Study reveals ancient microbial feasting

The first-ever snapshot of primitive organisms eating each other has been found in ancient fossils examined by a team of scientists at The University of Western Australia (UWA).

The fossils, preserved in 1900 million-year-old Gunflint chert from Lake Superior, Canada, capture ancient microbes in the act of feasting on a cyanobacterium-like fossil called Gunflintia - with the perforated sheaths of Gunflintia being the discarded leftovers of this early meal.

UWA postdoctoral research fellow and study leader Dr David Wacey said, instead of absorbing carbon dioxide, the microbes were eating previously formed organic matter and breaking it down, much as humans do after dinner, in a manner of feeding called heterotrophy.

The research, published in Proceedings of the National Academy of Sciences, provides both physical and chemical clues to primitive heterotrophy. Dr Wacey and UWA colleagues from the ARC Centre of Excellence in Core to Crust Fluid Systems, the Centre for Microscopy, Characterisation and Analysis and the Centre for Exploration Targeting, and researchers from the University of NSW (UNSW), Oxford University and Bergen University in Norway, analysed the microscopic fossils, ranging from about 3-15 µm in diameter.

They used a battery of new techniques and found that Gunflintia was more perforated after death than other kinds of fossils, consistent with them having been eaten by bacteria. In some places many of the tiny fossils had been partially or entirely replaced with iron sulfide (‘fool’s gold’) a waste product of heterotrophic sulfate-reducing bacteria that is also a highly visible marker.

The team also found that these Gunflintia fossils carried clusters of even smaller (around 1 µm) spherical and rod-shaped bacteria that were seemingly in the process of consuming their hosts. Similar processes of heterotrophic consumption are still happening today and can often be detected by the smells they emit, such as the rotten egg smell of hydrogen sulfide.

Dr Wacey said recent geochemical analyses had shown that the sulfur-based activities of bacteria could likely be traced back 3500 million years or so - a finding reported by his group in Nature Geoscience in 2011.

“But until now it has proved more difficult finding ancient fossil evidence for this heterotrophic mode of feeding. Whilst the Gunflint fossils are only about half as old, they confirm that such bacteria were indeed flourishing by 1900 million years ago. They were also highly particular about what they chose to eat, appearing to prefer to snack on Gunflintia as a ‘tasty morsel’ in preference to another bacterium (Huroniospora).”

The study was carried out using the Australian Microscopy & Microanalysis Research Facility at UWA and UNSW.