NSFGEO-NERC: Novel imaging, physiology and numerical approaches for understanding biologically mediated, unsteady sinking in marine diatoms

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Diatoms account for up to 40% of oceanic primary production sinking behavior is an important species-specific property that can determine the community composition, aggregate formation and the amount of material lost to depth. Although they are unable to swim, diatoms are far from passive, controlling their sinking speeds over long time scales in response to environmental factors, such as nutrient concentration, irradiance, and temperature and biological factors, such as reproductive state. Early work on diatom sinking demonstrated the capacity of diatoms to regulate their buoyancies over hours to days in response to changing environmental conditions. However, some species can also control their sinking speeds over much shorter time scales of seconds, performing a recently discovered unsteady sinking behavior. To date, diatom suspension studies have largely used bulk measurements such as settling columns (SETCOLSs) due to the ease of measurement and assumption that bulk rates adequately capture the essential characteristics of this group. We offer evidence that this assumption is not justified and propose a series of laboratory experiments using advanced optical techniques, physiological tools that measure processes around single cells and numerical approaches to investigate taxonomic and morphological variability in unsteady sinking over a range of environmental conditions and examine the implications the observed differences. This project will leverage an interdisciplinary collaboration involving innovative optical techniques, advanced cell physiology tools and numerical modeling approaches to characterize diatom suspension properties at the individual cell level. The shift from time-averaged sinking measurements to small time scales has indicated that sinking speeds can vary orders of magnitude over seconds. We will address several aspects of instantaneous velocity control behavior in order to determine the adaptive significance of unsteady sinking. This novel observation suggests that models of diffusion limited transport need to be revised in order to accommodate species-specific differences. We will use individual cell level experimental data from a variety of species and environmental conditions to inform 3D numerical models which will be used to characterize what, if any, adaptive significance unsteady sinking in diatoms has and why it may be constrained to certain taxonomic groups.