Are all single-celled organisms Bacteria?

Are all single-celled organisms Bacteria?

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I read that "Bacteria are one-celled organisms that can multiply by division", are all one-celled organisms bacteria or are there any more narrow definitions?

No, many unicellular organisms are not bacteria. Examples include (but certainly not limited to); some fungi, chlorella algae, and archaea.

Bacteria are one of three domains in the classification of life. You can find more about the bacteria domain at the wikipedia page (It's a long and complex history which is hard to summarise here) and about the domains here. This last defines bacteria loosely consisting of "prokaryotic cells possessing primarily diacyl glycerol (DAG) diester lipids in their membranes and bacterial rRNA, no nuclear membrane, traditionally classified as bacteria."

The composition of lipids in their membrane (DAG-esters) differentiates them from another major class of prokaryotes - Archaea, which have isoprenoid lipids in their membranes.

Initially Archaea was called Archaebacteria. Because of significant differences, including the membrane composition, they are now grouped into a different kingdom.

No, as @rg255 already mentioned, bacteria are one of the main kingdoms of life:

The Bacteria and Archaea are all unicellular organisms (though there are strange Archea like the Pyrodictium genus that are borderline multicellular). The Eukaryota include all plants, fungi and animals but there are also unicellular Eukaryotes. For example, Paremecium is a unicellular animal, brewer's yeast (S. cerevisiae) is a unicellular fungus and Chlorella is a unicellular plant.

In other words, all bacteria are unicellular but not all unicellular organisms are bacteria.

List of Single-Cell Organisms

The cell is the smallest living organism that contains all the features of life, and most all life on the planet begins as a single-cell organism. Two types of single-celled organisms currently exist: prokaryotes and eukaryotes, those without a separately defined nucleus and those with a nucleus protected by a cellular membrane. Scientists posit that prokaryotes are the oldest form of life, first appearing about 3.8 million years, while eukaryotes showed up about 2.7 billion years ago. The taxonomy of single celled organisms falls into one of the three major life domains: eukaryotes, bacteria and archaea.

TLDR (Too Long Didn't Read)

Biologists classify all living organisms into the three domains of life starting with single-cell to multicellular organisms: archaea, bacteria and eukaryotes.

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Staying Healthy

Bacteria should never be considered a good or bad thing. In nature, bacteria is just an organism that happens to exist – sometimes inside your body. Just like people, you will find forms of bacteria that can do your body good, and forms of bacteria that can do your body harm.

If you stay healthy and vigilant though, most harmful bacteria will be nothing to worry about. If you want to learn how to stay healthy and change your life, try the Udemy course Seeds 4 Change: A Path to Health and Healing.

Essay on Taxonomy (For School and College Students) | Organisms | Biology

Are you looking for an essay on ‘Taxonomy’? Find paragraphs, long and short essays on ‘Taxonomy’ especially written for school and college students.

Essay # 1. Meaning of Taxonomy:

The science of taxonomy and systematics involves the classification of organisms according to evolutionary relationships how closely they are related to each other. Before scientists were able to use DNA sequencing to examine evolutionary relationships, organisms were classified based on physical similarities and differences. Modern systematics combines data from many sources, including- the fossil record, comparative homologies (similarity of struc­tures due to shared ancestry), and comparative sequencing of nucleic acids (DNA and RNA) among organisms.

Essay # 2. Types of Taxonomy:

a. Evolutionary Taxonomy:

It is based on the fossil materials collected from the field. In constructing a hierarchy, a tra­ditional and very flexible combination of criteria was used. Firstly, morphological resem­blance and then phylogenetic relationships, the way in which the animals actually related to each other, in terms of the regency of a common ancestor (as far as could be determined).

The order of succession in the rock record (biostratigraphy) and the geographical distribution may play an important part in deciding these relationships. This practical approach, which takes all factors into consideration, has long been the basis of paleontological classification, and is still seen as the best method by many.

b. Numerical Taxonomy:

Evolutionary method has limitations like uncertainties and subjectivity of classification by observation, along with the preservation of the fossil record. To avoid this, numerical taxonomists attempt to use quantified observations of the animal in an attempt to decide on natural groupings. They consider that if enough characteristics are measured, and computed, then represented by the use of ‘cluster scatters’ (a form of graph) followed by the distance between clusters can be used as a measure of their differences.

