Cyanobacteria

Architects of the earliest microfossils, atmospheric oxygen, and plastids.

The Cyanobacteria

The structure of a cyanobacterium (right):
1. cytoplasmic membrane
2. cell wall - gram negative
3. capsule
4. mucoid sheath
5. paired thylakoid membranes studded with phycobilosomes
6. cyanophycin granules
7. nuclear material
8. carboxysomes (polyhedral structures that resemble bacteriophage heads. Carboxysomes comprise 5-6 proteins forming a shell around the ribulose bisphosphate carboxylase. Carboxysomes are believed useful in situations of low carbon dioxide concentration because they concentrate CO2 inside the structure, increasing the efficiency of ribulose bisphosphate carboxylase.
9. 70s ribosomes
10. cytoplasm

The cyanobacteria were formerly called "blue-green algae" because of their ecology and their resemblance to the algae. However, cyanobacteria are prokaryotes, lacking a nuclear membrane and membrane-bound organelles, though they do have internal membranes. The algae are eukaryotes, possessing both a nuclear membrane and membrane-bound organelles.

A filamentous Cyanobacterium, Lyngbya sp. (left), compared to the filamentous algae, Spirogyra (right). Click on image for larger photomicrograph.


The serial endosymbiosis theory (SET) of the prokaryotic origin of eukaryotic chloroplasts and mitochondria is widely accepted. There is little doubt that the chloroplasts of green plants are derived from Cyanobacteria.

Although the Cyanobacteria were not the first cells to evolve on the planet, the "self-fossilizing" activity of Cyanobacterial mats has left us with the oldest fossils.

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Photochemistry

Cyanobacteria contain chlorophyll a, which is also found in plants and algae. Other photosynthetic organisms contain chlorophyll b (green algae and the plants) or chlorophyll c (dinoflagellates and photosynthetic Chromista).

Chlorophylls a and b differ only in the character of a group attached to the Mg-porphyrin ring moiety of the molecule: -CH3 for chlorophyll a, and -COH for chlorophyll b. Chlorophylls are embedded in the thylakoid membrane of the Cyanobacteria. (Because Cyanobacteria gave rise, through serial endosymbiotic transfers, to chloroplasts, chlorophylls are associated with the thylakoid membrane within chloroplasts in plants and Protoctista.)

Phycobilins are water-soluble pigments, so they are found in the cytoplasm of Cyanobacteria, or in the stroma of the chloroplast. Phycolibins occur only in Cyanobacteria (phycocyanin and phycoerythrin) and the "red algae", the Rhodophyta (phycoerythrin).

The overall reaction of oxygenic photosynthesis is:

6H2O + 6CO2 → C6H12O6 + 6O2

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phycobilosomes

The Cyanobacteria possess photosynthetic thylakoid membranes studded with phycobilin-containing phycobilosomes (PBSs).

Marine Synechococcus spp. possess one of the most sophisticated antenna complexes found in the Cyanobacteria. It is proposed that, homologous with freshwater Cyanobacteria, PBSs possess six rods radiating around an allophycocyanin core, and that, at low light, rods comprise a single phycocyanin basal hexamer (disk), two phycoerythrin I disks, and three phycoerythrin II disks maintained by linker polypeptides (figure). Two new linkers have been characterized – MpeC and MpeD – that are involved in the binding of phycoerythrin (PE) disks. MpeC binds the middle PEII disk to the proximal disk, and MpeD binds the distal PEI disk to the proximal PEII disk. Phycobiliproteins APC, PC, PEI and PEII are colored pigments that enable these antenna complexes to capture and transfer photons of various energies to the reaction centers.

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Phylogenetic relationships

The Cyanobacteria are a monophyletic group within the Eubacteria, and are closely related to the purple bacteria and gram positive bacteria.

A monophyletic taxon or clade includes only and all descendents of a shared common ancestor. A monopyletic group is genetically homogeneous and reflects evolutionary relationships.

The Cyanobacteria and Archaea belong to separate lineages, having diverged from an unknown last universal common ancestor (LUCA, "?").

