Microbial and Fungal Diversity

This is Unit Nine of Biology, Part Two: bacteria and archaea. Two domains, and what separates them is not shape or size. Both include spheres, rods and spirals. The structural grounds are membrane chemistry — bacteria link fatty acids to glycerol one way, archaea link branched chains another — and cell wall composition, where archaeal walls never contain peptidoglycan at all. The Gram stain sorts bacteria by how much peptidoglycan their wall holds, and that is the whole of what it tells you. It says nothing about shape, metabolism or how dangerous an organism is. And a Gram-negative cell's thinner wall is not a weaker one — it carries an extra outer membrane instead. Nutrition works on two independent axes. One pair of prefixes names the energy source, the other names the carbon source, and they combine freely into four categories. Mixing up which pair means which is the most common error here. Then the weighting correction that runs through the second half. Only a small fraction of prokaryotic species are pathogens. Most are neutral or beneficial — fixing nitrogen no eukaryote can fix, cycling carbon, and living in us. By the end you will be able to do five things. One: name what separates the two domains. Two: say what a Gram stain sorts by. Three: say why fission gives no diversity. Four: sort a prokaryote by nutrition. Five: say what antibiotics do to resistance.

Three entries. What the cell contains, the shapes it comes in — which turn out to be neither exhaustive nor diagnostic — and the membrane chemistry that is one of the real grounds for two separate domains.

Inside the cell.

Bio 1941 Prokaryotic Cell Structure, Reused and Extended

A prokaryotic cell keeps its D N A in a nucleoid and often carries extra-chromosomal loops called plasmids, which replicate independently alongside the main chromosome — a plasmid can carry an antibiotic-resistance gene. A plasmid is not required for survival; it carries supplemental genes while the chromosome carries the essential ones.

Three shapes.

Bio 1943 Three Prokaryotic Cell Shapes

Prokaryotic cells fall into three basic shapes — spherical cocci, rod-shaped bacilli and spiral spirilli — though a curved rod shows the three are not exhaustive. Shape alone does not sort prokaryotes into bacteria or archaea; both domains include all three forms.

Two chemistries.

Bio 1945 Bacteria versus Archaea: Membrane Lipid Differences

Bacterial membranes link fatty acids to glycerol by an ester bond; archaeal membranes link branched chains by an ether bond, and some archaeal membranes form a single layer rather than the bilayer bacteria and eukaryotes share. This lipid difference, rather than shape or size, is one of the structural grounds for two separate domains.

Three entries on cell walls and the stain that reads them. The stain is a century-old workhorse and its limits are sharp — it answers one question about wall structure and no others.

A stain.

Bio 1947 The Peptidoglycan Cell Wall and the Gram Stain

Bacterial walls contain peptidoglycan, and the Gram stain classifies a bacterium by whether its wall retains crystal violet — thick peptidoglycan holds the stain and turns purple, thin loses it and takes the pink counterstain. The stain sorts by wall structure and says nothing about shape, metabolism or virulence.

Two walls.

Bio 1949 Gram-Positive versus Gram-Negative Wall Structure

A Gram-positive wall is a thick peptidoglycan layer alone; a Gram-negative wall is a thin peptidoglycan layer wrapped in an outer membrane containing lipopolysaccharides, which can trigger a strong immune response. The thinner wall does not mean a weaker cell — the extra outer membrane is a separate protective layer.

A third kind.

Bio 1951 Archaeal Cell Walls

Archaeal cell walls never contain peptidoglycan, building instead from pseudopeptidoglycan, polysaccharides, glycoproteins or protein alone — pseudopeptidoglycan resembles the bacterial material but uses different sugars, resisting enzymes that break the true form down. Presence or absence of peptidoglycan, not thickness, is the defining line.

Two entries. How a prokaryotic species boundary is drawn — which turns out to be a convention rather than a natural line — and reproduction that generates no variation at all on its own.

Drawing a species.

Bio 1953 Species Delimitation: Ribosomal RNA Sequencing

Prokaryotic species are delimited by comparing ribosomal R N A gene sequences rather than by morphology, with finer markers distinguishing strains within a species. Where to draw the species line is a sequence-similarity convention, not a natural boundary in the organisms themselves.

Splitting in two.

Bio 1955 Binary Fission and Horizontal Gene Transfer, Reused

Prokaryotes reproduce asexually by binary fission, splitting into two clones, and gain new gene combinations only through horizontal transfer — the three classical routes being uptake of free D N A, viral transfer and direct contact. Binary fission alone produces no diversity; any recombination comes from transfer.

Three entries. A classification built on two independent questions, and then two global cycles prokaryotes run — one of which they run exclusively.

Four categories.

Bio 1957 The Four Prokaryotic Nutritional Categories

Prokaryotes are classified by combining energy source and carbon source into four categories — a photoautotroph uses light for energy and carbon dioxide for carbon, a chemoheterotroph uses chemical compounds for both. The auto and hetero prefixes name the carbon source, photo and chemo name the energy source, and the two pairs combine independently.

Carbon.

Bio 1959 Prokaryotes and the Carbon Cycle

Prokaryotes cycle carbon between organic and inorganic forms, acting as producers that fix carbon dioxide or decomposers that release it through respiration — marine photosynthetic bacteria removing it, decomposers returning it from dead organisms. Decomposers are essential participants rather than incidental cleanup; without them carbon stays locked in dead biomass.

Nitrogen.