However, although useful in some instances, the operator still needs to (subjectively) choose how best to analyze the measurements taken, and possibly give greater precedence (weight) to certain more- important characteristics again, a subjective choice. Thus, numerical taxonomy is not as ob­jective as it may first appear.

Essay # 3. Basic Taxonomic Categories:

The seven basic taxonomic categories are:

Kingdom being the broadest category, while species being the most specific. It can be easily remembered by the mnemonic sentence “King Philip came over for green soup”.

Living organisms are subdivided into 5 major kingdoms:

Prokaryotes (i.e., without a nucleus) Unicellular and colonial, including the true bacteria (eubacteria) and cyanobacteria (blue-green algae) [

Unicellular protozoans and unicellular and multicellular (macroscopic) algae with 9 + 2 cilia and flagella (called undulipodia[

Haploid and dikaryotic (binucleate) cells, multicellular, generally heterotrophic, without cilia and eukaryotic (9 + 2) flagella (undulipodia) [

Haplo-diploid life cycles, mostly autotrophic, retain embryo within female sex organ on parent plant [

Multicellular animals, without cell walls and without photosynthetic pigments, form diploid blastula [

Classes are grouped into:

i. Phyla (the plural of phylum), and

Within the kingdom Animalia, the most common phyla are:

(c) Chordata (e.g. fishes, amphibians, reptiles, birds, mammals)

(d) Platyhelminthes (e.g. tapeworms)

(e) Nematoda (i.e. unsegmented worms)

(f) Annelida (i.e. segmented worms)

(g) Cnidaria and Ctenophora (e.g. jellyfish)

(h) Echinodermata (e.g. starfish)

The class is a major division within the Kingdom, and forms the basis on which most fossil study is based.

For example, the phylum Molluscas contains 4 classes:

Families are grouped into orders, whose individuals may vary in many ways such as the order of Carnivora – which includes cats, dogs and weasels. Orders begin with a capital and usually end in “a” – but not always.

Genera are grouped into families, which are major groups of generally similar or­ganisms such as Felidae, which includes all cat-like animals from domestic cat to wild lynx to tiger to cheetah to jaguar to snow leopard. Family names always end in the letters “ae”, but are not printed in any special way.

The generic name refers to the genus, which is a group of species that are fairly closely related – such as the genus Equus which includes several species, such as the Equus caballus, Equus asinus and Equus zebra (domestic horse, wild ass and zebra respectively). ‘Genus’ is the taxonomic classification lower than ‘family’ and higher than ‘species’. In other words, genus is a more general taxonomic category than the species.

It is the fundamental unit of taxonomy. This is a group of very similar individuals that typically have similar anatomical characteristics and have the potential to interbreed freely, to produce fertile offspring – but cannot interbreed successfully with individuals from other species.

A mule, for example, is not a distinct species. It is an infertile hybrid of a male donkey (Equus asinus) and a female horse (Equus caballus). There are, as ever, exceptions where the rule breaks down, especially in the plant Kingdom. However, in the majority of cases, interbreeding of species does not produce fertile offspring.

Essay # 4. Domains and Kingdoms of Life:

In 1990 American molecular biologist Carl Woese proposed a new category, called a Do­main, further highest level- Archaea, Bacteria, and Eukaryota. Archaea are a group of organ­isms that are adapted to live in extreme habitats like thermal volcanic vents, saline pools, and hot springs (Fig. 2.1). Though, they are quite similar in appearance to bacteria, molecular studies have shown that they are biochemically and genetically very different.

Bacteria are simple single-celled organisms that generally lack chlorophyll (an exception is cyanobacte­ria). Bacteria generally obtain energy for survival by breakdown organic matter through fer­mentation and respiration. They are generally heterotrophic. Bacteria such as cyanobacteria and those belonging to the genus Rhizobium play an important role in the fixing of atmos­pheric nitrogen.

