Award: OCE-1334365

Diatoms play an important role in the uptake and sequestration of carbon in the ocean. They fix CO2 into organic matter and because cells tend to be slightly denser than the ambient seawater they sink, removing the organic carbon from contact with the atmosphere. Thus, understanding the processes that affect sinking rates and removal of cells from the upper mixed layer of the ocean is central to the understanding of cycling of carbon in the ocean. Vertical velocities of settling particles in the ocean are determined by the size of the particle, its density (mass/volume) and interactions with the ambient flow (turbulence). Densities of diatom cells have been rarely measured directly and interactions of sinking cells with turbulent flows are not fully understood. It has been suggested that turbulence can significantly enhance sinking velocities of phytoplankton cells compared to their still-water velocities, but the underlying mechanism(s) remain speculative. This study was designed to address these gaps in knowledge. We used a density gradient approach to quantify cell densities of three species of diatoms (Coscinodiscus radiatus and Coscinodiscus weilesii and the chain forming Stephanopysis turris) and found that within a healthy population of cells cell density varies greatly (<1.05 to >1.4 g cm-3). We compared densities of three clones of Coscinodiscus that vary in cell size and found an inverse relationship between cell size and density. Furthermore, cell density is influenced by the nutritional status of cells and our results show that cell density slightly decreased when cells became light-limited. By quantifying cell density and its variance, this study provides valuable data to constrain models on phytoplankton sinking and dynamics in the mixed layer. To quantify effects of turbulence on sinking velocities we designed a new lab apparatus that allows us to track 3D trajectories of cells in a turbulent flow and calculate their vertical velocities. A numerical model was also developed as part of this project to further identify mechanisms and factors that influence the settling of cells in a turbulent flow (i.e., aspect ratio of the particle and the ration between sinking velocity and the Kolmogorov velocity scale). Results from empirical measurements and the numerical model show that turbulence affects vertical trajectories of non-spherical cells but the enhancement of sinking velocities is much smaller than previously reported. For elongated cells (modeled as prolate spheroids), we show that cell rotation (Jeffery?s orbit), drag and lift cause cells to translate into downwelling regions of the flow, resulting in the slight enhancement of their sinking velocities and weak clustering. The effect increases with the aspect ratio of the spheroid but saturates at a value of 10. Mechanisms that affect sinking and dispersion of diatoms in turbulent flows hold significant implications for a variety of process that affect carbon and energy transfer in marine ecosystems. Previous studies have focused on the settling of phytoplankton in still-water but in nature sinking cells interact with ambient turbulence and result from this study shed new light on the nature of the interaction, highlighting the importance of cell shape. The behavior of suspended particles in turbulent flows is of considerable interest to a variety of processes including dispersion of aerosols and collisions of water droplets in the atmosphere, dispersions of oil spills treated with dispersants and sediment transport, and results from this study expand the parameter space in which particle-turbulence interactions are being studied. Last Modified: 12/07/2017 Submitted by: Lee Karp-Boss