The Cyanobacteria are associated with the first fossils. Formerly called the "blue-green algae", they are not directly related to eukaryotic algae, though it is widely accepted that an endosymbiosis transfer involving Cyanobacteria generated the chloroplasts of photosynthetic eukaryotes.

The Cyanobacteria comprise a familiar bacterial clade, found in freshwater and aquatic environments, soils, hot-springs, Antarctica, and modern stromatolites, to name a few. Cyanobacteria are characterised by oxygenic photosynthesis with chlorophyll a.

A single genus, Gloeobacter, is recognisably basal to all others in that it lacks thylakoids. All other Cyanobacteria are placed in the clade Phycobacteria, which is characterized by chlorophyll a contained in thylakoids. Phycobacteria have traditionally been divided into five orders on the basis of morphological colony characters (below). Chloroplasts are derived from Phycobacteria, though from which subclade is still unknown.

The phylogenetics of early branching within the Life on Earth is not yet elucidated. Two alternative trees are currently considered possible alternatives (left). It is likely that the earliest cell was a heterotroph, employing environmental molecules to meet its energy requirements.

Modern extremophiles are included within the Archaea grouping.

Evolutionary relationships within the prokaryotes are blurred by the phenomenon of horizontal gene transfer, the phenomenon of cross-species genetic transfer peculiar to prokaryotes. Analyses of the tufA gene provide a broad perspective on eubacterial evolution. In conjunction with published rRNA trees, the analyses point to "at least two major radiations within eubacteria and their descendants: one of many eubacterial phyla, a second of cyanobacteria, and possibly a third radiation early in plastid evolution." Diagram

"Cyanobacterial nomenclature has been traditionally based on morphological and reproductive characteristics. It was proposed that they be separated into five Subsections, with I, II and III comprising the nonheterocystous strains and Subsection IV (12 genera) and V (6 genera), the heterocystous strains. Subsection ii and IV more recently were split into two subgroups each based on morphology, with a total of 56 genera across all sections. Another scheme classifies them into 6 orders, with 7 distinct families and 66 genera." [R]

Links: Phylogenetic Classification and the Universal Tree / Phylogenetics / BioMed Central Full text A genomic timescale for the origin of ... / Palaeos Bacteria: Bacteria / A phylogenomic approach to microbial evolution pdf /

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Fossilized cyanobacteria

The cyanobacteria were not the first cells to evolve on the planet - those were probably heterotrophic prokaryotes. However, the "self-fossilizing" activity of mats of cyanobacterial prokaryotes has left us with the oldest fossils yet found.

3.5 Ga stromatolite cyanobacterial fossils and compare with Anabaena or Hapalosiphon filament

Found within oil bubbles in quartz dated at about 1000 million (1 billion) years: remnants cell walls of cyanobacteria trapped in oil bubbles, 1 Ga

Fossilized cyanobacterial endoliths: Eohyella dichotemata of Late Protetozoic and Eohyella fossil assemblage or Eohyella campbellii ~ at 1.5 Ga, the oldest known microbial endolith compare with Hyella stella ~ similar to Eohella dichotemata of Late Proterozoic /

Compare Precambrian cyanobacterial colony in stromatolic carbonaceous chert, NWT, 2 Ga with Cyanobacterial cocci or Cocci of cyanobacteria or Coccal cyanobacteria or Entophysalis granulosa or Pleurocapsa sp. or Synechoccus

Compare cyanococci Bitter Springs chert with Chroococcus

Compare a modern cyanobacteria
with fossil thin-section of colonial chroococchalean cyanobacterium, Bitter Springs chert, Late Proterozoic

Compare a modern Lyngba sp.
with fossil thin-section of filamentous Palaeolyngbya, Bitter Springs chert, Late Proterozoic

View comparison photomicrographs and thin sections in comparison of fossil and modern cyanobacteria.

"The earliest microscopicallyrecognisable microfossils were reported from the 3.49 Ga Dresser Formation and from the 3.46 Ga Apex Chert of the Pilbara craton, Australia. Although their biogenecity has recently been questioned, whole rock carbon isotopic compositions with RuBisCO-signatures and morphological affinity of microfossils to recent cyanobacteria and bacteria, together with laser Raman identification of carbon within the fossil bodies, testify to their authenticity." Read article that examines criteria for concluding that fossilized structures are of biological origin:
Archean microfossils: a reappraisal of early life on Earth by Wladyslaw Altermann, Józef Kazmierczak.
Altermann W, Kazmierczak J. Archean microfossils: a reappraisal of early life on Earth. Res Microbiol. 2003 Nov;154(9):611-7.

