Seeing new depths: Three-dimensional flow of a free-swimming alga

A swimming microorganism stirs the surrounding fluid, creating a flow field that governs not only its locomotion and nutrient uptake, but also its interactions with other microorganisms and the environment. Despite its fundamental importance, capturing this flow field and unraveling its biological implications remains a formidable challenge. In this work, we report the first direct, time-resolved measurements of the three-dimensional (3D) flow field generated by a single, free-swimming microalga, Chlamydomonas reinhardtii, a widely studied model organism for flagellar motility. Supported by hydrodynamic modeling and simulations, our measurements resolve how well-established 2D flow features such as in-plane vortices and the stagnation point emerge from and dictate the 3D structure of the algal flow. More importantly, we uncover novel flow processes---such as hairpin flow, traveling vortices, and topological transformations---that significantly enrich our understanding of low-Reynolds-number fluid mechanics. Biologically, access to the complete 3D flow field enables rigorous quantification of the alga's energy expenditure, as well as its swimming and feeding efficiency, substantially improving the precision of these key physiological metrics. Taken together, our study reveals unexpectedly rich vortex dynamics in inertialess flows and advances fundamental knowledge of microhydrodynamics and its influence on microbial behavior. The work also establishes a powerful method for visualizing the 3D fluid environment sculpted by the beating flagella of motile cells.