199 - Maintenance energy: a general model for energy-limited and energy-sufficient growth

Even when a microbe is actively growing, it is not devoting all of the energy it gathers toward growth; some always goes toward maintaining basic functions. This is called "maintenance energy." When there's plenty of food around, this constant consumption (being relatively small) is minute and hard to perceive, but when food is scarce, it becomes more apparent.

In this paper, John Pirt (who apparently is one of my academic forebears) defined a number of ways to determine and use the maintenance energy in continuous culture experiments.

Through a series of equations are difficult to conceptualize except mathematically, Pirt derives a theory that only if maintenance energy were zero could growth rate and substrate utilization efficiency reach their theoretical maximum values. If maintenance is greater than zero, which it always is, the maximum growth rate is reduced some amount called a, which is the specific growth rate at which the actual growth yield is half of the maximum theoretical growth yield, so substrate utilization is not as efficient as it could be.

However, this assumes that the maintenance energy requirement stays constant over different rates of growth. But if maintenance decreases as growth rate increases, the formula is different. In this case, maintenance energy falls to zero as growth rate increases up to the reduced amount mentioned above. In this case, while growth rate never reaches its theoretical maximum, substrate utilization does.

Allow me to quote, since I don't think I could word this concept better:

"The specific maintenance rate (a) may be regarded as an endogenous metabolism rate which results in decrease in the biomass and expenditure of the corresponding amount of maintenance energy."

But this article goes beyond theory and tries to use data from live cells to test this concept. The data are take from this study.

The data show the behavior of bacteria, Klebsiella aerogenes, growing in conditions where either carbon, nitrogen, phosphorus, or sulfur is insufficient. In a graph with oxygen consumption on the y-axis and growth rate on the x-axis, a comparison between actual culture data and lines drawn on the graph based on theoretical patterns, the data fit the lines amazingly well. This is especially true for carbon, sulfur, and nitrogen; not so much with phosphorus, in which the oxygen consumption doesn't increase as much as expected at higher growth rates.

Overall, the maximum growth rate and oxygen consumption don't change between different limitations, either in theory or in real data (possibly excepting P). This is working under the assumption mentioned above, that maintenance energy decreases as growth rate increases. But at low growth rates, the different limitations have very different effects on the oxygen consumption: there is much lower consumption when carbon is limiting compared to the others, especially phosphorus.

To explain the divergence from theory in the P-limiting experiment, Pirt suggests that the cells may be storing excess nutrients in their cells, which the researchers that collected the data did not consider or control for. This is one thing about cells that can be tricky: they change their behavior in certain conditions. And this could be how the maintenance energy requirement changes.

And that's about it. So... I'm not sure I understand this completely, but probably better than before.

Citation: Pirt, S. J. Maintenance energy: a general model for energy-limited and energy-sufficient growth. Arch. Microbiol. 133, 300–302 (1982).