Learning Objectives
In this section, you will explore the following questions:
- What is the evolutionary history of prokaryotes?
- What are distinguishing features of extremophiles?
- Why is it difficult to culture prokaryotes for study purposes?
Connection for AP® Courses
In the recent past, scientists grouped living organisms into six kingdoms based on several criteria, including the absence or presence of a nucleus and other membrane-bound organelles, the absence or presence of cell walls, multicellularity, and so on. As we learned earlier, prokaryotes were the first cells to evolve on Earth 3.5 to 4.5 millions of years ago and lacked a nucleus and cytoplasmic organelles. In the late 20th century, scientists compared sequences of small-subunit ribosomal RNA, which resulted in a more fundamental way to group organisms on Earth. Based on the differences in the structure of the cell membranes and in rRNA, Carl Woese and his colleagues proposed the three domain system. The domain Bacteria comprises all organisms in the kingdom Eubacteria (bacteria), the domain Archaea comprises the rest of the prokaryotes, and the domain Eukarya comprises all eukaryotes, including protists, fungi, plants, and animals. Prokaryotes exhibit great diversity in form and function and are abundant and ubiquitous. In addition to inhabiting moderate environments like the human digestive system, prokaryotes are found in extreme conditions (extremophiles), from boiling hot springs to the Great Salt Lake and to frozen environments in Antarctica.
Because prokaryotes inhabit many different environments, they have evolved multiple strategies for meeting their energy demands, including chemosynthesis, photosynthesis, anaerobic cellular respiration, and aerobic cellular respiration. Hot springs and hydrothermal vents may have been the environments in which life began, and there is fossil evidence of the presence of microbial mats about 3.5 billion years ago. During the first two billion years of Earth’s history, the atmosphere lacked sufficient quantities of oxygen, and only anaerobic organisms were able to live. Cyanobacteria evolved from early phototrophs and began the oxygenation of the atmosphere. The increase in oxygen concentration allows the evolution of other life forms with other metabolic processes. Most prokaryotes prefer to live in communities, often forming biofilms.
Teacher Support
As students read about and discuss microorganisms that live in extreme environments, remind them that prokaryotic species are also very well adapted to “normal” habitats—all of the lands and waters in which most other species are found. Guide students to understand that prokaryotes’ ability to adapt to such a broad range of environments helps to explain why they are the most abundant organisms on Earth. You may wish to offer a few statistics that demonstrate this or invite students to research and report statistics to the class. (For example, total prokaryotic biomass is about ten times that of total eukaryotic biomass; more prokaryotes inhabit a handful of fertile soil than the total number of people that have ever lived, and so on).
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.
| Big Idea 1 | The process of evolution drives the diversity and unity of life. |
| Enduring Understanding 1.D | The origin of living systems is explained by natural processes. |
| Essential Knowledge | 1.D.1 There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. |
| Science Practice | 6.3 The student can articulate the reasons that scientific explanations and theories are refined or replaced. |
| Learning Objective | 1.29 The student is able to describe the reasons for revisions of scientific hypotheses of the origin of life on Earth. |
| Essential Knowledge | 1.D.1 There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. |
| Science Practice | 6.5 The student can evaluate alternative scientific explanations. |
| Learning Objective | 1.30 The student is able to evaluate scientific hypotheses about the origin of life on Earth. |
| Essential Knowledge | 1.D.1 There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. |
| Science Practice | 4.4 The student can evaluate sources of data to answer a particular scientific question. |
| Learning Objective | 1.31 The student is able to evaluate the accuracy and legitimacy of data to answer scientific questions about the origin of life on Earth. |
| Essential Knowledge | 1.D.2 Life on Earth evolved between 3.5 and 4.5 billion years ago. |
| Science Practice | 4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question. |
| Learning Objective | 1.32 The student is able to justify the selection of geological, physical, and chemical data that reveal early Earth conditions. |
The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.35][APLO 2.36][APLO 2.37]
Prokaryotes are ubiquitous. They cover every imaginable surface where there is sufficient moisture, and they live on and inside of other living things. In the typical human body, prokaryotic cells outnumber human body cells by about ten to one. They comprise the majority of living things in all ecosystems. Some prokaryotes thrive in environments that are inhospitable for most living things. Prokaryotes recycle nutrients—essential substances (such as carbon and nitrogen)—and they drive the evolution of new ecosystems, some of which are natural and others man-made. Prokaryotes have been on Earth since long before multicellular life appeared.
