The first photosynthetic oxygen-producing organisms on Earth were cyanobacteria. Their evolution radically changed the Earth, allowing oxygen to accumulate in the atmosphere for the first time and further allowing the evolution of oxygen-using organisms, including eukaryotes. Eukaryotes include animals, but also algae, a large group of photosynthetic oxygen-producing organisms that now dominate photosynthesis in modern oceans. However, when did algae start to occupy marine ecosystems and compete with cyanobacteria as important phototrophic organisms?
In a new study, Zhang et al use the molecular remains of ancient algae (called biomarkers) to show that algae played an important role in marine ecosystems 1,400 million years ago, some 600 million years earlier than what was previously recognized.
The specific biomarkers explored by Zhang et al are a group of sterane molecules derived from sterols which are important components of cell membranes in eukaryotic organisms. A particular difficulty in analyzing old sterans is that samples are easily contaminated with sterans from other sources. Sources of contamination range from steranes introduced during sampling, transport and processing of samples, to geological contamination of steranes when fluids flow through rocks.
Zhang et al carefully monitored each of the sources of contamination and, like others, found that no sterane was released when using standard protocols to extract biomarkers from these ancient rocks, in this case the formation 1400-million-year-old Xiamaling in the north. China.
However, Shuichang Zhang, the lead author of the study hypothesized that “there is fossil evidence of eukaryotic algae 1400 million years ago or even earlier, so we wondered if sterans in these rocks might be more tightly bound to kerogens and not readily released during standard biomarker extraction. ”Therefore, Zhang et al used a step-heating protocol in which samples were slowly heated in tubes gold in 9 steps from 300 ° C to 490 ° C. The organic molecules released in each of the nine steps were extracted and sterans indicating the presence of red and green algae were released, especially at higher temperatures.
Zhang continues, “Many will fear that the sterans we found were the product of some kind of contamination. We were also concerned about this, but we analyzed samples in parallel that were heated to high temperatures over the course of this process. their geological history and therefore did not contain any biomarkers. We did not find any steranes in these. This means that our protocols were clean, and therefore we are confident that the steranes we found were native to the rock. . “
It is not yet entirely clear why sterans were so tightly bound to kerogen and not released during standard protocols. However, the conclusions of Zhang et al. show that groups of green and red algae were present in marine ecosystems 1400 million years ago. This is 600 million years earlier than what has emerged from previous studies on biomarkers. This work shows that the red and green algae lineages certainly evolved 1400 million years ago, and this should be a useful constraint for timing the overall evolutionary history of eukaryotes. This work also shows that at least some ancient marine ecosystems functioned more similarly to modern ecosystems than previously thought, at least when it comes to the types of photosynthetic organisms that produce organic matter. This further means that there were enough nutrients and oxygen available to promote the presence of ecosystems containing algae.
Professor Don Canfield, Nordic Center for Earth Evolution, University of Southern Denmark, co-author of the study adds: “We hope our study will inspire others to use similar techniques to better disentangle the complete history of the evolution of eukaryotes through geological time. “
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Material provided by University of Southern Denmark. Original written by Birgitte Svennevig. Note: Content can be changed for style and length.