Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Phylogeny

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other animals for vision. Bacteriorhodopsin and halorhodopsin

are embedded in the cell membranes of halobacteria and each

pigment consists of retinal, a vitamin-A derivative, bound to a

protein. Irradiation of these pigments causes a structural

change in their retinal. This is referred to as photoisomeriza-

tion. Retinal photoisomerization leads to the synthesis of ATP.

Halobacteria have two additional rhodopsins, sensory

rhodopsin-I and sensory rhodopsin-II. These compounds regu-

late phototaxis, the directional movement in response to light.

See alsoEvolutionary origin of bacteria and viruses

PPhotosynthetic microorganismsHOTOSYNTHETIC MICROORGANISMS

Life first evolved in the primordial oceans of Earth approxi-

mately four billion years ago. The first life forms were prokary-

otes, or non-nucleated unicellular organisms, which divided in

two domains, the Bacteriaand Archaea. They lived around hot

sulfurous geological and volcanic vents on the ocean floor,

forming distinct biofilms, organized in multilayered symbiotic

communities, known as microbial mats. Fossil evidence sug-

gests that these first communities were not photosynthetic, i.e.,

did not use the energy of light to convert carbon dioxide and

water into glucose, releasing oxygen in the process. About 3.7

billions years ago, anoxygenic photosynthetic microorganisms

probably appeared on top of pre-photosynthetic biofilms

formed by bacterial and Archaean sulphate-processers.

Anoxygenic photosynthesizers use electrons donated by sul-

phur, hydrogen sulfide, hydrogen, and a variety of organic

chemicals released by other bacteria and Archaea. This ances-

tor species, known as protochlorophylls, did not synthesized

chlorophylland did not release oxygen during photosynthesis.

Moreover, in that deep-water environment, they probably used

infrared thermo taxis rather than sunlight as a source of energy.

Protochlorophylls are assumed to be the common

ancestors of two evolutionary branches of oxygenic photo-

synthetic organisms that began evolving around 2.8 billion

years ago: the bacteriochlorophyll and the chlorophylls.

Bacteriochlorophyll gave origin to chloroflexus, sulfur green

bacteria, sulfur purple bacteria, non-sulfur purple bacteria,

and finally to oxygen-respiring bacteria. Chlorophylls origi-

nated Cyanobacteria, from which chloroplasts such as red

algae, cryptomonads, dinoflagellates, crysophytes, brown

algae, euglenoids, and finally green plants evolved. The first

convincing paleontological evidence of eukaryotic microfos-

sils (chloroplasts) was dated 1.5 at billion years old. In oxy-

genic photosynthesis, electrons are donated by water

molecules and the energy source is the visible spectrum of

visible light. However, the chemical elements utilized by

oxygenic photosynthetic organisms to capture electrons

divide them in two families, the Photosystem I Family and the

Photosystem II Family. Photosystem II organisms, such as

Chloroflexus aurantiacus(an ancient green bacterium) and

sulfur purple bacteria, use pigments and quinones as electron

acceptors, whereas member of the Photosystem I Family,

such as green sulfur bacteria, Cyanobacteria, and chloroplasts

use iron-sulphur centers as electron acceptors.

It is generally accepted that the evolutionof oxygenic

photosynthetic microorganisms was a crucial step for the

increase of atmospheric oxygen levels and the subsequent

burst of biological evolution of new aerobic species. About 3.5

billion years ago, the planet atmosphere was poor in oxygen

and abundant in carbon dioxide and sulfuric gases, due to

intense volcanic activity. This atmosphere favored the evolu-

tion of chemotrophic Bacteria and Archaea. As the populations

of oxygenic photosynthetic microorganisms gradually

expanded, they started increasing the atmospheric oxygen

level two billion years ago, stabilizing it at its present level of

20% about 1.5 billion years ago, and additionally, reduced the

carbon dioxide levels in the process. Microbial photosynthetic

activity increased the planetary biological productivity by a

factor of 100–1,000, opening new pathways of biological evo-

lution and leading to biogeochemical changes that allowed life

to evolve and colonize new environmental niches. The new

atmospheric and biogeochemical conditions created by photo-

synthetic microorganisms allowed the subsequent appearance

of plants about 1.2 billion years ago, and 600 million years

later, the evolution of the first vertebrates, followed 70 million

years later by the Cambrian burst of biological diversity.

See alsoAerobes; Autotrophic bacteria; Biofilm formation

and dynamic behavior; Biogeochemical cycles; Carbon cycle

in microorganisms; Chemoautotrophic and chemolithotrophic

bacteria; Electron transport system; Evolutionary origin of

bacteria and viruses; Fossilization of bacteria; Hydrothermal

vents; Plankton and planktonic bacteria; Sulfur cycle in

microorganisms

PPhylogenyHYLOGENY

Phylogeny is the inferred evolutionary history of a group of

organisms (including microorganisms). Paleontologists are

interested in understanding life through time, not just at one

time in the past or present, but over long periods of past time.

Before they can attempt to reconstruct the forms, functions,

and lives of once-living organisms, paleontologists have to

place these organisms in context. The relationships of those

organisms to each other are based on the ways they have

branched out, or diverged, from a common ancestor. A phy-

logeny is usually represented as a phylogenetic tree or clado-

gram, which are like genealogies of species.

Phylogenetics, the science of phylogeny, is one part of

the larger field of systematics, which also includes taxonomy.

Taxonomy is the science of naming and classifying the diver-

sity of organisms. Not only is phylogeny important for under-

standing paleontology (study of fossils), however,

paleontology in turn contributes to phylogeny. Many groups of

organisms are now extinct, and without their fossils we would

not have as clear a picture of how modern life is interrelated.

There is an amazing diversity of life, both living and

extinct. For scientists to communicate with each other about

these many organisms, there must also be a classification of

these organisms into groups. Ideally, the classification should

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