Prokaryotes, the First Inhabitants of Earth
When and where did life begin? What were the conditions on Earth when life began? Prokaryotes were the first forms of life on Earth, and they existed for billions of years before plants and animals appeared. The Earth and its moon are thought to be about 4.54 billion years old. This estimate is based on evidence from radiometric dating of meteorite material together with other substrate material from Earth and the moon. Early Earth had a very different atmosphere (contained less molecular oxygen) than it does today and was subjected to strong radiation; thus, the first organisms would have flourished where they were more protected, such as in ocean depths or beneath the surface of the Earth. At this time too, strong volcanic activity was common on Earth, so it is likely that these first organisms—the first prokaryotes—were adapted to very high temperatures. Early Earth was prone to geological upheaval and volcanic eruption, and was subject to bombardment by mutagenic radiation from the sun. The first organisms were prokaryotes that could withstand these harsh conditions.
Microbial Mats
Microbial mats or large biofilms may represent the earliest forms of life on Earth; there is fossil evidence of their presence starting about 3.5 billion years ago. A microbial mat is a multi-layered sheet of prokaryotes (Figure 22.2) that includes mostly bacteria, but also archaea. Microbial mats are a few centimeters thick, and they typically grow where different types of materials interface, mostly on moist surfaces. The various types of prokaryotes that comprise them carry out different metabolic pathways, and that is the reason for their various colors. Prokaryotes in a microbial mat are held together by a glue-like sticky substance that they secrete called extracellular matrix.
The first microbial mats likely obtained their energy from chemicals found near hydrothermal vents. A hydrothermal vent is a breakage or fissure in the Earth’s surface that releases geothermally heated water. With the evolution of photosynthesis about 3 billion years ago, some prokaryotes in microbial mats came to use a more widely available energy source—sunlight—whereas others were still dependent on chemicals from hydrothermal vents for energy and food.
Stromatolites
Fossilized microbial mats represent the earliest record of life on Earth. A stromatolite is a sedimentary structure formed when minerals are precipitated out of water by prokaryotes in a microbial mat (Figure 22.3). Stromatolites form layered rocks made of carbonate or silicate. Although most stromatolites are artifacts from the past, there are places on Earth where stromatolites are still forming. For example, growing stromatolites have been found in the Anza-Borrego Desert State Park in San Diego County, California.
The Ancient Atmosphere
Evidence indicates that during the first two billion years of Earth’s existence, the atmosphere was anoxic, meaning that there was no molecular oxygen. Therefore, only those organisms that can grow without oxygen—anaerobic organisms—were able to live. Autotrophic organisms that convert solar energy into chemical energy are called phototrophs, and they appeared within one billion years of the formation of Earth. Then, cyanobacteria, also known as blue-green algae, evolved from these simple phototrophs one billion years later. Cyanobacteria (Figure 22.4) began the oxygenation of the atmosphere. Increased atmospheric oxygen allowed the development of more efficient O2-utilizing catabolic pathways. It also opened up the land to increased colonization, because some O2 is converted into O3 (ozone) and ozone effectively absorbs the ultraviolet light that would otherwise cause lethal mutations in DNA. Ultimately, the increase in O2 concentrations allowed the evolution of other life forms.
Science Practice Connection for AP® Courses
Early Earth was inhospitable to most life as we know it today, with a substantial amount of geological upheaval, volcanic activity, and an anoxic atmosphere (which means it lacked oxygen). Therefore, the origin of life on Earth is of great interest to scientists.
One of the earliest hypotheses of how life originated on Earth was by panspermia, which dates back to the 5th century BC. Panspermia is the idea that life is distributed to different parts of the universe on objects such as meteoroids and asteroids. If this were to occur, organic compounds and microorganisms would be able to survive the extreme conditions of space.
However, in the 1960s, scientists Stanley Miller and Harold Urey conducted laboratory experiments that showed it was possible for organic compounds, including amino acids, to be formed under certain conditions from inorganic molecules and energy, both of which would have been present in the early conditions of Earth. Subsequently, fossilized evidence of microbial mats by deep sea hydrothermal vents that dates back 3.5 billion years was discovered. Consequently, there are now several hypotheses regarding how life could have evolved on Earth.
Activity
Perform internet research using reliable webpages containing current scientific evidence supporting the ideas that 1) life originated on Earth, and 2) that life arrived on Earth from an extraterrestrial object, such as an asteroid or comet. Then decide which idea has more supporting evidence. Be able to justify your conclusion.
