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Learning Goals
By the end of this reading you should be able to:
- Discuss how some prokaryotes could become adapted to extreme environments
- Give examples of symbiotic relationships that involve prokaryotes
- Describe the nature of the human microbiome and how it benefits humans
- Explain the formation of a biofilm and how it benefits the organisms involved
- Outline how quorum sensing works and how it is thought to have evolved
Introduction
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.
Microbes Are Adaptable: Life in Moderate and Extreme Environments
Other bacteria and archaea are adapted to grow under extreme conditions and are called extremophiles (Figure 1), meaning “lovers of extremes.” Extremophiles have been found in all kinds of environments: the depth of the oceans, hot springs, the Arctic and the Antarctic, in very dry places, deep inside Earth, in harsh chemical environments, and in high radiation environments, 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. Other extremophiles, do not prefer an extreme environment but have adapted to survive in one.
Review Question:
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, ecosystem.
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.
Thinking Questions:
Microbial Symbioses
Symbiosis, strictly defined, refers to an intimate relationship between two organisms. Although many people use the term to describe a relationship beneficial to both participants, the term itself is not that specific. The relationship could be good, bad, or neutral for either partner. A mutualistic relationship is one in which both partners benefit, while a commensalistic relationship benefits one partner but not the other. In a pathogenic relationship, one partner benefits at the expense of the other.
Microbiomes
The human microbiome describes the genes associated with all the microbes that live in and on a human. All 1014 of them! The microbes are mostly bacteria but can include archaea, fungi, and eukaryotic microbes. These microbes can be found in locations such as the skin, upper respiratory tract, stomach, intestines, and urogenital tracts. Where do we get these colonizers? Colonization occurs soon after birth, as infants acquire microbes from people, surfaces, and objects that they come in contact with. It might help you to understand our biome better if you watch the following video:
Review Questions:
We are not the only organisms that have a microbiome and it’s not just animals. These communities of microbes are found everywhere. Each has its own unique participants and each has its own ecology. We are only just beginning to explore microbiomes and how they impact the organisms and environments in which they are found.
Biofilms and Microbial Mats
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. 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.
The basic steps for biofilm formation can be broken down into four steps:
Two: Colonization – cell-to-cell signaling occurs, leading to the expression of biofilm specific genes. These genes are associated with the communal production of extracellular polymeric DNA released by some cells that can be taken up by others, stimulating the expression of new genes.
Three: Maturation – the EPS (exopolysaccharidic) matrix, composed of polysaccharides and proteins, fully incases all the cells. The biofilm continues to thicken and grow, forming a complex, dynamic community. Water channels form throughout the structure.
Four: Detachment and sloughing – individual cells or pieces of the biofilm are released to the environment, as a form of active dispersal. This release can be triggered by environmental factors, such as the concentration of nutrients or oxygen.
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.
Biofilms are present almost everywhere and have huge impacts throughout many different types of industries. Medical implants ranging from catheters to artificial joints are particularly susceptible to biofilm formation, leading to huge problems for the medical industry. A type of biofilm that affects almost everyone is the formation of dental plaque, which can lead to cavity formation. 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.
Review Questions:
Microbial mats: These are large biofilms and 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 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 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.
Creating a quorum
The word quorum refers to having a minimum number of members needed for an organization to conduct business, such as hold a vote. Quorum sensing refers to the ability of some bacteria to communicate in a density-dependent fashion, allowing them to delay the activation of specific genes until it is the most advantageous for the population.
Quorum sensing involves cell-to-cell communication, using small diffusible substances known as autoinducers. An autoinducer is produced by a cell, diffusing across the plasma membrane to be released into the environment. As the cell population increases in the environment the concentration of autoinducer increases as well, causing the molecule to diffuse back into individual cells where it triggers the activation of specific genes. Essential what is happening is that the bacteria are actually using chemical signals to talk to each other. Sounds crazy right? This video is from a scientist who is not only studying the mechanisms of quorum sensing, she is working out how this system of communication might have evolved.
2) What type of molecule did they discover was the general communication molecule in all bacteria?
End of Section Review Questions:
2) In what order do these events in biofilm formation occur?
3) All of the examples in this reading demonstrate how diverse prokaryotes can be. They are only single-celled organisms but they display an amazing ability to adapted to many different conditions. Choose one example or come up with your own AND explain how it might have evolved? (Think about the mechanisms and processes of evolution)
References:
OpenStax, Biology. OpenStax CNX. Nov 7, 2018 http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@11.6. 22.1 Prokaryotic Diversity
Microbiology. Linda Bruslind http://library.open.oregonstate.edu/microbiology/
Image Attributions:
Figure 1. Image created and provided by D. Jennings
Figure 2. Image courtesy of CNX OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
Figure 3. Image courtesy of D. Davis [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
Figure 4. Image courtesy of Dr. Bob Embley, NOAA PMEL CC by 2.0 https://creativecommons.org/licenses/by/2.0/
1,A; 2,C; 3,B
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