Eukaryota are organisms that have a eukaryote type of cell.

This group of life includes the four primary kingdoms:

King­dom Protista is made up of single celled organisms and some of their simple multi-cellular close relatives. Some examples of unicellular protists include dinoflagellates, amoebas, Para­mecium, diatoms, and volvox. Slime molds, brown, red and green algae like Ulva are typical examples of multi-cellular forms of protests (Fig. 2.1).

According to recent estimation there are about 1.5 million different species of fungi exist in our planet. Most of these life-forms are multi-cellular. The biologists at large have grouped the fungi with plants. However, investigations of this life-form indicate that fungi are quite different from other eukaryotes in terms of feeding strategies, physiological organi­zation, reproduction and growth.

Many species of fungi are heterotrophic decomposers or they live in symbiosis with another species. Lichens are good examples of this type of biotic relationship (Fig. 2.2). Lichens involve the symbiotic relationship between a fungus and a photosynthetic alga.

The kingdom Plantae is composed of multi-cellular photosynthetic organisms that can convert inorganic elements, with the help of the sun’s energy into organic compounds. Plan­tae includes all land plants, including mosses, ferns, conifers, and flowering plants. Diversity in this kingdom is quite large with more than 250,000 species. Two other important traits associated with plants are cell walls made of cellulose and a large central cellular vacuole.

Animals are multi-cellular heterotrophic eukaryotes. Species in the kingdom Animalia must ingest produced organic molecules for food. Animals also differ from other forms of life by having two unique tissue types- nervous tissue and muscle tissue. Most animals pro­duce their offspring through sexual reproduction.

Researchers find bacteria causes a single celled organism to form colonies

Image from the research paper "Bacterial regulation of colony development in the closest living relatives of animals".

( -- Researchers working at a lab at Berkeley University, led by Nicole King, have uncovered the first example of a kind of bacteria that causes a single celled organism to form a colony, a finding that has implications for researchers looking into the origins of multi-celled organisms in general. The team has published their findings on the lab’s web site and their paper will appear in the first edition of the new open source journal eLIFE.

The team’s research centers on choanoflagellates, single celled organisms that swim around in water using their tails. In some settings, they swim around independently, while in others they form colonies in the shape of rosettes. King and her team spent several years trying to figure out why they sometimes go solo, and sometimes don’t. At one point, they applied an antibacterial agent to the environment in which specimens of organisms were living and found that afterwards, they all quit forming into colonies. That led to a search through some sixty strains of bacteria to determine which had caused the change. They finally found it, a new species, Algoriphagus machipongonensis. But of course, that was just the beginning. Next the team tore apart the bacteria trying to figure out what unique property it held that caused choanoflagellates to form into a colony. That led them to a lipid molecule they have named Rosette-Inducing Factor 1 (RIF-1). When choanoflagellates ingest the bacteria, they get a very tiny amount of RIF-1, and that is all it takes, apparently, for the daughter cells that are spawned to form the rosettes.

The findings have added importance because researchers have long thought that choanoflagellates are animals closest relative, which means that it’s possible that bacteria and the molecules they produce, could conceivably be part of the reason that single celled organisms first began to form colonies, leading eventually to multi-celled beings that evolved over time into all of the animals that are alive today.

Not everyone is onboard with that idea however, as over time, choanoflagellates have evolved just as have all the organisms that evolved from whatever the first multi-celled organisms actually were, but this new finding does hint at the possibility that we all owe our existence to a bacteria similar to A. machipongonensis living all those years ago.

There are still questions to be answered as well, such as what about RIF-1 causes the choanoflagellates to form colonies, and do they have to actually eat them to gain the ability to form the colonies or is just being around them enough? King’s team will no doubt be looking to answer such questions while other’s debate the significance of their findings.

