Award: OCE-1537546

Diatoms are a group of unicellular organisms and are part of the phytoplankton that are the basis of most marine food webs. They need both light and nutrients to grow and reproduce, and their access to both depends on their depth in the water, so understanding how they sink reveals a lot about their biology. A rich literature exists on the effects of various environmental factors on sinking, but the entire field is underpinned with the assumption that sinking occurs at a constant rate over minutes to hours. Our results challenge this assumption. Using sophisticated high-speed videography, we have tracked individual diatoms as they sink and found that some display an unsteady sinking behavior in which their sinking rates change dramatically over a time-scale of seconds (see figure). Multiple species exhibit this behavior and there may be a link between general shape of the cell and presence/absence of the behavior; all of the diatom species that have been found to exhibit unsteady sinking to date are large, centric (cylindrical) diatoms. The model species Coscinodiscus wailesii goes from terminal velocity to a complete stop in milliseconds and then resumes sinking after several seconds. Unsteady sinking behavior appears to be under metabolic control and varies in response to changes in light and nutrient conditions, but the mechanism is unknown. One outcome of this project is that the traditional view of how nutrients diffuse to and are acquired by phytoplankton assume a steady sinking rate. Introduction of unsteady sinking rates changes how these calculations are made and provides a possible mechanism to explain how larger phytoplankton are able to compete with smaller phytoplankton for nutrients. Four undergraduates and one postdoctoral Research Fellow was supported on the project with results disseminated in both peer-reviewed journals and national conferences. Last Modified: 09/27/2019 Submitted by: Tracy A Villareal