We marvel at the flight of hummingbirds, the songs of humpback whales, or a mountain goat leaping from ledge to ledge. Good! But small things are worth some contemplation too, even extremely small things. In particular, I’m considering some very small organisms called cyanobacteria (formerly known as blue-green algae): specifically, the roles they are thought to have played in the history of life on earth and their continuing roles in earthly ecology.
There are many kinds of cyanobacteria, inhabiting a wide array of environments and living different lifestyles. Some live as independent cells in soils and water or form biofilms on rocks and other surfaces; others live in hot springs, inside of rocks, or under Antarctic ice. And some live inside other organisms, as symbionts.
From our perspective, cyanobacteria are ridiculously small, typically measuring just tiny fractions of a millimeter in size, so small that millions could fit into one milliliter of water. Like other bacteria, they don’t even have organelles such as nuclei that direct their activity, or mitochondria that run their metabolism, or chloroplasts that do photosynthesis (the process by which plants use the sun’s energy to combine water and carbon dioxide to produce carbohydrates and oxygen). Because they lack these internal structures, they are sometimes considered to be evolutionarily simple or primitive. Yet they have their own internal organization and are very active in certain ways, have a lively metabolism, and photosynthesize. Lots of power in microscopic, single-celled, seemingly simple organisms!
Three or four billion years ago, cyanobacteria (and a few other kinds of bacteria) were probably the only forms of life on our planet. At that time, the earth’s atmosphere was anaerobic (without oxygen). But cyanobacteria captured energy from the sun and made carbohydrates to be used in their own metabolism, releasing oxygen as a byproduct. After they had done so for many millions of years, the earth’s atmosphere was quite different; oxygen produced by their photosynthesis comprised a significant part of the atmosphere, eventually reaching its current level of around 20 percent.
That change produced a revolution! Sometimes called the Great Oxygenation Event, it caused the extinction of most extant organisms, which were obligate anaerobes to which oxygen was lethal. But a few were able to tolerate an increase of oxygen, and they survived, forming a basis for subsequent evolution of great diversity. That free oxygen also linked up with atmospheric methane, a strong greenhouse gas, producing carbon dioxide—a less effective greenhouse gas. The resulting reduction of the greenhouse effect cooled the earth so much that glaciers covered much of the planet for millions of years, no doubt causing further extinctions. Geologically, new minerals were formed, as existing types became oxygenated (e.g., iron to iron oxide).
Somewhere along the line, in some little-understood way, some cyanobacteria apparently became incorporated in other single-celled organisms (those that could tolerate oxygen)—or perhaps into each other? — and became chloroplasts, the organelles that conduct photosynthesis in the cells of the vast majority of plants. In the oxygen-rich atmosphere that began to prevail, this set the stage for the evolution of the plant kingdom and the huge diversity of living and extinct plants. Many researchers believe that some cyanobacteria that were incorporated into cells became mitochondria, the organelles that govern metabolism, and are found in most types of organisms (except bacteria). This begs several questions: is having chloroplasts somehow a better way to do photosynthesis and is having mitochondria somehow better than doing cellular metabolism the old way? Or are these organelles just another way of conducting the work of a cell?
In addition to their major role in the history of life on earth, cyanobacteria are also very important for ecology on earth today. Free-living cyanobacteria in the ocean, especially one known as Prochlorococcus, are said to account for over half the photosynthesis in the open ocean and the majority of photosynthesis on earth, thus providing much of the oxygen that almost all of earth’s life depends on. Cyanobacteria are symbiotic in certain lichens, often joining green algae within a fungal partner, and lichens have important ecological roles in communities of terrestrial organisms. Cyanobacteria are also photosynthetic symbionts in some invertebrates, such as certain corals, sponges, and diatoms (unicellular planktonic algae), providing carbohydrates to the host organism.
Another ecologically important role of many cyanobacteria in the present-day earth is the fixation of atmospheric nitrogen (a gas) into ammonia and nitrates. Nitrogen is an essential plant nutrient, but atmospheric nitrogen is metabolically unavailable to plants. Ammonia and nitrates become available to plants indirectly in the soil and air or, in the case of symbiotic cyanobacteria, directly to their partners. Nitrogen fixation provides plants with usable forms of nitrogen, which is essential for the building of proteins, enzymes, DNA molecules, and so on. There are some other, quite different, nitrogen-fixing bacteria, such as those that live with alders and lupines, but these appear to require their symbiotic partners, whereas many (but perhaps not all) cyanobacteria are capable of free-living in water or soil. Interestingly, nitrogen fixation cannot occur in an oxygen-rich atmosphere, so all of these nitrogen-fixing bacteria enclose their nitrogen-fixing machinery in special, thick-walled cysts that keep out oxygen.
Cyanobacteria that are symbiotic in invertebrates may provide nitrogen as well as carbohydrates to their hosts. They are not known to occur very often in green plants, but they inhabit the aquatic fern Azolla, which thrives in rice paddies, where the cyanobacterial nitrogen-fixing activity helps fertilize the rice crop. They live inside the stem of a hefty, mostly-tropical flowering plant called Gunnera, where they reportedly fix nitrogen that is used by the plant; supposedly the cyanobacteria enter the plant through glands at the base of the leaves. These examples prompt the question of why cyanobacteria are found only in certain organisms, and apparently so rarely in ferns and flowering plants.
All is not sunshine and roses, however; some results of cyanobacterial activity may be viewed as negative. Consider that in the process of oxygenating the atmosphere and facilitating the evolution of most of the organisms we know, cyanobacteria were the agents of at least two episodes of major extinction in early earth history. Furthermore, some cyanobacteria are reported to be toxic to humans and other consumers, and great ‘blooms’ of aquatic cyanobacteria can cause die-offs of aquatic animals.
There is much yet to be learned, as always. That keeps things interesting. In the meantime, one lesson is this: Scorn not the small and inconspicuous, for they can create powerful effects–and they may inherit the earth!
• Mary F. Willson is a retired professor of ecology.