Bacterially-produced small molecules exert profound influences on animal health, morphogenesis, and evolution through poorly understood mechanisms. In one of the closest living relatives of animals, the choanoflagellate Salpingoeca rosetta, we find that colony development is induced by the prey bacterium Algoriphagus machipongonensisand its close relatives in the Bacteroidetes phylum. Here we show that a rosette inducing factor (RIF-1) produced by A. machipongonensis belongs to the small class of sulfonolipids, obscure relatives of the better known sphingolipids that play important roles in signal transmission in plants, animals and fungi. RIF-1 has extraordinary potency (femtomolar, or 10^-15 M) and S. rosetta can respond to it over a broad dynamic range – nine orders of magnitude. This study provides a prototypical example of bacterial sulfonolipids triggering eukaryotic morphogenesis and suggests molecular mechanisms through which bacteria may have contributed to the evolution of animals.

Reproduction of Bacteria

Even though they are single-celled microorganisms, bacteria can reproduce prolifically, if the conditions are right. Like most of the other single-celled organisms, bacteria too undergo reproduction through binary fission, which is a type of asexual reproduction. In this case, identical generations are formed, because the daughter cells are identical clone cells only. Budding is another form of asexual reproduction in bacteria. Even genetic recombination occurs in different types of bacteria, through transduction, transformation, and conjugation.

Binary Fission

This is a form of asexual reproduction, which is common among bacteria. In this type of reproduction, a single parent cell divides into two, and forms two daughter cells, which will be replicas of the parent. In order to undergo binary fission, the bacterial cells must grow to a fixed size. Once they reach that size, each cell produces a replica of the genetic material, and form two DNA molecules that attach to the cell membrane in different locations. The cell membrane starts growing inwards in such a fashion that the two daughter clone cells are formed with the two DNA molecules. It has been observed that, if the conditions are right, bacteria can double in number through binary fission, within a short time of about ten minutes!


This is another form of asexual reproduction in bacteria. Some types of bacteria reproduce through budding, which is otherwise known as fragmentation. In this case, the mother cell forms a bud at one end, and also makes a nucleus for the bud, through the process of mitosis. The bud grows to the same size as that of the mother cell, that remains constant in size. The bud separates from the mother, and forms a different organism. Even though the nucleus of the bud is formed through mitosis, it has been observed that sometimes the characteristics of the offspring may differ from that of the mother.

Sexual Reproduction

Though sexual reproduction is rare in bacteria, in some cases, genetic recombination is facilitated through conjugation, transformation, or transduction. One of the reasons is that bacteria formed through asexual reproduction have the same genetic material, and can get affected by the same antibiotics. So genetic recombination helps them in creating bacteria with variations in genetic material. The latter category may be resistant to the particular antibiotics, or may be adapted to the changing environment.

In case of conjugation, the genetic material will be transferred between bacteria, as one bacteria connects to the other through a tube called pilus. In case of transformation, DNA is collected from the remnants of dead bacterial cells. In this case, the bacterial cells get attached to the DNA of the dead bacteria, and this DNA is transported through the cell membrane and incorporated to the genetic material of the live bacteria. In case of transduction, genetic material is transferred through bacteriophages (viruses that attack bacterial cells). As a bacteriophages attach to the bacterial cell, it inserts its genetic material into the bacterial cell. This results in the formation of replicated bacteriophages inside the bacterial cell, which opens up to release the former. The genetic material of this host bacterial cell can attach to the DNA of any other bacterial cell, that is attacked by these new bacteriophages.

To summarize, bacteria reproduce both sexually and asexually. Asexual method of reproduction, especially binary fission, is commonly found in bacteria. Some types of bacteria may resort to budding too. Sexual reproduction, though rare, happens in bacteria in some special circumstances.

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Scientists Find Single-Celled Microorganisms in Deep, Hot Subseafloor Sediments

An international team of researchers has discovered microbial life, in particular bacterial vegetative cells, in up to 1.2-km-deep and up to 120 degrees Celsius hot sediments in the Nankai Trough subduction zone off Cape Muroto, Japan.

A microbial cell (center of the picture) detected from a sediment core sample at the depth of 1176.8 m at 120 degrees Celsius. Scale bar – 20 μm. Image credit: JAMSTEC / IODP.