Bio 1961 Prokaryotes and the Nitrogen Cycle

Prokaryotes drive nearly every step of the nitrogen cycle — fixation converting atmospheric nitrogen to ammonia, nitrification oxidising it, denitrification returning it to air, with nitrifying soil bacteria converting fertiliser into the form roots take up. No eukaryote can fix atmospheric nitrogen; every step of biological fixation on Earth depends on prokaryotes.

One entry covering two things at once — the range of conditions prokaryotes tolerate, and the communal structure most of them actually live in, which turns out to matter for treatment.

Extremes and communities.

Bio 1963 Extremophiles and Biofilms

An extremophile thrives in heat, salinity or acidity that would kill most life, and many prokaryotes live inside biofilms rather than freely suspended — thermophilic archaea in near-boiling vents, dental plaque as a biofilm in a gummy matrix. A biofilm's shared matrix makes its members more resistant to antibiotics than free-floating bacteria.

Two entries, and both correct an overreach. Disease has no single shared mechanism, only a small fraction of species cause any, and antibiotic use does something narrower to resistance than the popular account suggests.

Several mechanisms.

Bio 1965 Bacterial Disease Mechanism, Overview

A pathogenic bacterium causes disease by damaging tissue, releasing toxins, or triggering a harmful immune response — no single shared mechanism, with some species invading directly and others acting mainly through a released toxin. Only a small fraction of prokaryotes are pathogens; most species cause no disease in humans.

Selection at work.

Bio 1967 Antibiotic Resistance Mechanism

Antibiotic resistance spreads when a resistance allele — already present at low frequency or transferred in — is selected for as susceptible bacteria die off, and conjugation can move such a gene between bacteria faster than mutation alone. Antibiotic use does not create resistance genes; it selects among variants that already exist or arrive by transfer.

Five entries restoring the balance. Two mutualisms, a misconception entry and its correction, and one application. The pair in the middle is the book's usual twin structure — an overgeneralisation stated, then the picture it omits.

In root nodules.

Bio 1969 Nitrogen-Fixation Mutualism

Rhizobia bacteria fix atmospheric nitrogen inside legume root nodules, trading fixed nitrogen for photosynthetic sugars — soybean nodules housing bacteria that supply usable nitrogen while the plant supplies carbohydrates. The legume does not fix nitrogen itself; only the bacterial partner carries the enzyme machinery.

Living in us.

Bio 1971 The Human Microbiome

The human microbiome is the community of prokaryotes inhabiting the skin and digestive tract, aiding digestion, producing nutrients and helping train the immune system — gut bacteria producing vitamins the body cannot synthesise. Disrupting the community, as heavy antibiotic use can, may let a harmful species overgrow unchecked.

The overgeneralisation.

Bio 1973 "Germs Are All Bad"

Treating every prokaryote as a harmful germ overstates the case: only a small fraction of prokaryotic species are pathogens, and the vast majority of gut, soil and skin prokaryotes cause no disease at all. This states the overgeneralisation; the next entry restates the mechanisms it overlooks.

The correction.

Bio 1975 Beneficial Prokaryotes, Correctly Weighted

Most prokaryotes are neutral or beneficial: they fix nitrogen, cycle carbon, aid digestion and support the immune system — nitrogen-fixing rhizobia, gut digestive bacteria and decomposers all sustaining rather than harming the organisms around them. The germ misconception describes only the pathogenic minority; this restores the full picture.

Cleaning up.

Bio 1977 Bioremediation

Bioremediation uses a prokaryote's metabolism to break down or remove pollutants from a contaminated environment rather than removing them mechanically — oil-degrading bacteria metabolising hydrocarbons into simpler compounds as ordinary metabolism. It works within existing metabolic capability, so it depends on matching a species' diet to the pollutant.

One procedure applying the two nutritional axes in a fixed order. Energy source first, carbon source second — and the warning names exactly which prefix pair people attach to the wrong axis.

Three isolates.

Bio 1979 Classifying a Prokaryote by Its Nutritional Category

Three unknown bacteria, sorted. The first harvests energy from sunlight and builds molecules from carbon dioxide — light plus inorganic carbon makes it a photoautotroph. The second cannot use light, oxidising inorganic sulfur for energy while still fixing carbon dioxide — chemical energy plus inorganic carbon, a chemoautotroph. The third uses light for energy but absorbs organic compounds for carbon — a photoheterotroph. Energy first, carbon second sorts any prokaryote without knowing its taxonomy. Reading auto as an energy source is the common error.

Five things to carry out of Part Two. Bacteria and archaea are separated on structural grounds that have nothing to do with shape — both domains include spheres, rods and spirals. Their membranes differ in how lipids attach to glycerol, and their walls differ absolutely: bacterial walls contain peptidoglycan, and archaeal walls never do. The Gram stain sorts bacteria by whether a thick or thin peptidoglycan layer retains crystal violet, and that is all it reports. It says nothing about shape, metabolism or virulence. A Gram-negative cell's thin wall comes with an outer membrane, so thinner does not mean weaker. Binary fission produces two clones, so it generates no diversity by itself. Every new gene combination in a prokaryote arrives by horizontal transfer — taking up free D N A, viral delivery, or direct contact — which is why those three routes matter as much as reproduction does. Nutritional category combines two independent axes. Photo and chemo name where the energy comes from; auto and hetero name where the carbon comes from. Work them in that order, energy then carbon, and any prokaryote sorts into one of four categories without knowing its taxonomy. And antibiotic use does not create resistance. Resistance alleles are already present at low frequency or arrive by transfer, and the antibiotic selects for them as susceptible bacteria die off. Conjugation can spread such a gene faster than mutation would — which is selection acting, not novelty being made. Part Three turns to the protists — a group defined mostly by what it is not.