Bacteria and Archaea: Kindred Microbial Cousins
Earth’s microbes contribute to the dynamic chemical waltz carried out in the subsurface. The lush pliable soils soaking up the sun outside your window are teaming with microbes, invertebrates, and plant debris.
The same phenomenon happens deep beneath the bottom of the ocean, where hundreds meters below cold oceanic crust briny fluids deliver nutrients for microbes to feast upon resulting in physical and chemical changes to the rocks. All microbes, mainly bacteria and archaea, are intimately associated with geology on Earth.
The microbial lexicon:
A quick terminology primer moving forward.
When the word “microbe” is used, it is typically referring to bacteria, although sometimes archaea are colloquially included in this grouping.
Both bacteria and archaea are prokaryotes. Those are the simple single-celled organisms lacking membrane-bound organelles you might remember from middle and high school biology. Bacteria and archaea, however, are very different on a molecular level; sourced from different evolutionary pathways, and represent two distinct domains of life.
Eukaryotes, which include all multi cellular life, plants, animals, and fungi are the third major division (or domains) of life on Earth.
Viruses, unlike bacteria, archaea, and eukaryotes, don’t have a formal domain of life. Their nebulous existence, wholly dependent on hosts and existing cellular machinery results in them being an evolutionary important outlier relegated to existence without formal incorporation into our tree of life.
Geological drivers
From a geological perspective both bacteria and archaea are key drivers in geological processes. Their very small scale (typically less than a micron in diameter), coupled with the fact they’ve been on earth for over 3 billion years allows them to play a direct role in nearly all chemical cycles on the planet.
Both bacteria and archaea use a wide range of aerobic (oxygen-based) and anaerobic (without oxygen) metabolisms to catalyze a very diverse range of chemical reactions including iron and sulfur oxidation and reduction, carbon oxidation, and redox cycling of many heavy metals.
Despite their morphological similarities and similar size and metabolic capacities, bacteria and archaea vary widely at the genetic level. Bacteria have been more widely studied due to their ubiquity and the fact reports of bacterial life date back to the 1670s when Anthony van Leeuwenhoek outlined bacterial life.
Archaea, on the other hand, were first discovered in the late 1960s to 1970s by Carl Woese in pursuit deeply rooted to the heart of scientific discovery.
Bacterial investigations have had a 300-year head start. Although this is practically nothing in geological time, from our brief human perspective, it’s enough to make bacteria the main star of the microbial show. Consequently, most times the word “microbe” is thrown around, bacteria are typically the subject of interest despite the potential for archaea to contribute.
Geologically both archaea and bacteria have unique, sometimes overlapping niches in modern environments and in the rock record. From a very broad perspective, archaea tend to thrive in more extreme environments: obscenely hot temperatures, wide ranges of pH and salinity, and very high pressures, such as in symbiosis with shrimp on the Von Damm Spire, 7,500 feet underwater.
This tendency towards extremophile behavior is due to the unique properties of archaeal cell structure and selection bias. Bacteria are also capable of thriving in a very broad range of conditions, encompassing many of the same extremes as archaea, yet typically archaea dominate the extreme niches.
Despite broad functional similarities, the most glaring difference between archaea and bacteria lies in the chemical composition of the cellular membrane, where bacteria have a carbon-oxygen bond known as an ether, which archaea are lacking. Bacteria and archaea have some varying cellular machinery, like the RNA polymerases, and responses to antibiotics. These differences are primary rooted in the distinct evolutionary pathways for these two different domains of life.
In fact, much evidence supports the theory that archaea were the precursors to eukaryotic life. An ill-fated archaeal cell was engulfed by another cell, and overtime evolved into the chloroplasts and mitochondrial that power modern plant and animals cells. Known as endosymbiosis this scientific theory is supported by very strong molecular and genetic evidence within the DNA of all domains of life.
Together bacteria and archaea drive many of the processes that us humans take for granted. The free oxygen we breathe, sulfate-rich oceans that harbor diverse sea creatures, and our own immune systems primed with microbial influence are all directly created with microbial overprints.