Can biosignature give us information about ancient Earth’s climate and life? How RuBisCO enzyme is involved in the Earth’s history?
LONG-TERM EVOLUTION and BIOSIGNATURE
Biosignature is the main tool used to detect and reconstruct past biological activity on our planet. There can be different biosignatures depending on biotic and abiotic factors: organics elements, minerals, large- and small-scale rock, isotopes;
Is not easy to reconstruct ancient life. But fossil biosignature can preserve amazing evidence about archaic existence. Isotopes are one of the most important “types” of biosignature. The oldest signal of biological activity is carbon isotope (13C/12C) stored in carbonaceous material as shreds of evidence for driven carbon metabolism developed during the early metabolic processes. Carbon isotope records can be used to fingerprint specific metabolic processes and the organisms correlated. The looped exchange of carbon compounds between all the environments (oceans, atmosphere, biosphere and lithosphere) can give us precious information about processes that characterized the antecedent Earth’s climate and habitats.
CARBON ISOTOPE RECORD
Scientists observed, despite the continuous ecosystem turnover and the increasing organism’s complexity, a constant steady-state of carbon isotope records. Despite > 3 billion years of ecosystem turnover, 25‰ of inorganic and organic 13C/12C values have remained unchanged. Only two negative excursions had been observed (~2.7 and ~2 billion years) probably due to an increase in methanogenic activity. There is a wide range of variability of isotope composition that can be used as an instrument to discriminate different carbon metabolism as a biosignature of ancient carbon cycling processes. However, there are many different factors that can influence carbon isotope records that must take to into consideration. It wouldn’t be accurate to consider only a few factors as we would risk oversimplifying a very complex network of mechanisms that originated billions of years ago.
RuBisCO and BIOSIGNATURE
How RuBisCO enzyme is involved in the Earth’s history?
RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) is the main enzyme involved in the reduction of inorganic carbon (in the form of CO2) as the first step of carbon incorporation through the Calvin-Benson-Bassham (CBB) cycle for the production of organic compounds by photoautotrophic organisms.
RuBisCO is the most important driver of carbon fixation in Earth’s history. Emerging studies have hypnotized a correlation between molecular adaptation of RuBisCO on carbon isotope records and its fundamental role in biosignature processes. Scientists know that RuBisCO efficiency varies as a function of external CO2 levels. So, with the change in atmospheric CO2, we expect a change in RuBisCO molecular composition.
Different analysis shows that changes in catalytic processes were probably manifested in changes in carbon isotope effects considering the latter half of Earth’s history. Scientists talked about the latter half of the Earth’s history because was characterized by an increase in oxygen concentration (oxygenation process of the atmosphere) and the increasing need for better use of carbon by organisms dependent on RuBisCO-catalyzed CO2 fixation (in the past the atmospheric CO2 concentrations was probably up to 2500 times higher than present day). Furthermore, is fundamental to understand the different environmental factors that can impact RuBisCO molecular composition and its regulation in terms of evolution as a footprint of microbial ancient life; signs that can be encountered in the carbon composition and biosignature in geologic records.
GENERAL CONSIDERATIONS
Is important to consider the limits of the use of biological features as a proxy for archaic life. Is difficult to do a measure in terms of subcellular components related to isotope composition without a general point of view about the early life that existed billions of years ago. During centuries of scientific research, we learn that is hardly possible to resume an evolutionary process in light of a single or few factors. Biology is a network of circumstances that co-produce a single event of change that can become a sequence of subcellular alterations which we call evolution. The integration between different fields will be fundamental to reconstruct ancestral behaviours based on geological, biological and molecular activity.
A fundamental issue with the interpretation of carbon isotope biosignatures is that it is not known to what extent the isotope discrimination behavior or modern biology can serve as a proxy for past life.
Garcia, A.K., Cavanaugh, C.M. & Kacar, B. The curious consistency of carbon biosignatures over billions of years of Earth-life coevolution.
Sources:
- Garcia, A.K., Cavanaugh, C.M. & Kacar, B. The curious consistency of carbon biosignatures over billions of years of Earth-life coevolution. ISME J 15, 2183–2194 (2021). https://doi.org/10.1038/s41396-021-00971-5;
- Garcia, A.K., Kedzior Mateusz, Taton Arnaud, Li Meng, Young N. Jodi, Kaçar, B. System-level effects of CO2 and RuBisCO concentration on carbon isotope fractionation. bioRxiv 2021.04.20.440233; doi: https://doi.org/10.1101/2021.04.20.440233;
Images:
- Cover image <a href=’https://www.freepik.com/photos/marble-wallpaper’>Marble wallpaper photo created by rawpixel.com – www.freepik.com</a>;
- Fig. 1 – https://app.biorender.com/biorender-templates/t-60ba497151055400a258ed3b-calvin-cycle;
KEYWORDS: carbon fixation, carbon isotope, RuBisCO, biosignatures, ancient life
Author: Marta Riva
