12.3: Microbes in Charge

At the end of the twentieth century and the beginning of this century, marine science came to recognize that ocean ecosystems were dominated by microscopic organisms in a manner that would also change our perception of what constitutes the true foundation of life on our planet. To understand this, we first need to review just a few of the startling findings that have now revealed the dominance of microbial organisms on our planet. Note that microbe and microbial are terms to identify those groups of tiny ocean organisms that are commonly less than about 60 μm, which is smaller than the approximately 100 μm smallest object that a normal human eye can see clearly. First, we have learned that there are about 10⁶ bacteria in each milliliter of ocean water (or 5•10⁶ per teaspoon) and viruses are about 10 times more abundant. This means that Earth’s ocean is estimated to contain 10²⁹ bacteria which is a number one hundred million times greater than the total estimated number of stars in the universe (10²¹). The total mass of bacteria and other single-celled organisms in the ocean exceeds the combined mass of all living organisms that are non-microbial by 5 to 10 times. 

Microbial organisms perform essentially all of the primary production in the oceans, a role played by plants in terrestrial ecosystems and despite their small size, total primary production in the oceans is estimated to approximately equal total primary production on land. One type of primary producer, a cyanobacteria belonging to the genus Prochlorococcus is among the smallest primary producers (about 0.5 to 0.7 μm) but, despite its small size it is estimated to be responsible for about 20% of the oxygen in Earth’s atmosphere. Prochlorococcus is able to do this because it is believed it may be the most plentiful genus of species on our planet, with an estimated 10²⁷ individuals. 

If it were not for microbes, no animals, including humans, would ever have lived on the planet. Earth’s early atmosphere and oceans had no free oxygen. Microbial photosynthesis is thought to have developed in cyanobacteria during an event known as The Great Oxygenation Event that began about 2.45 billion years ago and lasted between 200 and 400 million years. During this event, the abundant free oxygen in the atmosphere and oceans that all animals depend on was generated by microbes (cyanobacteria) in the oceans and the atmospheric and ocean oxygen concentrations reached levels comparable to the current concentrations. 

The facts about ocean microbes listed above demonstrate the dominance of microbial organisms in the oceans. However, the dominance of microbial organisms is probably much greater still because they have the ability to metabolize at rates far exceeding those of larger organisms. This is because the ratio of surface area to volume generally is greater for smaller than for larger organisms. We can visualize why if we consider an apple cut into two halves. The apple’s total volume does not change when it is cut, but the total surface area is increased by the area of the newly exposed interior surface of each apple half. If the apple is cut again, the total volume still remains the same, but the total surface area again increases. All organic and inorganic nutrients, oxygen, and waste products have to pass through the cell surface. Because of its much greater ratio of surface area to volume a microorganism 1 micrometer (one thousandth of a millimeter) in diameter can metabolize about 100,000 times faster than a human. To put this in context, for one oceanic bacterium that divides every 10 min. (not unusually fast for bacteria if appropriate nutrients are available) the energy throughput of a mass of the bacteria equal to a human would be about 20 megawatts, enough to provide power and heat for about 10,000 homes in the U.S.

Because they are a large fraction of the total biomass and because they have high metabolic rates when nutrients are available, microbes dominate the movement of energy and biologically important elements in the oceans. Because the microbial community includes primary producers, consumers, and decomposers all with fast metabolic rates, most of the total primary production in the oceans is recycled through microbes and only a small fraction moves far enough up the food chain to reach non-microscopic eukaryotes. It is estimated that less than 1 to 2% of the total production is available for non-microscopic eukaryotes.

The rapid rate of division of microbial species also suggests that these microbial species are far more adept at adjusting to changing environmental conditions than larger longer lived organisms. It has been found, through genetic studies like metagenomics (CC19), that ocean microbial biomes are as small as a single drop of water and that complex ecological and evolutionary processes take place in each such biome. Additionally, recent studies have shown that viruses, especially those that infect bacteria and archaea, play a much larger role in microbial biomes than previously thought. Thus, although global climate change may cause many species to become extinct, ocean microbes will almost certainly evolve, adapt, and survive as they have done for billions of years.