S}\)) corresponding to microbial maintenance (\(\epsilon_{M}\)), and nutrient uptake (\(\epsilon_{UT}\)). Any leftover energy can be directed into biomass synthesis (\(P_{G}\)).

\[P_{G} = \epsilon_{UT} \epsilon_{M} P_{S}\]

The maintenance efficiency reflects the total cost of microbial maintenance processes - ones which are necessary for survival but do not directly contribute to growth. These could include maintaining a specific internal pH (\(P_{pH}\)), repairing biomacromolecules as they break down with temperature (\(P_{T}\)), and defending against adverse salinity (\(P_{SAL}\)) to give a few examples. Mathematically it takes this form:

\[ \begin{align}\begin{aligned}\epsilon_{M} = 1 - \frac{ P_{main} }{ P_{s} }\\P_{main} = P_{pH} + P_{T} + P_{SAL} + ...\end{aligned}\end{align} \]

The nutrient uptake efficiency is more complex to compute. It represents the effect of limited availability of carbon, hydrogen, nitrogen, oxygen, phosphorus, or sulfur (CHNOPS) elements on the amount of biomass an organism can actually make per unit time.

The core concept behind NutMEG is that if energy still remains after applying \(\epsilon_{M}\) and \(\epsilon_{UT}\) both the energy and nutrients are available in the local environment not just for an organism to survive, but for it to grow as well.