Dr. Uria Alcombri, Swiss Federal Institute of Technology, Zurich
20/12/2021 13:00

Title: “Unravelling the microscale mechanisms driving particle degradation in the ocean”

Abstract: The sinking of organic particles to the ocean depths is the main driving force of the
biological pump, the process responsible for the export of more than 50 Gt of fixed CO2 annually
and one of the major fluxes in the oceans’ carbon cycle. Yet, the mechanisms determining the
magnitude of the pump remain poorly understood, limiting our ability to predict this carbon flux
in future ocean scenarios. Current ocean models assume that the biological pump is governed by
the competition between sinking speed and degradation rate, with the two processes
independent from one another. In this talk, I will demonstrate that contrary to this paradigm,
sinking itself is a primary determinant of the rate at which bacteria enzymatically degrade
particles in the ocean. By combining video microscopy and microfluidic experiments to directly
observe and quantify bacterial degradation of individual organic particles in flow, I will show that
even modest sinking speeds of 8 meters per day enhance degradation rates more than 10-fold. I
will further discuss the molecular mechanism behind the sinking-enhanced degradation, as well
as possible ways by which bacteria can slow the sinking of particles. Finally, using the results
obtained from a mathematical model, I will show that the flow associated with sinking is a major
contributor to the observed magnitude of the vertical carbon flux in the ocean, and will outline
major open questions in the field.

Host: Benjamin Podbilewicz