“Water boils on the (Earth’s) surface at 100 degrees Celsius, and we found organisms living in sediments at 120 degrees Celsius,” said team member Dr. Arthur Spivack, a scientist in the Graduate School of Oceanography at the University of Rhode Island.

In October 2020, a team of researchers announced that microbial diversity below the seafloor is as rich as on Earth’s surface.

They discovered 40,000 different types of microorganisms from core samples from 40 sites around the globe.

Dr. Spivack and colleagues focused on the Nankai Trough off the coast of Japan, where the deep-sea scientific vessel, Chinkyu, drilled a hole 1,180 m deep to reach sediment at 120 degrees Celsius.

“Only a few scientific drilling sites have yet reached depths where temperatures in the sediments are greater than 30 degrees Celsius,” said Professor Kai-Uwe Hinrichs, a researcher in the Center for Marine and Environmental Sciences (MARUM) at the University of Bremen.

“The goal of the T-Limit Expedition, therefore, was to drill a thousand-meter deep hole into sediments with a temperature of up to 120 degrees Celsius — and we succeeded.”

“Surprisingly, the microbial population density collapsed at a temperature of only about 45 degrees Celsius,” said Dr. Fumio Inagaki, a researcher in the Research and Development Center for Ocean Drilling Science and Kochi Institute for Core Sample Research at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).

“It is fascinating — in the high-temperature ocean floor, there are broad depth intervals that are almost lifeless.”

“But then we were able to detect cells and microbial activity again in deeper, even hotter zones — up to a temperature of 120 degrees Celsius.”

While the concentration of vegetative cells decreases sharply to a level of less than 100 cells per cm 3 of sediment at over 50 degrees Celsius, the concentration of endospores increases rapidly and reaches a peak at 85 degrees Celsius.

Endospores are dormant cells of certain types of bacteria that can reactivate and switch to a live state whenever conditions are favorable again.

“Some specialist types are able to adapt to these severe conditions and persist over geological time spans in a sort of deep sleep,” Dr. Inagaki said.

“The findings of our expedition are surprising,” said Dr. Verena Heuer, a researcher of MARUM.

“They show that at the lower boundary of the biosphere lethal limits coexist with opportunities for survival. We didn’t expect that.”

“And this new understanding would not have been possible without the strong interdisciplinary team and its dedicated spirit of cooperation.”

“We found chemical evidence of the organisms’ use of organic material in the sediment that allows them to survive,” Dr. Spivack said.

“This research tells us that deep sediment is habitable in places that we did think possible.”

While this is exciting news on its own, the research could point to the possibility of life in harsh environments on other planets.

“Like the search for life in outer space, determining the limits of life on the Earth is fraught with great technological challenges,” the scientists said.

At every step of the biological hierarchy, structure and function are connected. For example, different cells have specific components that help them carry out their duties. Red blood cells, which carry oxygen, are formed differently than the white blood cells that fight infection. The relationship between structure and function is also apparent in entire organisms and the physiological systems that serve them. A cat’s long, sensitive whiskers gather information from its environment.

All organisms interact with their environment, which includes both organic and inorganic components. Material and energy flow back and forth. For instance, green plants use water, carbon dioxide and sunlight from their environment to produce their own energy through photosynthesis, but they release oxygen as a byproduct.

Acknowledgement By the Royal Society of London

Leeuwenhoek was not into writing books but he communicated with the Royal Society of London through letters. He sent to the Royal Society his various recorded microscopic observations. In 1673, his observations about stings of bees were published in the Royal Society's journal. He soon established good reputation with the Royal society through his deep analysis and careful observations.

However, in 1676, his credibility was doubted by the Royal Society when he claimed in his correspondence about the discovery of microscopic one-celled organisms. At first, the Royal Society remained skeptic towards Leeuwenhoek's findings but then he convinced the Royal Society to confirm his results. The Royal Society team tested and thoroughly approved his observations. Leeuwenhoek was extended membership by the Royal Society in 1680.


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