Wednesday, April 23, 2014
Environmental Science and Engineering Seminar
A metabolic tree of life: from geochemical roots to oceanic ecosystems
Rogier Braakman, Postdoctoral Associate, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology
Metabolism forms a global biochemical network that mediates the biogeochemical cycles and supplies the energy and building blocks for all cells. Reconstructing its evolution helps us understand how chemistry constrains coupled biological, environmental and planetary dynamics. Reconstructing evolution from the genomic record poses significant challenges, but for metabolism we can take advantage of a simple yet powerful constraint: the continuity of life. This means that the flow of mass through all metabolic networks alive today has not been interrupted since their lineages diverged from the earliest cells. This allows us to build trees of functional metabolic networks. Because nodes in these trees are chemical phenotypes, we can often identify clear environmental driving forces for divergences. I will highlight these concepts using two examples. First I will discuss the early evolution of CO2-fixing pathways, which support all life on Earth. All extant pathways can be related to a single ancestral form, and simple physical-chemical driving forces can explain most divergences. Next I will discuss how metabolic evolution may impact ecological dynamics, using the marine cyanobacterium Prochlorococcus. Prochlorococcus is one of the most abundant organisms on Earth, and a dominant CO2-fixer in the nutrient poor tropical and sub-tropical oceans. Surprisingly, its core metabolic network appears to have evolved toward a strategy that facilitates increased exudation of small organic molecules, which may have important downstream ecological consequences. Together these examples illustrate how metabolism provides a lens for uncovering chemical feedbacks in the coevolution of Earth and the biosphere.