Think About It
If organic compounds, which eventually formed organisms, could have formed both extraterrestrially and on Earth, why would it be less complicated, and thus more likely, that life formed on Earth?
Think About It
Even though Miller and Urey showed that organic molecules could be produced from conditions of early Earth, why are there still different hypotheses about how life could have originated on Earth?
Teacher Support
Activity:
Be sure to guide students to reliable websites, such as online scientific journals, government websites and reliable scientific periodicals.
- This question is an application of AP® Learning Objectives 1.30 and 1.31, and Science Practices 6.5 and 4.4, because students are evaluating alternative hypotheses and using sources of data to answer questions about the origin of life
Think about it: Be sure to guide students to reliable websites, such as online scientific journals, government websites and reliable scientific periodicals.
- It would be more likely that life formed on Earth, because in both cases organic compounds would have had to be formed in some way. Life originating on Earth would be the more simplistic explanation because the organic compounds or microorganisms would not have had to survive traveling through space.
- This question is an application of AP® Learning Objectives 1.30 and 1.31, and Science Practices 6.5 and 4.4, because students are evaluating alternative hypotheses and using sources of data to answer questions about the origin of life.
- There are several hypotheses because the Miller-Urey experiment only showed that unloving organic compounds could have formed during the early history of Earth. It does not explain how these organic compounds could have formed into living cells. This “hole” has led to the development of other hypotheses, such as the idea that life arrived from extraterrestrial bodies.
- This question is an application of AP® Learning Objectives 1.29, and Science Practices 6.3 because students are evaluating reasons for the existence and revision of scientific hypothesis.
Microbes Are Adaptable: Life in Moderate and Extreme Environments
Some organisms have developed strategies that allow them to survive harsh conditions. Prokaryotes thrive in a vast array of environments: Some grow in conditions that would seem very normal to us, whereas others are able to thrive and grow under conditions that would kill a plant or animal. Almost all prokaryotes have a cell wall, a protective structure that allows them to survive in both hyper- and hypo-osmotic conditions. Some soil bacteria are able to form endospores that resist heat and drought, thereby allowing the organism to survive until favorable conditions recur. These adaptations, along with others, allow bacteria to be the most abundant life form in all terrestrial and aquatic ecosystems.
Other bacteria and archaea are adapted to grow under extreme conditions and are called extremophiles, meaning “lovers of extremes.” Extremophiles have been found in all kinds of environments: the depth of the oceans, hot springs, the Artic and the Antarctic, in very dry places, deep inside Earth, in harsh chemical environments, and in high radiation environments (Figure 22.5), just to mention a few. These organisms give us a better understanding of prokaryotic diversity and open up the possibility of finding new prokaryotic species that may lead to the discovery of new therapeutic drugs or have industrial applications. Because they have specialized adaptations that allow them to live in extreme conditions, many extremophiles cannot survive in moderate environments. There are many different groups of extremophiles: They are identified based on the conditions in which they grow best, and several habitats are extreme in multiple ways. For example, a soda lake is both salty and alkaline, so organisms that live in a soda lake must be both alkaliphiles and halophiles (Table 22.1). Other extremophiles, like radioresistant organisms, do not prefer an extreme environment (in this case, one with high levels of radiation), but have adapted to survive in it (Figure 22.5).
| Extremophile Type | Conditions for Optimal Growth |
|---|---|
| Acidophiles | pH 3 or below |
| Alkaliphiles | pH 9 or above |
| Thermophiles | Temperature 60–80 °C (140–176 °F) |
| Hyperthermophiles | Temperature 80–122 °C (176–250 °F) |
| Psychrophiles | Temperature of -15-10 °C (5-50 °F) or lower |
| Halophiles | Salt concentration of at least 0.2 M |
| Osmophiles | High sugar concentration |
Prokaryotes in the Dead Sea
One example of a very harsh environment is the Dead Sea, a hypersaline basin that is located between Jordan and Israel. Hypersaline environments are essentially concentrated seawater. In the Dead Sea, the sodium concentration is 10 times higher than that of seawater, and the water contains high levels of magnesium (about 40 times higher than in seawater) that would be toxic to most living things. Iron, calcium, and magnesium, elements that form divalent ions (Fe2+, Ca2+, and Mg2+), produce what is commonly referred to as “hard” water. Taken together, the high concentration of divalent cations, the acidic pH (6.0), and the intense solar radiation flux make the Dead Sea a unique, and uniquely hostile, ecosystem1 (Figure 22.6).
