24, 2008, Courtesy Indiana University and World
Bacterial decay was once viewed
as the mortal enemy of fossilization. But new
research suggests resilient colonies of bacteria,
called biofilms, may have actually helped preserve
the fossil record's most vulnerable stuff: animal
embryos and soft tissues.
Scientists have found that
bacteria can invade dying embryo cells and form
densely packed biofilms in those cells. These
completely replace the cell structure and generate
a replica of the embryo, say the researchers,
who call this formation of bacteria filling
the shape of an embryo a "pseudomorph."
The investigators, led by Indiana
University Bloomington biologists Rudolf and
Elizabeth Raff, report their findings in this
week's early online issue of the research journal
Pro-ceedings of the National Academy of
"The bacteria consume
and replace all the cytoplasm in the cells,
generating a little sculp-ture of the embryo,"
said Elizabeth Raff, the report's lead author.
But "certain conditions
must be met if the bacteria are going to aid
the preservation pro-cess." For one, she
explained, the embryo must have died in a low-oxygen
environment, such as the bottom of a deep ocean
or buried in lakeside mud. Oxygen would make
em-bryos self-destruct as digestive enzymes
break free and wreak havoc.
Then, "bacteria able to
survive in low-oxygen conditions must then infest
the cells of the dying embryo," Raff said.
The bacteria form biofilms, crowded assemblies
of bacterial cells held together by sticky fibers
made of proteins and sugars. As the biofilms
fill the embryo cells, the tiny bacteria insinuate
themselves between and among the structures
within the cells, forming a faithful representation
of the cell's innards.
This early-stage embryo
is protected by a fertilization envelope, seen
here as a white line encircling the embryo cells.
(Credit: E.C. Raff and R.A. Raff)
of a trove of half-a-billion-year-old embryo
fossils from China, offered evidence that bacteria
may have been involved in the preservation,
research-ers say. (Credit: F.R. Turner, E.C.
Raff, and R.A. Raff).
Last, the bacteria must leave
a permanent record, she added: in the case of
finely pre-served fossil embryos, the bacteria
likely excrete tiny crystals of calcium phosphate
which eventually replace the bacterial sculptures.
These crystals would provide the support for
embryo and soft tissue fossilization.
High resolution imaging of
a trove of half-billion-year-old animal embryo
fossils from Doushantuo, China, offered scientists
tantalizing evidence that bacteria may have
been involved in the preservation of the delicate
cells, she said.
The Raffs studied early-stage
embryos of two Australian sea urchin species,
Heliocidaris erythrogramma and Heliocidaris
tuberculata. Experimental results with modern
embryos were compared to the high resolution
images of fossil embryos prepared by colleagues.
Although it's impossible to
know whether bacteria aided the preservation
of the embryo fossils from Doushantuo and elsewhere,
the Raffs argue the evidence they gathered strongly
favors the view that bacteria are a fundamental
force in fossil formation, as rapid biological
processes must be available to convert delicate
cells into a stable form and trig-ger their
"This work is important
because it helps us understand fossilization
as a biological as well as geological process,"
Elizabeth Raff said. "It gives us a window
onto the evolution of the embryos of the earth's