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How Does Life Work?

Symbiosis and Interliving:
Crossing the Kingdoms
Bacteria and Archaea



Introduction to Six Life Kingdoms

Human Sciences depend on communication among the community of scientists. This requires common language and agreements of categories or classifications. In the life sciences, these categories show differences among various types of organisms. This study is called Taxonomy. The lines beween the major types of organisms live in human minds, not the Biosphere. Nature seems to care little for taxonomy. In truth, living organisms show a strong tendency to live together, often for mutual benefit. This makes the scientists' categories blurry, and many of them don't like that, and struggle to accept the facts of symbiosis.

In the twentieth century, biologists slowly discovered that our categories (taxonomies) of life are not nowhere near exclusive. Functionally, the divisions between kinds of life become more and more blurred as we learn. The large categories that most biologists use are called the Six Kingdoms. The image below suggests their evolutionary relationships among the kingdoms. The rest of this page, and more pages linked below, exemplify Nature's lack of concern with human classifications, and show many symbiotic associations where the participants cross kingdoms to assist each others' lives.

Six Kingdoms of Life
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (

Bacteria and archaea, the single-cell procaryotes that were the original life on Earth, are organisms that have been around and evolving for over four billion years, a span of time we cannot really imagine. During those long eons and epochs of time, procaryotes invented several kinds of metabolism; they invented photosynthesis, which created Earth's oxygen atmosphere; and most importantly, in our eyes, procaryotes came together in symbiotic association to gradually invent the Eucaryote Cell.

The eucaryote cell gave rise to many new kinds of life we call Algae and Protozoans, currently grouped together as the Kingdom Protists.The protists in turn slowly gave rise to the rest of the Kingdoms of Life : Fungi, Animals, and Plants.

A process of Symbiosis has gifted us large organisms with billions of cells; each cell is a cooperative of bacteria that have become completely dependent on each other.

Every day, it seems, scientists and their students discover new examples of symbiosis, and most of those have partners from different kingdoms. Partners in symbiotic associations are called symbionts.

Archaea and Bacteria "invented" the rest of life, and it turns out, they always came along for the evolutionary ride. Prokaryotes are the most common symbionts, apparently part of all symbioses. The partnerships we know most about concern the vital matters of nutrition and defense. Bacteria manufacture amino acids and vitamins for many animals. The distinction between Procaryotes and the other kingdoms of life has become blurred. You could say that all life is an elaboration of bacteria and archaea.

Bacteria and Archaea Procaryotes
Cross Kingdoms to become Symbionts
The symbioses below are just a tiny sample
of these mutually beneficial associations

Gut Bacteria and All Vertebrate Animals
Prokaryote and Animals

We evolved in concert with gut bacteria and archaea; humans have over 500 kinds in our guts, a huge menangerie of tiny beings, up to 100 trillion. Each kind of procaryote has a different lifestyle and preferred intestinal location.They digest some foods for us; they create some needed vitamins and amino acids; they help us get rid of toxins and infective microbes. Without microbial symbionts, few, if any, vertebrates could live. The microbes have always been part of us, and our ancestors, and all other backboned animals.

Intestinal villi in back;greatly magnified bacteria in foreground

Bacteria and Annelid Worms

Earthworms contain dense colonies of bacteria in some 200 nephridia, their version of kidneys, which help the worms regulate their water content and influence the excretion of castings and nitrogen. The earthworm gut also contains great numbers of microbes of varying kinds.

Bacteria and Crustaceans

The saltmarsh fiddler crab hosts symbiont bacteria which supply digestive enzymes which allow the crab to digest a range of foods. All marine crustaceans studied host bacterial symbiotes in their stomachs and hindgut.

Roundworms and Bacteria

Nematode worms that parasitize insects contain symbiont bacteria that kill and digest the insect host from the inside.The worm can't eat without its microbe partners.

Vent Worms and Bacteria

Chemosynthetic bacteria in hot vent ocean ecosystems are symbionts of tube worms, clams and mussels that don't even eat--all their energy needs are supplied by sulphur- and methane-reducing bacteria. In the picture notice large clams attached to some tube worms; both are fed by bacterial symbionts.

