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2011 Annual Science Report

Massachusetts Institute of Technology Reporting  |  SEP 2010 – AUG 2011

Biomechanics of the Rangeomorph Fauna

Project Summary

The oldest communities of fossil eukaryotes are found in the sedimentary rocks of Mistaken Point Newfoundland. These sediments were deposited in deep, slow-moving waters, at depths where light could not penetrate. Communities of fossil fronds preserved here reached up off the bottom, much like plants, but are thought to have lived by absorbing reduced compounds through their large surface area. In our work we show that growth off the seafloor provides an opportunity to reach higher flow velocity in this low flow environment. This exposure to flow breaks down diffusional limits, permitting more rapid uptake and growth. This opportunity is only available to larger-sized organisms, and this size advantage is exclusive to multicellular eucaryotes – not to competing bacteria with their smaller cell-size, and minimal multicellularity. Thus these communities and this advantage to multicellular form represents an important step in the evolution of complex multicellular life.

4 Institutions
3 Teams
1 Publication
0 Field Sites
Field Sites

Project Progress

A series of modeling exercises conducted this year clearly indicate that the primary advantage to the size of rangeomorphs resides in there increased exposure to flow with greater height off the bottom. Rangeomorphs are thought to be osmotrophs, that like bacteria, absorb reactants that are in solution form the environment. Models of flow based on the deep-water setting recorded in the sedimentary record, as well as the spacing and height of the individual rangeomorphs, reveal that canopy flows operate in this context Fig1. 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. These are the same Kelvin-Hemholtz vortices that make fields of grain wave in the wind. The efficient mixing in the community precludes explanations for eukaryote size-advantage that envision gradients or concentration differences effective at the scale of the height of the community. 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 are able to escape limits that constrain prokaryotes. These results were presented at the annual GSA meeting in Minneapolis, and a manuscript will be submitted this fall.

Fig 1 – Frontal Area, Velocity and Mixing in a Mistaken Point Rangeomorph Community. ​Top panel – frontal or laterally projected area of the faunal components and the total for Bedding plane D at Mistaken Point. Middle panel – Reconstructed velocity for the community. Lower panel – Mixing in the reconstructed community. Note that velocity is dramatically reduced but vertical mixing is greatly increased in the community, due to the Kelvin Helmoltz vortices induced by the community “canopy”.

Fig 2. – Uptake at the surface of Rangeomorphs. ​Uptake rate at an organismal surface as a function of flow velocity and intrinsic uptake “velocity” k. Note that in the flow velocities (dashed box) likely to be typical within the rangeomorph community, access to flow is far more critical to uptake than biological variation in k. Given the vertical flow structure (fig 1 top) the height of rangeomorphs off the bottom will dramatically increase their uptake of resources.

    David Jacobs David Jacobs
    Project Investigator
    Marco Ghisalberti Marco Ghisalberti
    David Gold David Gold
    Marc LaFlamme Marc LaFlamme
    David Johnston

    Roger Summons

    Objective 4.2
    Production of complex life.

    Objective 5.2
    Co-evolution of microbial communities

    Objective 6.1
    Effects of environmental changes on microbial ecosystems