Dynamic Sinking Behavior in Diatoms: Rapid Changes in Sinking Rates May Aid in Nutrient Uptake

Diatoms are an important group of phytoplankton and the problem of their suspension and retention in the euphotic zone remains a central concept in defining their abundance and distribution. In this study, we present evidence of rapid sinking rate changes and consider the implications for nutrient uptake. We use high resolution video, particle tracking software as well as a novel method of micro-particle image velocimetry to quantify sinking behavior in three species of large (100+ µm) centric diatoms isolated from the Gulf of Mexico (Coscinodiscus wailesii, C. cf. radiatus and Hemidiscus cuneiformis). All species exhibited a highly dynamic, sinking behavior we termed ‘start-stop’. Sinking speeds of individual cells can vary over an order of magnitude in only 200-300 milliseconds. In the species tested, sinking rates as traditionally measured would represent an integration of the time spent in these two states. We estimate that only a 1.5% change cell density is required to achieve the observed result, a difference well within the range of cell densities under metabolic control. No external control of sinking (i.e. polymer excretion) was evident in video or fluid measurements. Furthermore, we find this behavior to be under physiological control of the cell as both actin inhibitors and cellular physiological inhibitors cause diatoms to sink continuously at speeds similar to heat killed cells. The rapid start-stop behavior is regained when cells are rinsed and placed back in filtered seawater. During the brief periods of rapid sinking, Cosinodiscus cells are theoretically capable of increasing the flux of nutrients into the cell by over 160% compared to the average sinking rate. These results suggest a mechanism whereby large cells can successfully compete for nutrients and that cell-specific suspension properties need to be revisited in a variety of diatom species to better understand nutrient uptake and competitive abilities of large diatoms.