Photosynthesis and circadian rhythms regulate the buoyancy of marimo lake balls

Marimo are unusual, attractive and endangered spherical aggregations of the filamentous green macroalga Aegagropila linnaei (Figure 1A–E) [1]. 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.