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

NSF Award Abstract:

This is a project that is jointly funded by the National Science Foundation's Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (UKRI/NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own investigators and component of the work.

This project takes a small-scale approach to look at individual cells to investigate the sinking of marine diatoms, which on larger scales has implications for how food for larger organisms, carbon, and organic particles move throughout the ocean. Diatoms are a type of phytoplankton, cells that use photosynthesis in surface waters to produce roughly half of the world's oxygen and the food to support ocean food webs. They have a heavy, glass-like outer wall which causes them to sink and move up to 40% of particulate organic carbon from the ocean's surface to the deep sea. The investigators are using novel methods to determine how diatoms regulate their sinking quickly in response to different environmental conditions. These include state-of-the-art video measurements of individual cells, a micoelectrode approach to understand changes at cell surfaces, and microscopy to see changes inside and at the surface of cells. The resulting information will be used to build a model to understand how and why diatoms use unsteady sinking behavior based on their environment. The project supports early career investigators, provides training for a postdoctoral scientist and undergraduate students, and develops a collaboration between US and UK scientists. The team is also developing lesson plans in conjunction with local high schools with high populations of underrepresented students in STEM fields.

The problem of sinking and suspension of diatoms has received considerable attention because of its ecological, evolutionary and biogeochemical significance, yet understanding of the processes that regulate sinking rates remains rudimentary. The investigators have used new techniques to make preliminary observations showing that some species of diatom exhibit an unsteady sinking behavior that consists of rapid changes of buoyancy on time scales of seconds. However, it remains unclear how widely this behavior matters across species and ocean conditions. In this study, the team of investigators is using state-of-the-art video-based measurements of sinking rates of individual cells to assess the prevalence of unsteady sinking among centric and pennate diatoms of varying cell sizes and quantify how this behavior changes in response to sharp gradients in nutrients and light. The project leverages an interdisciplinary, international collaboration to combine innovative optical techniques, advanced tools to assess cell physiology, and numerical modeling approaches to characterize suspension properties for individual diatom cells. Results are likely to transform the way we think about the ecology of diatoms, their strategies for nutrient acquisition, and mechanisms to control their buoyancy, in particular the modulation of volume and membrane of the central vacuole. This project contributes to the development of novel tools for single cell physiological studies, most notably direct measurement of diffusive boundary layers around cells under varying flow conditions and numerical modeling of cell-level processes.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

NERC Award abstract:

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.