Bacteria
Bacteria may be small, and usually invisible to the human eye, but these seemingly insignificant organisms play crucial roles both on land and in the sea, and exert a major influence on all other living beings. The oxygen-rich atmosphere in which we live and breathe today is thought to have been created by microbes. It is the Earth’s third atmosphere. The first atmosphere of Earth’s formation had no oxygen available for breathing. Neither did the second, the atmosphere into which the first life forms evolved. The oxygen within our atmosphere today is largely released by plants as a byproduct of photosynthesis—it is oxygen they also need to consume, for cellular respiration and for energy supply. Yet the earliest marine microbes were capable of performing photosynthesis in the absence of oxygen; in order to extract energy from food, they relied on other elements. Researchers may now have discovered a clue to solving this mystery. Several research teams have published observations both on modern microorganisms and on fossil sedimentary rocks of biogenic origin (stromatolites). These indicate that the bacteria of the Precambrian oceans may have been breathing arsenic. With the rise of the ancestors of cyanobacteria, oxygen-producing photosynthesis gained momentum, in a long-term event now called the Great Oxygenation, or the Oxygen Catastrophe; what laid the foundation for life as we know it today was a disaster for many organisms then. As they were adapted to anaerobic conditions, the new atmosphere led to their mass extinction.
Sources: Visscher, Pieter t. et al., “Modern arsenotrophic microbial mats provide an analogue for life in the anoxic Archean”, Commun Earth Environ, vol. 1, issue. 24, (2020).
Sforna, Marie Catherine et al., “Evidence for arsenic metabolism and cycling by microorganisms 2.7 billion years ago”, Nature Geoscience, vol. 7, (2014): 811-815.
Hodgskiss, Malcolm S. W. et al, “A productivity collapse to end Earth’s Great Oxidation”, Proceedings of the National Academy of Sciences of the United States of America, vol. 116, issue. 35 (2019): 17207–17212.