ScienceWise - May/Jun 2006

From purple soup to clear blue seas

The complex animals and plants that inhabit the Earth today represent only a tiny fraction of the history of life. For most of the last two billion years, the planet was entirely populated by simple single-celled organisms such as bacteria and algae. Although they may not appear outwardly spectacular their presence caused profound changes to the environment and laid the evolutionary foundations for the complex organisms of today.

However, getting information about these early organisms is difficult. Under some circumstances they can leave microfossils but their tiny size means it’s rarely possible to derive much information from them. Chemical analysis of sedimentary rocks yields some data, but again the detail is lacking. Even recent advances in DNA mapping have little to offer to palaeobiologists because molecular clocks that predict the first appearance of organisms become increasingly unreliable as you go back millions of years.

To help fill in the gap, Dr Jochen Brocks from the Research School of Earth Sciences has been working on adapting a petroleum research technique. The method looks for lipids from the cell membranes of bacteria, some of which are both chemically stable and relatively indigestible. This means that they neither decay or are consumed by other organisms. Furthermore, the molecular structure of these “biomarker” lipids is closely linked to the bacterial gene sequences that create them. This means that even though you can’t directly detect an ancient organism’s DNA, you can deduce parts of its genome from the types of lipids it leaves behind.

The biochemistry is very complex and there are few convenient instances where a single lipid is species specific. However, by carefully correlating the ratios of the various lipids present, Dr Brocks has begun to build up a picture of the diversity and ecology of bacteria in billion-year-old oceans.

Not every oil-bearing sedimentary rock is suitable for this kind of study. Those rocks that have been pushed deep underground by subsequent geological forces get heated from within the earth. Once they reach 200°C, the biomarkers are destroyed and the information is lost, and this is the case with most sediments that were deposited in marine basins. However, there are some isolated regions such as the McArthur Basin in northern Australia where the rocks bearing the biomarker evidence have remained stable and cool for over a billion years.

Dr Brocks extracted oils from 1.6 billion year-old rock cores using organic solvents to isolate the chemical groups of interest. The samples were then analysed in a high sensitivity mass spectrometer. The process was complicated by the fact that as well as the biomarkers of interest, the samples contained millions of other hydrocarbons, oils and lipids.

A significant breakthrough occurred when Dr Brocks was checking that the distribution of biomarkers was uniform throughout a given date strata in the core samples. To his surprise he discovered that it wasn’t. After an exhaustive study, he concluded there was contamination caused by modern oils which inevitably seep into the cooling water on drilling rigs or accumulate from airborne fumes on drill core material during years of storage. Once the effects of contamination were eliminated, the data pointed to a highly stratified proterozoic ocean with oxygen producing bacteria in the upper layers and the ancient purple sulfur bacteria in the deeper anoxic sulfidic waters.

Scientists already know that oxygen first accumulated in the atmosphere around 2.3 billion years ago. However, it took a lot longer for the sea to become as oxygenated as it is today. Some scientists believe this is because oxygen in the atmosphere remained too low to penetrate the deep oceans. Any oxygen was quickly mopped up by reactive iron emitted from volcanoes at mid-oceanic ridges in the deep sea. Dr Brocks believes that in this mid-proterozoic ocean current mixing may have created what he describes as a “marble cake” of sulfidic and oxygenated waters each with their own populations of bacteria exploiting the chemistry that suited them best.

It’s still early days for palaeobio-geochemistry, but Dr Brocks is confident that we now have a better tool for probing life in Earth’s early oceans. This should ultimately help us understand how the sulfidic purple soup of the mid-Proterozoic era became today’s blue oxygen rich ocean teaming with complex and diverse life forms.

More info: Jochen.Brocks@anu.edu.au

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