Sedimentation of elongated phytoplankton in the ocean

Phytoplankton plays a vital role in the removal of CO$_2$ from the atmosphere, as they perform photosynthesis on the ocean surface and then sink to the bottom. This process drives almost half of the carbon sequestration (the process of removing carbon from the atmosphere) on Earth. It is known that their sizes and shapes affect their average sinking speed and how they are mixed by the flow in the ocean, but little is known about the underlying mechanism from a fluid dynamics perspective. Furthermore, depending on the particle shape, it is known that some species may tend to aggregate, which in turn can speed up their collective sinking.

Meanwhile, in fluid dynamics, it is known that when elongated particles sink, they tend to aggregate together in a plume-like streamer structure. While the phenomenon has been well-known for some time, there remain some unanswered questions regarding the width of such a structure. Our recent work suggests that the formation of this streamer structure is not unlikely that of the gyrotactic plumes. In fact, they are driven by the same mathematics as chemotactic collapse, another well-known phenomenon found in auto-chemotactic systems, such as a slime mould. This discovery opens new avenues of knowledge transfer between three different topics and may help further our understanding of phytoplankton sinking in the long term.