Going ballistic in the plankton: Anisotropic swimming behavior of marine protists

Abstract

Lay AbstractPlankton are a critically important group of oceanic organisms, comprising the base of marine food webs and accounting for half of all biological carbon fixation on Earth. Although plankton have traditionally been thought of as passive drifters in the ocean, many of these organisms have the ability to move of their own accord. The twisting paths that swimming protists (single‐cell eukaryotic plankton) traced out in previous laboratory studies have frequently been compared with the paths that a molecule follows as it moves randomly through space due to thermal energy in a process called diffusion. However, it is also well known that many protists in the ocean undergo large vertical migrations of several meters or more, which can be explained only if the cells were able to swim in essentially straight vertical lines, known as ballistic motion. Although there have been a few direct observations of ballistic motion of protists in the laboratory, little quantitative proof has been accumulated thus far. We assembled a large data set of relatively long‐duration, three‐dimensional swimming paths for several protist species, both plant‐ and animal‐like, and calculated how long they tended to swim straight before turning. Since almost all the cells moved primarily in the vertical direction, we separated the horizontal dimensions from the vertical in this analysis. All the protist species observed here did make frequent horizontal (left/right) turns, matching a diffusion model at scales greater than a few seconds or fractions of a millimeter. However, turns in the vertical direction were extremely rare for almost all species, indicating vertical ballistic motion (i.e., moving either up or down without switching direction) at swimming speeds that match the large‐scale field observations of migratory behavior. Our observations show ballistic swimming for much longer than in several previous studies—up to 120 s or 6 mm for organisms sized about the diameter of a human hair. The habitat and biogeochemical footprint of these planktonic protists are therefore much larger than one might expect based on their microscopic size.