Abstract: "The oldest fossils found thus far on Earth are c. 3.49- and 3.46-billion-year-old filamentous and coccoidal microbial remains in rocks of the Pilbara craton, Western Australia, and c. 3.4-billion-year-old rocks from the Barberton region, South Africa. Their biogenicity was recently questioned and they were reinterpreted as contaminants, mineral artefacts or inorganic carbon aggregates. Morphological, geochemical and isotopic data imply, however, that life was relatively widespread and advanced in the Archean, between 3.5 and 2.5 billion years ago, with metabolic pathways analogous to those of recent prokaryotic organisms, including cyanobacteria, and probably even eukaryotes at the terminal Archean." By authors: Publikationen

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Endosymbiotic Gene Transfer in Cyanobacterial Plastids

ScienceWeek: mod. "Amazingly some of the chloroplast's own genes were not simply lost from its genome but moved to the host nuclear genome. The evolutionary introduction of a transit peptide sequence into these genes resulted originally in cyanobacterial gene products ending up back in the chloroplast, carrying out their original function but under host control . . . The other bonanza from this symbiosis: the large number of genes from the original cyanobacterium that have ended up in the host nuclear genome without a role in chloroplast maintenance. These genes provided the raw genetic material for plant diversification and competition, possibly against the original cyanobacterium itself."

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Discovery Channel :: News :: 3-D Images Reveal Fossils Inside Rock

Discovery Channel :: News :: 3-D Images Reveal Fossils Inside Rock: "Paleobiologist J. William Schopf of the University of California, Los Angeles, and his colleagues have developed a technique that lets them see fossilized microorganisms and their chemical makeup. "

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The initiation of biological processes on Earth: summary of empirical evidence.

Entrez PubMed: "With the currently available geological record at band, the existence of life on this planet as from at least 3.8 Gyr ago seems so firmly established as to be virtually unassailable. Specifically, various disparate lines of evidence have merged to indicate (1) that the surface of the Archaean Earth had hosted prolific microbial ecosystems as is testified by a quasi-continuous record of microbialites ('stromatolites') and associated microfossils of prokaryotic affinity over 3.5, if not 3.8 Gyr of geological history, and (2) that the sedimentary carbon record has preserved the isotopic signature of autotrophic (notably photosynthetic) carbon fixation over the same time span. With the observed enrichment of isotopically light carbon in sedimentary organic matter largely consonant with the bias in favor of 12C during photosynthesis, the mainstream of the carbon isotope record can be best explained as geochemical manifestation of the isotope discriminating properties of the ribulose-1,5-bisphosphate (RuBP) carboxylase reaction of the Calvin cycle suggesting an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation. As a consequence, partial biological control of the geochemical carbon cycle was established already during Early Archaean times and fully operative by the time of formation of the Earth's earliest sediments."

The initiation of biological processes on Earth: summary of empirical evidence. Schidlowski M. Adv Space Res. 1992;12(4):143-56.

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Oxygenic photosynthesis by micro-organisms

ScienceWeek: "Microorganisms are important for many reasons, not the least
of which is their responsibility, direct or indirect, for the
production of nearly all of the oxygen we breathe. Oxygen is
produced during photosynthesis by a reaction that can be written
as CO(sub2) H(sub2)O --> CH(sub2)O O(sub2). Here,
'CH(sub2)O' is a geochemist's shorthand for more complex forms
of organic matter. Most photosynthesis on land is carried out by
higher plants, not microorganisms; but terrestrial
photosynthesis has little effect on atmospheric oxygen because
it is nearly balanced by the reverse processes of respiration
and decay. By contrast, marine photosynthesis is a net source of
oxygen because a small fraction (approximately 0.1%) of the
organic matter synthesized in the oceans is buried in sediments.
This small leak in the marine organic carbon cycle is
responsible for most of our atmospheric oxygen."

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