Samplings: June 2017

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X-ray tomography of the 0.5-millimeter-long, fossil thread-like red alga, Rafatazmia chitrakootensis, reveals well-preserved cells, each containing a rhombic platelet assumed to belong to the photosynthesizing apparatus.

Stefan Bengtson

Eukaryotes, the group of organisms that includes all plants, animals, and fungi, are thought to have diverged from single-celled prokaryotes at least one billion years ago. The exact timing of this transition has long been debated. Now, researchers led by paleobiologist Stefan Bengtson of the Swedish Museum of Natural History and the Nordic Center for Earth Evolution in Stockholm believe they have found 1.6-billion-year-old red algae fossils, suggesting that eukaryotic life began 400 million years earlier than previously thought.

Bengtson and his team discovered what appear to be fossils of two new species of red algae, or rhodophytes, in the Chitrakoot region of Uttar Pradesh and Madhya Pradesh in central India. The researchers found the fossils by manually sifting through residue of dissolved rocks from the Chitrakoot site. They then used synchrotron-radiation X-ray tomographic microscopy—essentially, X-rays on a microscopic scale—to view the fossils and create three-dimensional images.

The fossils of one species, a thread-like alga dubbed Rafatazmia chitrakootensis, are tiny tubes between 58 and 275 micrometers in diameter. The other species, Ramathallus lobatus, appears to have been bulbous, with each cell between five and fifteen micrometers across. 

The researchers determined these two species to be eukaryotes, and specifically red algae, based on their size, internal structures, and apparent growth patterns. The Rafatazmia cells were larger than those of filamentous bacteria with tubes of similar sizes. They also contained organelles that were 100 times larger than bacterial organelles. These organelles appear to have been pyrenoids, small compartments associated with chloroplasts in algae. In the center of each cell wall were other structures typically found in red algae. The Ramathallus cells showed a pattern of growth that suggests photosynthetic ability.

These findings shift our understanding of the evolution of the eukaryotic branches. Not only were eukaryotes present at least 1.6 billion years ago, but the two species seem to represent ancestral rhodophytes that had already diverged into a filamentous and a fleshy form. (PLOS Biology)