What sort of prokaryotes do we find in the Dead Sea? The extremely salt-tolerant bacterial mats include Halobacterium, Haloferax volcanii (which is found in other locations, not only the Dead Sea), Halorubrum sodomense, and Halobaculum gomorrense, and the archaea Haloarcula marismortui, among others.
Unculturable Prokaryotes and the Viable-but-Non-Culturable State
Microbiologists typically grow prokaryotes in the laboratory using an appropriate culture medium containing all the nutrients needed by the target organism. The medium can be liquid, broth, or solid. After an incubation time at the right temperature, there should be evidence of microbial growth (Figure 22.7). The process of culturing bacteria is complex and is one of the greatest discoveries of modern science. German physician Robert Koch is credited with discovering the techniques for pure culture, including staining and using growth media. His assistant Julius Petri invented the Petri dish whose use persists in today’s laboratories. Koch worked primarily with the Mycobacterium tuberculosis bacterium that causes tuberculosis and developed postulates to identify disease-causing organisms that continue to be widely used in the medical community. Koch’s postulates include that an organism can be identified as the cause of disease when it is present in all infected samples and absent in all healthy samples, and it is able to reproduce the infection after being cultured multiple times. Today, cultures remain a primary diagnostic tool in medicine and other areas of molecular biology.
Some prokaryotes, however, cannot grow in a laboratory setting. In fact, over 99 percent of bacteria and archaea are unculturable. For the most part, this is due to a lack of knowledge as to what to feed these organisms and how to grow them; they have special requirements for growth that remain unknown to scientists, such as needing specific micronutrients, pH, temperature, pressure, co-factors, or co-metabolites. Some bacteria cannot be cultured because they are obligate intracellular parasites and cannot be grown outside a host cell.
In other cases, culturable organisms become unculturable under stressful conditions, even though the same organism could be cultured previously. Those organisms that cannot be cultured but are not dead are in a viable-but-non-culturable (VBNC) state. The VBNC state occurs when prokaryotes respond to environmental stressors by entering a dormant state that allows their survival. The criteria for entering into the VBNC state are not completely understood. In a process called resuscitation, the prokaryote can go back to “normal” life when environmental conditions improve.
Is the VBNC state an unusual way of living for prokaryotes? In fact, most of the prokaryotes living in the soil or in oceanic waters are non-culturable. It has been said that only a small fraction, perhaps one percent, of prokaryotes can be cultured under laboratory conditions. If these organisms are non-culturable, then how is it known whether they are present and alive? Microbiologists use molecular techniques, such as the polymerase chain reaction (PCR), to amplify selected portions of DNA of prokaryotes, demonstrating their existence. Recall that PCR can make billions of copies of a DNA segment in a process called amplification.
The Ecology of Biofilms
Until a couple of decades ago, microbiologists used to think of prokaryotes as isolated entities living apart. This model, however, does not reflect the true ecology of prokaryotes, most of which prefer to live in communities where they can interact. A biofilm is a microbial community (Figure 22.8) held together in a gummy-textured matrix that consists primarily of polysaccharides secreted by the organisms, together with some proteins and nucleic acids. Biofilms grow attached to surfaces. Some of the best-studied biofilms are composed of prokaryotes, although fungal biofilms have also been described as well as some composed of a mixture of fungi and bacteria.
Biofilms are present almost everywhere: they can cause the clogging of pipes and readily colonize surfaces in industrial settings. In recent, large-scale outbreaks of bacterial contamination of food, biofilms have played a major role. They also colonize household surfaces, such as kitchen counters, cutting boards, sinks, and toilets, as well as places on the human body, such as the surfaces of our teeth.
Interactions among the organisms that populate a biofilm, together with their protective exopolysaccharidic (EPS) environment, make these communities more robust than free-living, or planktonic, prokaryotes. The sticky substance that holds bacteria together also excludes most antibiotics and disinfectants, making biofilm bacteria hardier than their planktonic counterparts. Overall, biofilms are very difficult to destroy because they are resistant to many common forms of sterilization.
Visual Connection
Footnotes
- 1Bodaker, I, Itai, S, Suzuki, MT, Feingersch, R, Rosenberg, M, Maguire, ME, Shimshon, B, and others. Comparative community genomics in the Dead Sea: An increasingly extreme environment. The ISME Journal 4 (2010): 399–407, doi:10.1038/ismej.2009.141. published online 24 December 2009.