Research at Virginia Tech has shown that the oldest complex life forms
— living in nutrient-rich oceans more than 540 million years ago –
likely fed by osmosis.
The researchers studied two groups of modular Ediacara organisms, the
fern-shaped rangeomorphs and the air mattress-shaped erniettomorphs.
These macroscopic organisms, typically several inches in size, absorbed
nutrients through their outer membrane, much like modern microscopic
bacteria, according to the cover story of the Aug. 25, 2009 issue of the
Proceedings of the National Academy of Sciences (PNAS),
"Osmotrophy in modular Edicara organisms," by Marc Laflamme, Shuhai
Xiao, and Michal Kowalewski. Laflamme, now a Postdoctoral Fellow in the
Department of Geology and Geophysics at Yale University, did the
research as a postdoc in Xiao’s lab at Virginia Tech. Xiao and
Kowalewski are professors of geobiology in the College of Science at
Virginia Tech.
The rangeomorphs had a repeatedly branching system like fern leaves
and the erniettomorphs had a folded surface like an inflated air
mattress to make tubular modules. "These organisms are unlike any life
forms since and so are poorly understood," said Laflamme.
Their feeding strategy has been a topic of controversy, with theories
ranging from parasitism to symbiosis to photosynthesis. "Some
hypotheses can be ruled out because the organisms lack feeding
structures, such as tentacles or mouths, and because many of them lived
in the deep ocean where there was no sunlight for photosynthesis" said
Xiao.
The researchers decided to simulate various morphological changes in
the overall construction of the organisms to test whether it would have
been possible for them to attain surface area to volume ratios on the
same order as modern bacteria that feed by osmosis. Theoretical models
were constructed to explore the effects of length, width, thickness,
number of modules, and presence of internal vacuoles, on the surface
area of the Precambrian fossils. "Modeling efforts suggest that internal
vacuoles – that is, voids filled with fluids or other biologically
inert materials – are a particularly effective way of increasing
surface-to-volume ratio of complex, macroscopic organisms," said
Kowalewski.
They discovered that the two groups (the repeatedly branching
rangeomorphs and the air-mattress like erniettomorphs) grew and
constructed their bodies in different ways; however both groups
attempted to maximize their surface-area to volume ratios in their own
way. "The increase in size was clearly accomplished primarily by
addition of modules for the erniettomorphs and repetitive branching and
inflation of modules for the rangemorphs," Laflamme said. "The repeated
branching system in rangeomorphs was essential to allow for a high
surface-area to volume ratio necessary for proper osmosis-based
feeding."
Today, only microscopic bacteria find it efficient to us only osmosis
to feed, although some animals, such as sponges and corals, use osmosis
as a supplementary food source. But in the Ediacaran period, 635 to 541
million years ago, with nutrient-rich oceans, "a diffusion-based
feeding strategy was more feasible," Laflamme said.
"We believe the Ediacarans were feeding on dissolved organic carbon,
which can come in many forms," he said. "It represents the organic
material originating from plants, fungi, animals — you name it, which
has dissolved into fats and proteins during natural organic decay. There
is a growing body of evidence that in Ediacaran times, due mainly to
the absence of animals with true guts capable of packaging organic
matter into fecal pellets, there was a much greater pool of dissolved
organic nutrients, especially in deeper waters. Without fecal pellets,
organic substances would have remained in suspension and decomposed into
fats and proteins capable of dissolution into marine waters," he said.
"We believe these compounds were then absorbed via osmosis through
Ediacaran "skin" due to the high surface-area to volume ratios."
The PNAS article concludes that today "giant sulfur bacteria, such as
Thiomargarita, thrive along the coastal area of Namibia, where constant
upwelling allows for greater access to (dissolved organic carbon) and
nutrients. Such nutrient-rich areas may be modern-day analogs to
Ediacaran deep oceans … suggesting that it may be more than
coincidental that the earliest rangeomorphs occurred in (dissolved
organic carbon)-rich deep waters."
The research was supported by the Natural Sciences and Engineering
Research Council of Canada (NSERC) and Laflamme’s Bateman Fellowship,
the NASA Exobiology and Evolutionary Biology Program, and the National
Science Foundation Sedimentary Geology and Paleobiology Program.
Source : Virginia
Tech
