Tiny fractals in the sea: the biogeochemistry of particle shape

Project Rationale:

Particles* are a critical aspect of life and chemistry in the ocean. Marine particles are hotspots of biological activity, and their sinking mediates a major vertical transport of mass and energy that connects the sunlit and dark ocean ecosystems and fundamentally influences global biogeochemistry. Marine particles tend to be formed by aggregating processes, so they tend to behave like fractals** [1]. Both particles’ sinking speeds and the chemical reactions that occur within them [2] are strongly influenced not only by their sizes but also by their shapes. Yet for simplicity particles are nearly always treated as spheres, neglecting any shape effects. The student’s goal will be to explore how particle shape in the ocean affects particle-mediated transport and the oxygen (and other chemical) gradients and cycling within particles, and thus ocean biogeochemistry as a whole. How different are the cycles of carbon, nitrogen and other elements as a result of particles being spherical, fractal, or other shapes altogether? This requires pairing microscale modeling of fractal particles with global-scale ocean biogeochemical modeling, complemented by laboratory and field data.

*objects ranging from ~micron to ~millimeter diameters
**objects whose key geometric properties (radius, area, volume…) have different relationships than ‘simple’ objects like spheres

Methodology:

The project will involve a multidisciplinary combination of theory, simulation, and experimentation. The student will use models of fractal aggregates from the physics and engineering literatures [1,3] to explore how particle shape and fractal dimension influence chemical cycling within particles as well as these particles’ sinking speeds. A careful meta-analysis of published data on particle size, shape, and sinking properties would also be tremendously useful for this purpose. The student will then apply these findings to the global scale by modifying a global numerical model to include non-spherical particles [2]. This will allow the student to estimate how global biogeochemical cycles are influenced by the shape of particles in the ocean. The student can also conduct laboratory experiments and/or field measurements to constrain or test predictions arising from the modelling efforts. The student could employ optical and bioimaging techniques to determine particles’ size and shape of field-collected or lab-generated particles, and in combination with molecular techniques to identify and characterize the microbial activities within. The initial focus will be on carbon and nitrogen cycling, particularly under reduced oxygen conditions, but can be expanded to other elemental cycles such as those of sulphur and trace metals.

Training:

The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at the National Oceanography Centre (NOC)within the globally renowned Ocean Biogeochemistry and Ecosystems group. The student will receive training in numerical modelling – one of the most in-demand skills in international environmental research – at both the microscale and the global scale. Trainings in (bio)imaging and molecular biological techniques will also be provided as needed, at the School of Ocean and Earth Science (University of Southampton) which shares the same building as NOC. The student will have the opportunity to participate in one or more research cruises to make relevant measurements on particles at sea using cutting-edge technologies, and to develop relevant laboratory experiments. The student will also be encouraged to present results at national and inter-national conferences and to attend workshops on the research topic. The student can attend appropriate graduate courses for background as needed.