  The motility of cilia and flagella is driven by thousands of dynein motors
that hydrolyze adenosine triphosphate (ATP). Despite decades of genetic,
biochemical, structural and biophysical studies, some aspects of ciliary
motility remain elusive, such as the regulation of beating patterns and the
energetic efficiency of these nanomachines. Here, we introduce an experimental
method to measure ATP consumption of actively beating axonemes on a single-cell
level. We encapsulated individual sea urchin sperm with demembranated flagellum
inside water-in-oil emulsion droplets and measured the axonemes ATP consumption
by monitoring fluorescence intensity of a fluorophore-coupled reporter system
for ATP turnover in the droplet. Concomitant phase contrast imaging allowed us
to extract a linear dependence between the ATP consumption rate and the
flagellar beating frequency, with ~2.3e5 ATP molecules consumed per beat of a
demembranated flagellum. Increasing the viscosity of the aqueous medium led to
modified beating waveforms of the axonemes and to higher energy consumption per
beat cycle. Our single-cell experimental platform provides both new insights
into the beating mechanism of flagella and a powerful tool for future studies.
