Alexander Petroff, Ph.D.Raymond and Beverly Sackler Fellow
Center for Studies in Physics and Biology
I am interested in understanding how mathematical forms appear in nature. In my past projects, I have investigated two examples of growth in a diffusion field: a network of streams that grows by attracting groundwater and a microbial community that grows by attracting nutrients. Through a combination of theory, experiments, and field observations, our results have provided insight into diverse phenomena, from dendritic networks of streams to billion-year-old fossils. My current work seeks to understand an example of how physical interactions between bacteria can give rise to cooperation.
My field site is in a salt marsh on the coast of Massachusetts. In this marsh—and many others the world over—the slow decay of buried seaweed produces hydrogen sulfide, which diffuses from the mud into the overlaying oxygen-rich water. Water is sufficiently viscous as to remain stagnant within a fraction of a millimeter above the bottom of the marsh. Within this so-called "diffusive boundary layer", chemical diffusion is the sole mechanism by which oxygen and sulfide mix. As the reaction of sulfide with oxygen releases energy, some bacteria have evolved to exploit this energy source. These "sulfur-oxidizing bacteria" live in the diffusive boundary layer.
An inevitable problem arises for bacteria living within the diffusive boundary layer. Because the diffusive transport of nutrients over the relevant distances is slow, the metabolic rate of the bacteria becomes limited by diffusion. Sulfur-oxidizing bacteria have evolved numerous remarkable strategies to overcome diffusion limitation. The bacteria I study form into massive communities extending over centimeters. Within these communities, each bacterium exudes a tether that the cell uses to tie itself to the community. As each tethered cell beats its flagellum, the community generates macroscopic flows that transport nutrients an order of magnitude faster than either by diffusion alone or by the flow created by an individual cell. In this way, the community is more than the sum of its parts. In my research, I seek to understand how the ability of a community to transport nutrients so effectively is related to the behavior and morphology of individual cells.
To these ends, I combine mathematical models with laboratory observations of the flow of water and nutrients around cells and communities. Thus far we have found that hydrodynamic interactions between tethered cells and the surfaces to which they attach allow these cells to transport water orders of magnitude more effectively than free swimming bacteria. Moreover, we have observed that long-range interactions between cells give rise to collective modes. We believe that the resulting density fluctuations are vital to the transport of nutrients over macroscopic distances. We are currently designing experiments to measure the influence of these fluctuations on nutrient transport.
After completing two bachelor’s degrees—one in physics and one in mathematics—at Carleton College, Dr. Petroff went on to earn his Ph.D. in geophysics from M.I.T. He has received many honors, including a 2010 American Geophysical Union Outstanding Student Paper Award and a 2008 Department Student Teaching Award from M.I.T. He joined Rockefeller in 2011 as a fellow in the Center for Studies in Physics and Biology.