Life comes in many shapes and sizes. There are the insects that scurry over the ground and fly through the air. The fishes that swim in the sea, various kinds of birds, and large mammals ranging from lions to whales. There are also small organisms such as bacteria present in the water, air, and soil.
What are the ranges of size of organisms? Starting from the big to small, and to be more scientific, measurements are done using the metric system and the unit of measurement for length is the meter. Organisms are measured, starting with the smallest in micrometers, a unit of measurement that is used for microorganisms which can be seen only with a microscope. Going up, in terms of size, are the organisms that can be measured in millimeters and centimeters and these include small insects, small mammals, and birds and these are the kind of organisms we see with our eyes and then we proceed up to the bigger organisms which includes the blue whale and the biggest plant which is a tree called the redwood.
Still the question to ask is what is the smallest organism known to biology that can perform all life functions? Before we find the smallest organism we must first review the characterstics of life , regardless of the size of organisms and I have written some blogs about what all life has in common and for those of you haven’t read my earlier blogs, I will summarize all that life has in common before we find the smallest organisms that is capable of carrying out all these functions.
The Many Characteristics Shared by all of Life
From a human being composed of many organ systems , each of which has a specific function such as the circulatory system delivering oxygenated blood and nutrients to all body cells, a digestive system for breaking down large food molecules into their simpler components, and a respiratory system taking in oxygen to all the way down to unicellular organisms such as an amoeba where all the functions of biochemistry are in one cell, life has a degree of organization and indeed there is nothing random at all in the organization of the components for each of them has a specific function or else it would cease being what we would recognize as life.
Living things are constantly surviving, and depending on the species, it could be the birth, life, and death of an individual multicellular organism or a single cell organism. Both of which have a variety of biochemical processes from the cellular to organ level that involves taking in food and producing wastes, maintaining a body temperature, and coping with changes in the environment. Collectively these various process define homeostasis or maintaining a core set of physiological processes to deal with changes in the environment.
One of the most important characteristics of life is its ability to form another (near) perfect copy of itself and life has been doing this, in what is called reproduction, for 3 billion years of the earth’s history. There are two forms of reproduction. One ( that we all know too well!) is sexual reproduction and this involves, at the cellular level, a process called meiosis where two cells, or gametes, are formed with half the number of chromosomes which are subcellular structures consisting of DNA and protein and these gametes with half the number of chromosomes are combined together through fertilization to form the complete organism that will develop into an organism that is almost like the parents. I say “almost” because there is no way that the process of replicating DNA is perfect and indeed such imperfections are what are called mutations and mutations can either be harmful or beneficial and such tiny changes could gradually result in a population of organisms different from the ancestral population ( I will talk about this in detail shortly).
It is accurate to consider life more as a verb than a noun. Life can do a lot such as in animals that swim, fly, or run and also of plants that break through the soil, spread leaves and in the case of angiosperms or flowering plants, unfurl their petals. All of this requires energy. Organisms do not create energy out of nothing. The energy comes somewhere and in another blog where I talk about life and energy, there are two sources of energy, one is sunlight coming from the sun and the other geochemical energy near hydrothermal vents. These two forms of energy are used by those organisms called autotrophs and they are organisms that can one form of energy to combine simple inorganic compounds into complex molecules. Plants are considered autotrophic because they carry out photosynthesis where energy in the form of sunlight is use to synthesize carbohydrates, a complex organic molecule using water and carbon dioxide. In the case of life near hydrothermal vents, this would be chemical energy using minerals such as iron sulfide in what is called chemosynthesis.
In addition to autotrophs, there are the other class of organism called heterotrophs and these organisms need organic molecules to survive. Animals are the perfect example of heterotrophs in that they actively search for food which consists of carbohydrates, proteins, and fats and derive energy by breaking them down and utilizing the building blocks of the molecules which in carbohydrates is glucose, amino acids from proteins, and fatty acids and glycerols from fats.
Photosynthesis is an example of a form of metabolism called anabolism and this involves building up complex molecules from simpler components. Another example that is important and universal in all of life (as we know it, so far) is protein synthesis and this is where protein molecules are built up from amino acids. Both of these processes involve an input of energy and for the former this is electromagnetic radiation in the form of visible light and the latter a source of chemical energy called adenosine triphosphate or ATP, a unit of energy used for every anabolic process.
The opposite of photosynthesis, respiration takes carbon and hydrogen rich molecules and in a complex series of processes together with oxygen, a waste gas released from photosynthesis, produce carbon dioxide and water as waste and energy locked up in ATP. With the ATP, which is found in all of life, is the source of energy needed for anabolism. The breakdown of molecules is known as catabolism and respiration is a form of catabolism.
