Microbial Communities and their Mayhem across Mass Extinction Events

Our presenter at this on-line meeting was Professor Kliti Grice, Distinguished Professor at John Curtin University and Fellow of the Australian Academy of Science. She is the Founding Director of the WA Organic and Isotope Geochemistry Centre, School of Earth and Planetary Sciences at Curtin University.

There have been five mass extinction events over geological time. They occurred at the end of the Ordovician, the Devonian, the Permian, the Triassic and the Cretaceous periods. The last of these is the one we have all heard of, which occurred 66 million years ago, when a meteorite hit the Earth at Chicxalub in Mexico’s Yucatan Peninsular, resulting in the extinction of all the non-avian dinosaurs and 75% of all species.

But that was a minor event compared to the biggest such event, the End-of-Permian extinction, 252 million years ago, in which 96% of marine and 75% of terrestrial organisms became extinct. This extinction occurred when the land mass of Pangea formed as continents aggregated in the process of tectonic plate movement, resulting in massive volcanic activity. This extinction event had a cascade of causes, including a CO2 greenhouse effect from the volcanism and an atmosphere 6 degrees hotter than now, with no ice at the poles. There were stagnant oceans with low levels of oxygen and toxic levels of hydrogen sulfide.

Kliti and her organic geochemical team at Curtin University have used the technique of identifying biomarkers, intact lipids and stable isotopes to help find what conditions existed that may have caused the extinctions. In particular, she has used biomarkers derived from the lipids and pigments of algae, bacteria and archaea in geological strata from the time of these extinctions. Soils are ground up and thousands of biomarkers extracted. These can be seen as molecular fossils.

Green sulfur bacteria are found in situations where there was sunlight and H2S but little or no oxygen – that is, where H2S was in the upper layers of the ocean in anoxic conditions. As in today’s Black Sea, H2S is produced by sulfate-reducing bacteria. The anaerobic green sulfur bacteria higher up in the water use this H2S instead of H20 to photosynthesise, using special pigments that capture the long-wavelength light that penetrates the water column. There are also green, brown and purple sulfur bacteria. The biomarker evidence suggests that euxinic conditions (ie low O2 and high H2S) in the photic zone occurred in the ancient seas at the times of the great extinction events.

Using biomarkers, the pigments of green sulphur bacteria have been identified from six sites which were on the continental margins of Pangea when the sediments were laid down. This demonstrates that these were anoxic environments. Such an environment – “killer seawater” – would have destroyed most organisms. These biomarkers have also been found in WA, where cores drilled by Origin and ARC Energy in 2003 span this Permian-Triassic interval.

In the most recent extinction event, the meteor hit a limestone reef platform. The impact was equivalent to several billion nuclear weapons and brought an end to the Cretaceous period. Researchers at Curtin Uni sampled material from the impact crater from a major International Ocean Drilling Program. They revealed unique markers of the tsunami and showed that photosynthetic processes recovered within 20,000 years after the extinction.

Kliti presented a great deal of detailed information that she and her team are generating using the geochemical research from cores in different parts of the globe, mostly using biomarkers as “fingerprints” of certain organisms. These in turn suggest the environment they existed in and the geological and biological processes that were occurring at the time.

I find it amazing how much detail of the deep past can be ascertained from this sort of research. It also has relevance to today’s environmental concerns, since ocean warming, reef death and algal blooms that occurred during those extinctions are occurring now in the Anthropocene. Kliti’s presentation will soon be available on the DRB NATS – WA Naturalists’ Club YouTube Channel.

Mike Gregson