Wednesday, December 20, 2006
The oldest-known animal eggs and embryos, whose first pictures made the cover of Nature in 1998, were so small they looked like bugs - which, it now appears, they may have been.
This week, a study in the same prestigious journal presents evidence for reinterpreting the 600 million-year-old fossils from the Precambrian era as giant bacteria.
The discovery 'complicates our understanding of microfossils thought to be the oldest animals,' said lead author Jake Bailey (homepage), a graduate student in earth sciences at the University of Southern California.
Bailey made his discovery by combining two separate findings about Thiomargarita*, the world's largest known living bacterium.
In 2005, Thiomargarita discoverer Heide Schulz, from the University of Hannover in Germany, showed that the bacterium promotes deposition of a mineral known as phosphorite**.
The fossils identified as eggs and embryos in 1998 came from southern China's Doushantuo Formation, which is rich in phosphorite.
The source for the rare mineral was unknown. Bailey wondered if an ancient relative of Thiomargarita might have been involved.
Continued at "Oldest Animal Embryos or Bacteria?"
Based on the Letter to Nature:
"Evidence of giant sulphur bacteria in Neoproterozoic phosphorites"
In situ phosphatization and reductive cell division have recently been discovered within the vacuolate sulphur-oxidizing bacteria. Here we show that certain Neoproterozoic Doushantuo Formation (about 600 million years bp) microfossils, including structures previously interpreted as the oldest known metazoan eggs and embryos can be interpreted as giant vacuolate sulphur bacteria. Sulphur bacteria of the genus Thiomargarita have sizes and morphologies similar to those of many Doushantuo microfossils, including symmetrical cell clusters that result from multiple stages of reductive division in three planes. We also propose that Doushantuo phosphorite precipitation was mediated by these bacteria, as shown in modern Thiomargarita-associated phosphogenic sites, thus providing the taphonomic conditions that preserved other fossils known from the Doushantuo Formation.
*Current info on Thiomargarita:
Discovered in 1999 off the coast of Namibia, Thiomargarita namibiensis has the distinction of being the largest bacterium in the world, at a remarkable volume of 3 million times that of a normal bacterial specimen. The long string of cells, named "Sulfur pearl of Namibia" because of the reflective white globules of sulfur contained in them, was found buried in the sulfur-abundant sea floor. Each ball-shaped cell can grow up to nearly 1 milimeter in diameter - a measurement usually taken in microns. Because of some unique adaptations, Thiomargarita namibiensis is able to survive in a high-sulfur environment with little or no oxygen.
**From the journal Science:
"Large Sulfur Bacteria and the Formation of Phosphorite"
by Heide N. Schulz and Horst D. Schulz
Phosphorite deposits in marine sediments are a long-term sink for an essential nutrient, phosphorus. Here we show that apatite abundance in sediments on the Namibian shelf correlates with the abundance and activity of the giant sulfur bacterium Thiomargarita namibiensis, which suggests that sulfur bacteria drive phosphogenesis. Sediments populated by Thiomargarita showed sharp peaks of pore water phosphate (equal to or less than 300 micromolar) and massive phosphorite accumulations (equal to or greater than 50 grams of phosphorus per kilogram). Laboratory experiments revealed that under anoxic conditions, Thiomargarita released enough phosphate to account for the precipitation of hydroxyapatite observed in the environment.