Monday, July 27, 2015

How Microbes Created Our Atmosphere

Paul G. Falkowski has produced a fascinating little book titled Life's Engines: How Microbes Made Earth Habitable.  He holds the Bennett L. Smith Chair in Business and Natural Resources at Rutgers University.  Falkowski’s intent with this book was to provide to the non-specialist an explanation of how all forms of life share common ancestors and how we owe our existence as humans to the efforts of the tiniest of creatures. 

Much of the text is devoted to illustrating the basic set of molecular engines (nanomachines in Falkowski’s terminology) that were developed to power cells and manufacture the proteins and other chemicals necessary for their function.  Falkowski makes clear that many of “life’s engines” that developed in bacteria remain the engines that drive life in all its current forms.  Along the way, he provides an interesting tale of how microbes played a role in providing the atmosphere that allowed animals like us to evolve.  That will be the topic here. 

By definition microbes are creatures too small to be visible to the naked eye.  Consequently, they were essentially unknown until the seventeenth century when Hooke and Leeuwenhoek acquired microscopes of sufficient power to detect them.  It was Leeuwenhoek who first identified bacteria and showed that we are not only surrounded by such creatures, we are also inhabited by them.  Despite the great significance of these discoveries, Falkowski indicates that it was only in the middle of the nineteenth century that the study of microbes began as an active area of science.  He attributes to Ferdinand Julius Cohn the prescience to recognize the potential these little creatures had to alter the earth’s environment.

“The renaissance of the study of microbes began only in the middle of the nineteenth century.  It was championed by an almost forgotten hero, Ferdinand Julius Cohn.  Cohn was a Jewish wunderkind who had been born in Breslau, Prussia (today’s Wroclaw, Poland), in 1828.  It is reported that Cohn learned to read before he was two years old, began high school at seven, and entered the University of Breslau at fourteen.”

“Cohn was not focused on the role of microbes in causing human disease.  Although he worked on microbial diseases of plants and animals, and was far less famous than Pasteur, he had an even broader vision.  He saw microbes as organisms that helped shape the chemistry of the Earth—the planet’s metabolism.”

Little creatures can accomplish great things if there are enough of them.  This source provides us with a way to visualize the numbers involved.

“There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water….forming a biomass which exceeds that of all plants and animals.”

“There are approximately ten times as many bacterial cells in the human flora as there are human cells in the body, with the largest number of the human flora being in the gut flora, and a large number on the skin.”

And that neglects the various viruses and other parasites that inhabit us.

The key requirement for the evolution of complex, multicellular structures was the availability of free oxygen in the atmosphere.  This source provides a chart illustrating the history of the atmospheric oxygen abundance.

The red and green lines indicate the highs and lows of the various estimates of oxygen abundance as a function of time.  The x-axis records billions of years in the past.

The Earth is thought to have been formed about 4.5 billion years ago.  There is evidence that single-celled bacteria existed about 3.5 billion years in the past, but oxygen as a free molecule doesn’t become available until about a billion years later.  Since life began before free oxygen was available, the first cellular organisms would have to use other elements to produce the materials needed for energy, nourishment, and reproduction.  For example, bacteria capable of ingesting hydrogen sulfide and producing sulfur as a waste product (with and without the utilization of sunlight to drive photosynthesis) predate the photosynthetic microbes capable of replacing hydrogen sulfide with water and producing oxygen.  These creatures would have consumed any resident oxygen as they died and their decay produced carbon dioxide.  Other sinks for free oxygen molecules assisted in limiting its concentration.

Oxygen is important for biological evolution because it reacts readily with many elements and provides considerable energy when it does.  That is exactly the property that would have rendered it rare as a free molecule in the early atmosphere.  It would be readily coupled with the abundant hydrogen atoms to form water or reacted with other elements to form oxides. 

Water is much more difficult to break down than hydrogen sulfide.  It would take a very long time before the mechanism capable of that feat would be developed.  From Falkowski:

“The first photosynthetic microbes were anoxygenic—that is they were not capable of splitting water.  It took several hundred million years before microbes evolved the ability to split water.  Water is an ideal source of hydrogen on Earth’s surface because it is far more abundant than any other potential electron donor, but splitting it takes a lot of energy.  The responsible nanomachines evolved only once among prokaryotes: in the cyanobacteria, or blue-green algae.  When these organisms finally were able to split water, they produced a new gaseous waste product: oxygen.  The biological production of oxygen changed the evolution of life on Earth forever.”

