Breath Taking. Michael J. Stephen

environments of freshwater bacteria and ocean bacteria are strikingly different. In inland bodies of fresh water, the temperature can vary widely, as can the amounts of nutrients and minerals. The summer and winter also produce very different living conditions in freshwater environments, which often host very different species depending on the season. By contrast, the ocean environment is exceptionally stable. The temperature variations are much smaller than they are in inland bodies of water, and the microenvironment of nutrients much steadier. Bacteria that thrive in a freshwater environment are what oceanographic scientists call “eutrophs,” organisms that can handle an abundance of nutrients and highly variable temperatures. Saltwater bacteria, “oligotrophs,” require lower levels of basic nutrients. So, somewhat contrary to intuition, saltwater bacteria are more sensitive, more fragile than their freshwater cousins.

      Over the course of a vexing year, Waterbury came to understand this. He fastidiously scrubbed all his culture flasks and test tubes, making sure not even a microscopic amount of calcium or other substance was left over. He then calibrated his culture medium to precisely reflect the nano-amounts of nutrients he had measured in the ocean water. Finally, after a year of meticulous work, and much to Waterbury’s delight, cyanobacterium from the ocean started growing for the first time outside of their natural habitat. The discovery of the species Synechococcus was official.

      The questions then remained: How much of this stuff is out there, and what is its habitat? From the end of a wooden dock in Woods Hole, Waterbury filled a few jars with salt water, a little murky but otherwise unremarkable. He looked at the specimen under his epifluorescence microscope; it was teeming with cyanobacteria.

      The study of cyanobacteria exploded over the next ten years. Hundreds of different species were identified in almost every ocean habitat on Earth. We now know that blue-green algae inhabit any body of water that is warmer than five degrees Celsius, usually in massive numbers, so massive that Waterbury refers to them as “those little beasts.”

      With their sheer numbers and diverse habitats, cyanobacteria are today recognized as the creatures primarily responsible for putting oxygen in our atmosphere. They do so through photosynthesis, the process by which plants, algae, and cyanobacteria capture sunlight and turn it into energy. The primary molecule that captures the sunlight is chlorophyll, which uses the energy from photons of light to drive the reaction of carbon dioxide and water to glucose and oxygen. The photosynthetic reaction also gives off energy that helps cyanobacteria convert atmospheric carbon dioxide to edible carbon, which is consumed initially by lower life forms but then carried up the food chain. This process makes cyanobacteria the source of a great deal of the food production on Earth. They are also responsible for a majority of Earth’s oil, natural gas, and coal, all of which derived from settled matter (dead cyanobacteria) that condensed at the bottom of the ocean over millions of years. Indeed, cyanobacteria as a group are the most abundant species on Earth, and one of the most important for the purpose of life.

      We tend to associate the process of photosynthesis with plants, but almost certainly cyanobacteria did this first. It is thought that, millions of years ago, ancestral cyanobacteria paired with larger cells, in a process called endosymbiosis, and evolved to become chlorophyll-containing chloroplasts, which allowed the larger cells to perform photosynthesis. These chloroplast-containing cells eventually bound together to make the forerunners of present-day plants and algae.

      The mastery of photosynthesis by cyanobacteria, and later by plants, is something, despite all our technological progress, we can still only marvel at. Humans figured out early how to burn carbon, but we still have not been able to produce it ourselves from carbon dioxide and light. If photosynthesis could ever be simulated artificially, it could be the golden key to solving our energy-production problems; it would also solve the problem of global warming, by making it possible to take carbon dioxide out of the atmosphere.

Looking back, we now know that the explosion of life during the Cambrian Period, some five hundred million years ago, was significantly fueled by rising oxygen levels in the atmosphere caused by oxygen production from cyanobacteria.21 Higher animal life forms would simply not be here without these little creatures, nor would most plant life forms.

      Our lungs developed to utilize oxygen and efficiently drive our metabolic reactions. We are aerobic creatures, and if the lungs are our most important organ, then oxygen is the most important gas in the atmosphere. Anaerobes exist, but they are constrained by an inefficient method of energy production. With oxygen, the possibilities of the world opened up. Almost every living creature on Earth is reliant on some method of oxygen extraction, and John Waterbury and others in the field of ocean bacteria helped show us where all this life came from.

      With the existence of the new gas in Earth’s atmosphere, the last five hundred million years of this planet have been radically different from the first four billion. The first period was marked by an absence of life, the second period by an abundance of it. The timing of those two appearances, of oxygen and life, is no accident. Oxygen is the life force, the source of life’s infinite possibilities.

      Along with the rise in oxygen from cyanobacteria, plant life began to flourish around this time. It first occurred in the ocean, and then inexorably these plant forms made their way onto the scorched orange land mass that, at the time, was completely devoid of anything except rock. First shallow moss colonized the rock, and then slowly more advanced plant life established itself. Trees came later, which further increased oxygen levels.

      In the oxygenated ocean, animal life became increasingly sophisticated. With more plants came more oxygen, and with that, worms, mollusk-like clams, and jellyfish came into being, using primitive gills or simple diffusion to extract oxygen from the ocean. Eventually, over the course of tens of millions of years, creatures made their way onto the land that had been colonized by plants. Insects, spiders, and worms were the first to take advantage of a nascent verdant landscape. But they couldn’t have made this remarkable transition without some kind of ability to utilize oxygen.

      Worms have no functioning respiratory system. They derive oxygen from the moist soil around them, letting it dissolve through their skin and into their blood. Dry a worm out and you suffocate it. Spiders and insects have a respiratory system, but it is simply a long pipe, going through their bodies, that allows oxygen to diffuse into the surrounding tissues. In all of these species, there is no muscular system to augment the utilization of oxygen, and no way to increase the supply of oxygen significantly in a time of need. These primitive systems are limited by their lack of efficiency. This limitation prevents the bodies and brains of these creatures from growing bigger. They are constrained by their lack of lungs.

As worms and spiders were creeping out of the sea, life was advancing at a much faster rate in the ocean than on land. Creatures were both growing bigger and developing more complex organs. Vertebrate animals, with an endoskeleton and a skin covering, evolved, as did fish with familiar organs like a brain, liver, heart, and alimentary canal. These sophisticated vertebrates began to colonize many different aquatic niches, from the highest river streams to the deepest ocean trenches. The Devonian Period, spanning from 420 million to 359 million years ago, is known as the age of fish for the explosion in the number of species and in the number of habitats they colonized.22

      Fish likely diversified because they developed an ability to utilize oxygen through an efficient circulatory system. A large part of that system is the gill. Most fish have a single slit on either side that allows water to pass through. As the water flows in, a vast network of capillaries embedded in the gills extracts oxygen from the water. The capillaries also expel carbon dioxide, in a gas exchange system parallel to our own. Most fish also have muscles around their gills that can cause them to flap and increase the stream of water and oxygen into the system as energy needs increase. It’s a worthy system of oxygen utilization, and it explains why fish have developed into some of the biggest creatures on Earth.

      In time, and only after they developed lungs as a way to extract oxygen from the atmosphere, fish made their way onto land. It is a single miraculous development, albeit spread out over tens of millions of years. It fascinates us because we can think of it as the moment of our birth, a symbol of when life as we know it was finally within reach. What made this transition possible was the creation of lungs, the organ that defines us as terrestrial creatures.

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