Many organisms are observed to show an increase in size and this is apparent in the plant and animal kingdom. A chick hatches from an egg which developed from an embryo inside the egg as a result of sperm and egg uniting together and the chick then grows up to a chicken. Likewise , a seed from a tree, in the soil, together with water and nutrients will grow up to become a tree with leaves. This depends ultimately on the cells and for eukaryotic organisms or cells with nuclei, one cell becomes two in a complex process called mitosis and even though it is a splitting of cells, the process anything but simple and there are several stages that a cell must go through in order to divide. This is the main reason for the growth of an organism from small to large.
Response and Stimuli
Any organism is never in complete isolation from its environment. An organism such as a bacterium must be able to sense what is present outside of it, whether there is glucose, a food source, or an antibiotic, a poison. These are the stimuli that will allow the cell to move towards for the former or away if the latter is present. Whatever stimuli the single cell senses, it will execute an appropriate series of behavior which are the responses. The behaviors and methods of sensing stimuli is much more sophisticated for multicellular organisms such as animals with a nervous system for taking in various kinds of senses such as sight and smell and with a muscular system for movement.
When life appeared on the planet about around 3.8 billion years ago, it was simple and unicellular but as the fossil records, many different life forms appeared throughout the history of planet earth, many of which became extinct while other species survived. Why is that? It is because organisms have a set of characteristics that allowed them to live in their environment and as long as the environment is stable, it is likely but by no means certain, that any species of life form will survive in its environment. This is know as adaptation and there is a mechanism, first realized by Charles Darwin, called natural selection where the environment selects those characteristics for survival and if in each reproductive cycle of each species, an organism has features that allow it to survive in its native environment, then natural selection will allow the organism to survive into the next generation whereas if in each generation , the offspring does not have these kinds of adaptations, then they will likely die. It is this process of filtering out those that can adapt, that gradually many organisms were able to exploit whatever natural resource which by this I mean, the presence of food and space for living and mating and many , if not all organisms, become more complex in organization and even behavior than there predecessors resulting in different species, some of which have survived but others, because of changing environments went extinct.
From the Large to the Small
Now that we have reviewed the major characteristics of life, we know begin to ask a question that is central to this blog and the question is “What are the kinds of life forms that are the largest to the smallest?” By asking this question, we eventually get to the main point of what the smallest organism is that has all the characteristics of life.
Starting from the largest organism, I will also explain why the laws of nature can permit as well constrain what the biggest organism can be and likewise I will do the same for the smallest organism as well.
To start, and keeping with the accuracy and precision that is characteristic of science, I will use the measurement of length, the meter, when expressing size.
Let’s begin with the kingdom of life forms that we all know so well and that’s the animal kingdom. We must ask “What is the largest animal that we know?”
The answer is a marine species adapted to life in the oceans and it is the mammal, the Blue whale. What is the average size of the blue whale? The average size is 24 meters or 79 feet. Compare this size to other whale species and the second largest below that of the blue whale, the North Pacific right whale. It’s size is 15.5 meters or 51 feet. The third largest below that of the North Pacific whale, the Southern Right whale, measures close to15.25 meters or 50 feet.
I have compared the sizes of three whale species , all of which live in the ocean. What about land animals? What is the largest land animal? That turns out to be the African Bush elephant with a length of 6 meters or 19.7 feet.
So much for large living animals. What about plants? What is the largest plant on record? That would be the aptly named giant sequoia and it measures on average to 70 to 85 meters in height or 230-280 feet.
Starting from the largest life form, is the giant sequoia and the second largest life form, the blue whale all the way to the African bush elephant. We then proceed to the small organisms that includes small fish to small frog down to small plants all the way down to life forms so small you need a microscope to see them and then we proceed from meters to centimeters, millimeters and down to the unit of measurement that is used in the field of microscopy, the micrometer which is a millionth of a meter.
At this level, we see the cells, and depending on the cell, we may come across an amoeba which is a unicellular organism known for its ability to shape shift because an amoeba is an active hunter and must consume bacteria in order to surviving. Another is the paramecium and it is an oval shaped cell surrounded by tiny hairs called cilia and with these cilia, this allows the cell to move quickly through water. Both amoeba and paramecia are fine examples of unicellelular eukaryotes and these are cells with nuclei and specialized structures called organelles which carry out different kind of biochemical functions. Amoeba and paramecia belong to the kingdom Protista and this is the kingdom of unicellular eukaryotes but recent methods of molecular biology have revealed that are even multicellular protists as well but we won’t get into that.
