Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Wednesday, January 10, 2024

The air that I breathe

A month ago I looked at a geological oddity called banded iron formations -- alternating gray and red bands of iron-rich sediments that have been found all around the world, and all seem to date from about the same time (2.4 billion years ago).  These sedimentary deposits are thought to be the fingerprint of the Great Oxidation Event, when photosynthetic organisms began to pump so much molecular oxygen into the atmosphere that it literally changed the chemistry of the entire planet.

To understand how this happened will take a little bit of explaining.

Photosynthesizers such as plants, phytoplankton, and cyanobacteria evolved a trick for harvesting energy and storing it for use later.  Prior to this, all organisms were heterotrophs -- they required pre-formed organic compounds, which (fortunately for them) were abundant in the early oceans, created by the reducing atmosphere (reduced is chemist-speak for "capable of donating electrons") and sources of energy like lightning and ultraviolet light.  Heterotrophy back then was a fairly inefficient process.  The kind of energy processing they did only produced two ATP molecules, the energy currency of all cells, for every molecule of glucose metabolized.  (Glucose is the most commonly used energy containing molecule.)

Then, around 2.4 billion years ago a new metabolic pathway evolved that could produce ATP directly, driven by the energy in sunlight, rather than by breaking down pre-existing organic molecules.

This process, which probably was created by mutations in a chemosynthesis pathway of the kind we still see today in hydrothermal vent bacteria, used light-capturing pigments like chlorophyll to initiate a chain reaction called photophosphorylation to create ATP by the boatloads.  It required a source of electrons -- nearly all of the energy transfer in cells relies on the movement of electrons in what are called oxidation/reduction reactions -- and the cells performing photophosphorylation found it in abundance.


The problem was, pulling the electrons from water molecules makes them fall apart.  The result is a pair of hydrogen ions, which can be used for other chemical reactions in the cell -- and free oxygen, which is given off as a waste product.

This led to a huge problem for the rest of life on Earth, because to put not too fine a point on it, oxygen is really freakin' dangerous.  It is, unsurprisingly given the name, a strong oxidizer -- it's really good at pulling electrons away from other substances, which makes them fall apart.  This had the effect of stopping the natural production of food molecules in the ocean; with oxygen in the atmosphere and dissolved in the water, organic compounds now fell to pieces as soon as they were produced.

The result was that in the flip from a reducing atmosphere to an oxidizing atmosphere, nearly all life on Earth died.

The only survivors were:
  1. The photosynthesizers -- i.e., the ones who caused the problem in the first place.  They were able to make their own food, so they didn't give a damn if everyone else starved.
  2. A handful of anaerobic heterotrophs who were able to escape the oxygen.  We still have them today -- they live in places like anaerobic mud at the bottom of lakes and ponds.
  3. A small number of cells that had a pathway to detoxify oxygen.  This pathway involved essentially reversing photosynthesis, combining oxygen with hydrogen ions to lock it up harmlessly as water.  A side benefit -- which ultimately became its major benefit -- is that this is a powerful energy-releasing pathway, and once you can hitch it to ATP production, it's capable of producing 36 ATP molecules per glucose instead of 2, increasing the efficiency of energy capture by a factor of eighteen.  It has to be done in steps -- oxidizing molecules all at once is called "combustion" -- but if it can be slowed down and harnessed, it's a fantastic way of processing energy.  This stepwise oxidation, called the electron transport chain, was such a tremendous advantage that this group -- the aerobic heterotrophs -- basically went out and took over the entire planet.  In fact, they're our ancestors and the ancestors of all the other life forms on Earth that are dependent on oxygen.
The reason all this comes up is a recent discovery I was alerted to by a friend and loyal reader of Skeptophilia.  Researchers analyzing sedimentary rocks from Australia and Canada found fossils of single-celled organisms dating to 1.75 billion years ago that contain traces of thylakoids -- the layered membranes inside chloroplasts on the surfaces of which the oxygen-releasing photophosphorylation reactions take place.  So what we have here are fossils so finely preserved that they retain details not only of cells, nor the organelles inside cells, but the structures inside organelles inside cells.

And not just any old structures.  These, or ones very like them, are the same things that caused all the havoc during the Great Oxidation Event.

Emmanuelle Javaux, of the Université de Liège, who led the study, said, "Their production of oxygen led to accumulation of oxygen and profoundly modified the chemistry of the Earth’s oceans and atmosphere, and the evolution of the biosphere, including complex life."

It's astonishing that traces of these delicate organelles could last in the fossil record for 1.75 billion years.  It gives us a lens into an Earth we wouldn't even recognize, a time when there was nothing whatsoever living on the land, the most complex life was composed of simple clusters of cells, and the oceans were a rapidly-thinning soup of organic monomers.  In a very real sense these microscopic structures created the Earth we see around us today.  Without these tiny pancake-like membranes, the Earth would be a very different place -- one we would not find survivable.


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