Insects and Archaea

Several kinds of archaea are symbionts of aphids; other kinds are symbionts of cockroaches. These archaea digest nutrients that their hosts can't, and some synthesize nutrients such as vitamins that their host require.

Molluscs and Bacteria

Wood-eating "shipworms", or teredos, are bivalves, not worms. Their shells have become drill bits. Symbiont bacteria that live in their gills digest cellulose for them. In the photo, note the white shell halves; these make a powerful boring tool when rotated.
Like termites and mammal grazers, teredos could not live a day without their cellulose-digesting symbionts.

The teredo-drilled holes in wood show the precision of their borings. Some holes are apparently drilled perpendicular to each other, which is a pure mystery.

Plants and Cyanobacteria

Gunnera plants are able to thrive in nitrogen-poor soils because their symbiont cyanobacteria are photosynthetic and can also
fix nitrogen. Gunnera developed 95 million years ago, and lived in the swamps where dinosaurs browsed.

Insects and Bacteria

All insects have bacterial symbionts that provide nutrients; for example, aphids feed on plant sap, which contains few essential amino acids. Their symbiont Buchnera bacteria create the amino acids for them. An insect group that includes cicadas, spittlebugs, leafhoppers and sharpshooters, has multiple diverse symbionts, up to 33 types. Bacterial symbioses date back many millions of years in such insect groups as ants, weevils, aphids, whiteflies, tsetse flies, psyllids, and mealybugs.

This little leafhopper is less than half an inch long, but it contains millions of symbiotic microbes

Nutrition and Reproduction
Insects and Bacteria

Insects often transmit symbiont bacteria from mother to child. These bacteria help digest food, and create amino acids and vitamins their symbionts require to thrive. Stinkbugs include in their egg clutches a number of specially-evolved bacteria-filled capsules that then innoculate the larvae as they chew through their eggshells. This is called vertical transmission of a symbiote, through reproduction.


Nutrition and Defense:Protists and Bacteria

The toxic dinoflagellate protozoan Ostreopsis lenticularis hosts a variety of bacterial symbionts. These bacteria not only provide nutrients but also create the toxins that poison fish that eat the tiny dinoflaggelates. Message?
Don't eat my symbiont!

Marine Worm and Bacteri

A worm that lives in shallow ocean sands, Olavius algarvensis, has no mouth and no digestive system. It has instead an array of bacterial symbionts just under its skin that feeds it carbon compounds, all of its amino acids and vitamins, and then eats the nitrogenous waste that results. The worm migrates through sands with varying nutrient sources, each type satisfying different microbe partners---a sequential symbiosis.

Nutrition and Reproduction

Rice seedling blight, caused by the fungus Rhizopus microsporus, depends on the toxin rhizoxin, which is made by the fungus endosymbiont bacteria Burkholderia. This symbiosis is so profound that the fungus cannot reproduce without its bacteria. The fungal spores contain the symbote bacteria, ensuring the future reproduction of the fungus.

Adaptation to Locale

Dunegrass has fungal endo-symbionts that allow it to tolerate high amounts of salt. How much of the fungus an area of dunegrass contains depends on the distance from high tide lines. These fungi adapt too. Invasive plants that succeed on the beach have acquired the same fungi as the dunegrass.

Multiple Bacterial Symbionts
at Hot Vents

Several kinds of hot vent animals survive courtesy of bacteria that create food chemosynthetically using sulfur and/or methane that flows from the vents. Just recently in 2008, scientists discovered six different symbiotic bacteria functioning in hot vent mussels. Some of the six can derive food from sulfide, some from methane. Three of these bacteria are newly known, and seem to provide their mussel hosts with energy from a yet unknown source.

Giant hot vent mussels
Flashlight fish,Photoblepharon, has bright organs under its eyes filled with Vibrio fischeri bacteria that make light. The fish can draw muscle "shutters" over the light to hide. The light attracts zooplankton food.
Image courtesy
Little bobtail squid use Vibrio fischeri bacteria to make themselves invisible to predators below. Their light organ matches the light that hits the squid from above and bounces it down via an internal mirror.
Image credit M.J McFall-Ngai and E.G Ruby, U.HI

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