Marimo are unusual, attractive and endangered spherical aggregations of the filamentous green macroalga Aegagropila linnaei (Figure 1A–E) . Globally rare, marimo populations persist in cold freshwater lakes in Japan, Iceland and Ukraine. Marimo occupy both the lake bed and rise to the lake surface [2,3]. Here, we show that marimo buoyancy is conferred by bubbles arising from photosynthesis. We find that light-induced acquisition of buoyancy by marimo is circadian-regulated. We identify that there are circadian rhythms of photosynthesis in marimo, which might explain the circadian rhythm of buoyancy in response to light. This identifies a circadian-regulated buoyancy response in an intriguing and little-studied plant. Cano-Ramirez et al. show that marimo lake balls are buoyant in the light because photosynthesis causes bubbles to form on the balls. They identified a circadian rhythm in the time marimo take to become buoyant, and identified circadian rhythms of photosynthesis in marimo. These processes might underlie movements of marimo within the water column.