Producing a source for molecular oxygen does not immediately produce a ready supply.  The sinks for oxygen would have to be overcome.  In fact, the algae that produce oxygen using photosynthesis will respire carbon dioxide and water when no light is available, thus negating a fraction of the oxygen production.  These same bacteria will die and decay and thus consume more of the oxygen.  For oxygen to build up in the atmosphere, decaying creatures would have to be stashed somewhere where they would not have access to oxygen.  This is where all that oil and gas we have been using for so long enters the story.

“By far the most important storage areas are in shallow seas and along the coasts of continents.  But even there, on average less than 1% of the organic matter produced by phytoplankton reaches the sea floor, and only about 1% of that is subsequently buried in sediments.  This means that less than 0.01% of organic matter is actually buried, but over millions and millions of years, this very small fraction becomes significant on a global scale….”

Having these sediments ultimately incorporated into sedimentary rock still does not necessarily protect the oxygen gas from being consumed.  Yet another mechanism is required to keep it safe.

“Some of the sedimentary rocks, which contain the organic matter, are subsequently uplifted onto the continents to form mountains….Without uplifting the organic matter onto continents, the organic matter would be subducted into the interior of the Earth by tectonic processes, heated, and returned to the atmosphere as carbon dioxide from volcanoes—and little or no oxygen would accumulate.”

Producing an atmosphere containing a significant oxygen component was no easy matter.  For the planet itself it was revolutionary.  As in most revolutions, death and chaos ensued.  The emergence of oxygen in the atmosphere led to what is known as the Great Oxygenation Event.

Many species of microbes had evolved in a low oxygen or oxygen free environment.  An excess of oxygen was poisonous to many.  Species that produced methane were common and methane was a significant component of the atmosphere.  The availability of oxygen not only harmed the methanogenic microbes it also reacted easily with methane and removed it from the atmosphere.  Unfortunately methane is a very efficient greenhouse gas and the Earth needed it to maintain its surface temperature.  As this source puts it:

“Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Additionally, the free oxygen reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, possibly the longest snowball Earth episode in the Earth's history.”

Note that after the Great Oxidation Event the oxygen content of the atmosphere leveled off at just a few percent.  Further buildup would require additional evolution.  That would come in the development of multicelled eukaryotes (made up of cells with nuclei), particularly the phytoplankton.  These could be larger and produce more oxygen and, more importantly, could sink faster and more efficiently hide its decaying bodies in sediments.  This is the mechanism that generated the rise in oxygen that began about 700 million years ago in stage 4 on the above chart.

Yet more magic is required to get us to our current state.  At this point in time there are no land-based plants or animals.  As the oxygen level rises, simple animals that now depend on oxygen can arrange themselves in more complex configurations without worrying about oxygen limitations.  Stage 5 contains the Cambrian Explosion when nature experimented with many species, discarded most, and kept the ones that provided the major species classifications of today.  This unique period occurred around 540 million years ago and lasted a few tens of millions of years. 

Falkowski provides this perspective:

“The evolution of animals seems to have preceded the evolution of plants on land by approximately 200 million years….Terrestrial plants are derived from a single group of green algae and began to colonize land about 450 million years ago.”

The rise of oxygen respiring plants greatly boosted the oxygen level to higher values than currently exist.  The result was that animals began to invade land from the oceans encouraged by this abundant supply.  They would provide a healthy sink for oxygen as they consumed it and respired carbon dioxide and water.  Eventually equilibrium would be established.

What to take away from this story?  One thing is that life forms were created early in the earth’s history, but life as we know it, life based on an oxygen-rich atmosphere, occurred only through a sequence of not inevitable events.  Quite a bit of luck was involved.

“Oxygen is unique to Earth’s atmosphere.  The gas has not been found in high concentrations on any other planet in our solar system, nor has it been found in the surrounding neighborhood of stars that have planets.  Although it is highly likely that other planets will be discovered to have oxygen, it does not seem to be a common gas on terrestrial planets.”

The evolution of life forms may not be that difficult on other planets; the evolution of life forms like ours might be quite a bit more unlikely

Another conclusion one might draw is that Earth is a complicated place and small changes can produce dramatic effects.  The pollution we generate and the carbon dioxide we spew into the air are hardly noticeable to us.  However, they are major factors affecting our oceans.  Acidity levels are rising, food chains are being disrupted, and species populations are being altered.  This is risky business.  The oceans produced life; they can also destroy it.  Since the Cambrian period there have been at least five mass extinction events.  Our current era has already been referred to as the sixth.  There is no law of nature that prevents humans from being victims.

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