They reconstruct a microbiological community "mummified" thousand years old

Microbial mats are groups of microorganisms, composed of bacteria, archaea, eukaryotes and viruses, which form a vertical structure and live in community giving rise to a complex microecosystem. The analysis of the biological remains of ancient and preserved microbial mats allows us to reconstruct the biological community that it had in the past, as well as the relationships between individuals and with the environment.

Scientists have reconstructed the biological and metabolic community of a microbial mat from Antarctica.

The work has been carried out by a team of researchers from the Center for Astrobiology (CAB), dependent on the Higher Council for Scientific Research (CSIC) and the National Institute of Aerospace Technology (INTA), all these entities in Spain, in collaboration with the Autonomous University Madrid in Spain and McGill University in Canada.

In this study, which was published in the academic journal Frontiers in Microbiology, the team reconstructed the biological community and metabolisms of an approximately 1,000-year-old microbial mat. The study mat was found completely dry and “mummified” on the side of a small ice mountain near Bratina Island, on the McMurdo Ice Shelf, Antarctica.

On the surface of the McMurdo Ice Shelf is a network of meltwater ponds. Due to the continuous pressure exerted by the shelf on Bratina Island and the deformation capacity of the ice, the topography of the shelf is changing.

These changes in topography can eventually cause ponds to disappear from one year to the next, thus leaving microbial mats that once grew on the shores disconnected from the water. These dried and preserved microbial mats today serve as an archive of the conditions that were conducive to their growth in their time.

One of the meltwater ponds near the location of the ancient microbial mat analyzed in this study. Active microbial mats within the pond and on the shore can be seen in the photograph. (Photo: Antonio Quesada)

The analysis of biomolecules, such as DNA, proteins or lipids, is decisive for studying the biological composition of a sample (modern or ancient), since they are present in all the cells of all living beings. Through the joint analysis of these three classes of biomolecules, it has been possible to overcome the information gaps derived from the analysis of each type of biomolecule separately in old samples.

According to María Ángeles Lezcano, a researcher at the Center for Astrobiology who has led the study, determining the microbial composition in an old sample is very complex due to several factors. The first is that each biomolecule has a different preservation capacity over time, being higher in lipids, followed by proteins, and lower in DNA. On the other hand, DNA has greater taxonomic specificity, that is, it allows individuals to be classified up to the species level, while lipids only offer a generic classification; they do not allow differentiation at the species level, but the presence of some specific lipids makes it possible to distinguish between groups of organisms more generally, such as cyanobacteria, diatoms, sulfur-reducing bacteria, plants, etc. Finally, the different analysis methods used for each type of biomolecule lead to analytical biases. Therefore, interpreting the biological composition of an ancient sample with the analysis of a single type of biomolecule may contribute to an incomplete reconstruction of the sample and its past ecological setting.

To overcome these obstacles, the team has analyzed DNA, proteins and lipids using massive sequencing techniques of certain genes, metaproteomics (a technique to identify all the proteins preserved in the microbial community), analysis of lipid biomarkers and fluorescence microscopy (to visualize microorganisms capable of photosynthesis).

DNA analysis showed a microbial profile dominated by microorganisms resistant to adverse environmental conditions (such as the order of Clostridial bacteria, which are spore-forming). In contrast, the proteins made it possible to identify microorganisms that were not detected by DNA analysis, such as cyanobacteria. The most resistant to degradation lipids (alkanes) confirmed the presence of cyanobacteria and also suggested the presence of mosses and/or remains of vascular plants from a period in which the climate in Antarctica was warmer than today (middle Miocene and/or or Eocene). (Source: CAB / INTA / CSIC)