What are biosignatures?
Any evidence of life
Why are they useful for?
Look for past life, look for life on other planets, to understand life.
Why ‘microbial’?
From all the life we know, prokaryotes (archea and bacteria) are the most widespread organisms, in time and space. They are the first organisms that populated the Earth, and would likely be the first dwellers on other planets where life may have developed. They have also the widest limits of environmental tolerance (temperature, pH, radiation, desiccation, etc.), the widest variability in metabolic strategies, and they occupy all the known ecological niches. Additionally, they have been the dominant form of life for about 70% of the geologic time. Thus, they should be the starting point for the search of ancient life on Earth and beyond.
Evidence of the existence and activity of microbes in the fossil record consists primarily on stromatolites, microfossils, and biomolecules, whose antiquity can go far back to the Archean (~3.5 Ga). These microbial signatures can be traced almost countinually over time since their appearance on Earth, but the older they are, the hardest to prove them biogenic and the easiest to confuse them with abiogenic structures. Hence, because their biogenicity becomes less evident, not one, but many biosignatures should be retrieved from the study objects, and these biosignatures should converge into a unique conclusion supporting a life-originating process. Otherwise, the object could be discarded as life-related.
What kind of fossil biosignatures exist?
Life can be manifested in several ways, and thus traced using:
- Biomarkers : chemical compounds produced inside cells
- Biominerals : minerals produced by their influence on the environment
- Bioisotopes : isotopes derived from metabolic activity
- Ichnofossils, microbialites or biofabrics : sedimentary structures biologically originated
- Microfossils : any cell remains
The fact is:
- We need to understand life to understand biosignatures
- Biosignatures must prove biogenicity or non abiogenic origin
- Lots of biotic and abiotic processes, their products and effects are still unknown
- Morphology may be valid, but more independent biosignatures confirming biogenicity are encouraged
- As technology, knowledge and interdisciplinarity advance, more biosignatures will be able to detect, and more things known about them.
Biosignatues of terrestrial microorganisms
Biological Soil Crusts(BSCs) are organo-sedimentary structures formed by microbes(mainly dyanobacteria, but also fungi, byrophytes, and algae) that cement topsoil sediments forming a crust.
Modern BSC are important because they:
- increase/decrease runoff of rain water
- hydrate topsoil for a longer time
- water from below escape at lower rates
- prevent/decrease soil erosion
- contribute to C, N and other nutrients in soils
- enhance seed anchoring and germination
- are distributed all over the world(principally in arid to semiarid areas
Hypothesis
Because cyanobacteria, the main builders of biological soil crusts, are a very old taxa (~3,500 Ma), very well adapted to UV-light and desiccation conditions, they were probably dwellers on the early Earth’s land. They likely colonized and formed crusts in incipient soils as they do today. But yet, there is no evidence for that. To prove that they existed in the Precambrian would have deep evolutionary and ecological implications.
How to distinguish BSC in rocks
Many other structures that resemble BSCs are found in the rock record (varves, mud cracks, cohesive sand layers, etc.). Thus, unique features of modern BSCs must be recognized in the rocks.
We study sedimentological, chemical and biological features of a variety of BSCs and other structures. The architecture of modern BSCs, its variability within depositional settings, and the morphological changes under diagenetic processes may provide useful elements to distinguish crust-like structures from the rock record.
We found that the crust tends to be enriched in metals as compared to the soil underneath the crust and the uncrusted soils. Minerals are slightly different also, and characteristic sedimentary structures develop only when microbes are present. So, microbes are leaving behind their signature in many of the aspects we study. The metabolic diversity should be further studied so to understand more on the interactions between microbes and particles.
So far, we can say that soil type, water regime, local temperature and wind may influence the interaction between microbes and sediments, which together determine the structure and constitution of BSCs. Differences among BSCs from different sedimentary environments comprise crust thickness, topology, porosity, and layering. Layers of fine sediment, formed by microbial action, can be preserved and recognized after compaction. These features may be useful as indicators of microbially-produced crusts and may be identifiable in the rock record.
Given the great diversity of abiotic sedimentary structures found in nature, individual features alone cannot be counted as biosignatures. Several biosignatures (biominerals, bioisotopes, biomarkers, microfossils, etc.) must be considered when looking for BSC in the rock record. Finally, a definition of BSCs should encompass an array of characteristics that distinguish them from other crust-like structures. This work is a preliminary approach to that goal.