Eukaryotic cells, because of their complex structure tend to be much larger than another set of cells, which are the prokaryotes. Prokaryotes are the opposite of eukaryotes and that is because they have no nucleus and no organelles. The DNA is arranged in chromosomes like eukaryotes but the difference is that in eukaryotes the DNA is wrapped around in special proteins called histones and that results in a complex organization of the chromosomes whereas in prokaryotes it is only DNA. In both eukaryotes and prokaryotes, both of these cells have what are called ribosomes and these are protein RNA complexes where proteins needed by cells are synthesized and even there is difference in size. Ribosomes in eukaryotes are much larger than ribosomes in prokaryotes.
Despite their different organization, prokaryotes are nonetheless capable of all the characteristics of life so what is the smallest prokaryote, the one capable of all life functions?
From Mycoplasma to Nanoarcheum
Since prokaryotes are much simpler in terms of cell structure compared to eukaryotes, the class of bacteria that have the smallest size are the Mycoplasmas, more specifically a bacterium called Mycoplasma genitalium, a disease causing bacteria, was at first reported to be a smallest organism known to biology capable of carrying all the properties of life and there is one way of quantifying the size in terms of the organism’s genome.
Recall that in the cell structure, all cells possese DNA as the carrier of genetic information. DNA is present in both eukaryotes and prokaryotes and it is the sequence of base pairs or when adenine pairs with thymine, and guanine pairs with cytosine , is what determines the genetic information.
Indeed there is a quantifiable unit, the base pair, which can be used to determine the size of the genome and that is all the organism’s genes. There is even a rough if not exact correlation between the complexity of an organism and the size of the genome although there is no exact correlation between the size of genome and the complexity of the organism. An example is that some organisms have even more DNA than humans. In fact, in the case of eukaryotic DNA, much of the DNA doesn’t code for any proteins and is aptly named “junk DNA”.
If we consider DNA that codes for proteins while ignoring the rest that does not, then as we proceed from eukaryotes where the DNA in chromosomes are divided into regions called introns which are the genes that do not code for proteins and the exons where the genes do, then as we move towards the prokaryotes there are more exons and less , if any introns.
The size of the genome can be measured in terms of the number of base pairs and one such unit of measurement is called megabase pairs or Mbp for short and it is equal to a million base pairs.
If we use that criteria for judging the size of prokaryotes then what is the size of the genome of Mycoplasma or rather we should be asking: What is the smallest genome size that an organism posseses that is still capable of performing all the key requirements for the definition of life?
The answer turns out to be within the range of 525 genes and since each gene that codes for proteins requires no more than 3 bases, which are any of the four bases, adenine, thymine, guanine, and cytosine, the total number of base pairs is 580, 070 or the total number that codes for all proteins needed for protein synthesis, DNA replication, in short all that is needed to satisfy the requirements of life , so from the perspective of base pairs, this would make this life form the smallest form of life but is it really the smallest life form known so far?
In 2003, another smaller microbe with a somewhat smaller number of genes and still with the capacity to perform all the seven characteristics of life was discovered and named Nanoarchaeum equitans. The base pairs for Nanoarchaeum is 490,885 base pairs. Given the relatively small number of base pairs than Mycoplasma genitalium, and like Mycoplasma genitalium, it is the only Archaen or prokaryote adapted to extreme environments that is a parasite and must depend on another Archaen, Ignicoccus, for survival since because of its small genome, it doesn’t have enough genes to carry out most of other vital functions but must be dependent on another.
What about organisms with even smaller genomes? They wouldn’t be free living organisms but they would be known as viruses and these organisms live between the living and non living in that on their own, they are incapable of carrying out all life functions but depend on other organisms to do so.
It seems that organisms within the base pair ranges of 587,070 to 490,885 is the limit by which they can carry out the necessary biological functions but they are more likely to be parasitic in that due to the size of their genomes, they do not have enough genes to live a fully independent life and for Mycoplasm genitalium with a total of 525 genes, must resort to living on cells of another species, in this case human, and for Nanoarchaeum, an even smaller genome size forces it to depend totally on another living cell.
Since all of life can be (almost) traced to a single universal ancestor and if it was a small unicellular organism composed of a single cell, its genome would likely to be much larger than 587,070 base pairs but no larger and through evolution, the base pairs would up increasing whenever each new species evolved into various kinds of habitats. Life had small beginnings.
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