2012 Annual Science Report
Massachusetts Institute of Technology Reporting | SEP 2011 – AUG 2012
Biomechanics of Rangeomorph Fauna
The oldest evidence of complex communities of lifeforms come from Newfoundland, Canada. The fossil beds discovered there are dominated by rangeomorphs, which look superficially like underwater plants, but probably got their nutrition by direct uptake of dissolved resources in the water. Here, we use models of water flow in the community to see how these complex organisms could have competed with bacteria for organic matter or reduced compounds in the water. Ultimately, we have determined that by sticking up off the sea floor, rangeomorphs could take advantage of sheer forces created by moving water, and gain an advantage over bacteria in competition for nutrients. The benefits of sticking up into water flow may have driven the early evolution of complex life on earth. Ongoing work seeks to clarify the transitions between flow regimes across stages of community succession from prokaryotic mats to eukaryotic communities
This year, we completed a publication modeling the flow in Precambrian rangeomorph communities. Rangeomorphs were a clade of benthic eukaryotes that dominated the first known communities of complex multicellular eukaryotic life on earth. It has been hypothesized that rangeomorphs were osmotrophs that, like bacteria, absorb reactants dissolved in the water. The size of rangeomorphs could confer advantage relative to bacteria by permitting access to a mixed regime above the bottom or by accessing flow in a low flow environment, which would facilitate uptake. Our research indicates that the primary advantage of rangeomorphs size relative to bacteria was their increased exposure to flow in low-flow deep-water settings. This is an important contribution to understanding the factors that confered size advantage to early mulitcellular eucaryotes.
The sedimentary record, as well as the spacing and height of the individual rangeomorphs, reveal that canopy flows operate in this context. In such a flow environment, the already low velocities in this deepwater setting are further reduced near the bottom. However, counter intuitively, vertical mixing increases in the presence of the community. This is a function of the vortices produced in canopy flow. Further modeling of the submillimeter diffusive boundary layer at the surface of organisms reveals that at the low velocities present in the community, any incremental increase in exposure of the organism to higher velocity yields a concomitant increase in uptake rate. In this context, velocity serves to mitigate the limits on uptake imposed when diffusion operates alone (called the unstirred condition in experimental biology). Thus, with the increased size provided by multicellular architecture, eukaryotes were able to escape limits that constrain prokaryotes. We are continuing our modeling efforts to better understand the transitions in the system between prokaryotic mats and eukaryotic communities in the marine environment
PROJECT INVESTIGATORS:David Jacobs
Project InvestigatorMarco Ghisalberti
PROJECT MEMBERS:David Johnston
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Production of complex life.
Co-evolution of microbial communities
Effects of environmental changes on microbial ecosystems