Dynamic Unsteady Sinking Behavior in Marine Diatoms: Rapid Responses to Changing Nutrient Conditions

Sinking is an important determinant of a diatom population's distribution and of an individual diatom's access to light and nutrients. It has long been known that populations of diatoms change their mean sinking speeds in response to environmental conditions, such as nutrients concentration and light availability, and to their own physiological conditions. Direct observation of sinking behavior through videography has revealed diatom sinking to be more complex than previously recognized. Population mean sinking speeds represent cells with individual mean speeds that can range over an order of magnitude. Furthermore, some species of large, centric diatoms exhibit unsteady sinking behavior, rapidly oscillating their sinking speeds within seconds. This unsteady sinking behavior change is dynamic, responding to changing environmental and physiological conditions. Faster and more unsteady sinking can increase nutrient fluxes by thinning the diffusion boundary layer around a cell, but faster sinking also decreases the amount of time a cell spends in the euphotic zone. Past studies have shown that Coscinodiscus wailesii diatoms sink faster and more unsteadily when deprived of nutrients. Here we show that nutrient-deprived cells can further increase the speed and unsteadiness of their sinking behavior when exposed to a spike of nutrients. After depriving C. wailesii cultures of nitrate or silicate and adding a spike of the missing nutrient, cells showed a large increase in sinking speed and unsteadiness within the first 2 h that declined over the next 22 h. Phosphate-deprived cultures did not respond similarly to exposure to phosphate. Faster and more unsteady sinking may allow diatoms to increase nutrient fluxes upon encountering high concentrations of limiting nutrients.