Microcalorimetry reveals bacterial motility as an expensive trait during starvation
Quantifying the energetic costs of cellular traits is important to understand their evolutionary constraints and ecological function. However, direct measurements of energy expenditure of distinct traits are rare. Here, we present direct measurements of the power dissipation due to bacterial flagellar motility using microcalorimetry, based on measuring the power dissipation of a motile population and a non-motile mutant. For the single-flagellated marine bacterium \textit{Vibrio coralliilyticus}, we determined the power dissipated by motility in cultures to be 0.91 W/mL/OD. Per swimming cell this amounts to 2.0E-15 W, or a turnover of 2.5E4 ATP molecules per second. The power dissipated for motility equals 35 % of total metabolism during starvation, indicating this is likely the most expensive cellular trait during starvation. The dissipation for motility is in good agreement with that predicted using resistive force theory, indicating that any thermodynamic inefficiencies upstream of the flagellar motor, such as metabolic inefficiencies, are small. We further show that the multiflagellated bacterium Escherichia coli dissipates 0.70 W/mL/OD when in motility buffer, comparable to marine bacteria despite the much lower swimming speed, and hence indicating lower efficiency of swimming. Finally, we show that the energy spent by Vibrio coralliilyticus on motility can have major consequences on their survival under starvation, causing a 40 % reduction in survival after 3 weeks of starvation of a motile compared to non-motile mutant. These results reveal the large costs involved in an individual trait that shapes microbial life in heterogeneous environments.