Cyanobacteria , also known as Cyanophyta,
is a phylum of bacteria that obtain their
energy through photosynthesis. The name "cyanobacteria"
comes from the color of the bacteria = blue).
They are often called blue-green algae, but
some consider that name a misnomer as cyanobacteria
are prokaryotic and algae should be eukaryotic,
although other definitions of algae encompass
prokaryotic organisms.
By producing oxygen as a gas as a by-product
of photosynthesis, cyanobacteria are thought
to have converted the early reducing atmosphere
into an oxidizing one, which dramatically
changed the composition of life forms on Earth
by stimulating biodiversity and leading to
the near-extinction of oxygen-intolerant organisms.
According to endosymbiotic theory, the chloroplasts
found in plants and eukaryotic algae evolved
from cyanobacterial ancestors via endosymbiosis.
Ecology
Cyanobacteria can be found in almost every
terrestrial and aquatic habitat—oceans,
fresh water, damp soil, temporarily moistened
rocks in deserts, bare rock and soil, and
even Antarctic rocks. They can occur as planktonic
cells or form phototrophic biofilms. They
are found in almost every endolithic ecosystem.
A few are endosymbionts in lichens, plants,
various protists, or sponges and provide energy
for the host. Some live in the fur of sloths,
providing a form of camouflage.
Aquatic cyanobacteria are known for their
extensive and highly visible blooms that can
form in both freshwater and marine environments.
The blooms can have the appearance of blue-green
paint or scum. These blooms can be toxic,
and frequently lead to the closure of recreational
waters when spotted. Marine bacteriophages
are significant parasites of unicellular marine
cyanobacteria.
Characteristics
Cyanobacteria are a photosynthetic nitrogen
fixing group that survive in wide variety
of habitat, soil and water. In this group
photosynthetic pigments are cyanophycin, allo-phycocyanine
and erythro-phycocyanine. Their thallus varies
from unicellular to filamentous, filamentous
heterocystous. They fix atmospheric nitrogen
in aerobic condition by heterocyst, specialized
cell, and in anaerobic condition.
Nitrogen fixation
Cyanobacteria include unicellular and colonial
species. Colonies may form filaments, sheets
or even hollow balls. Some filamentous colonies
show the ability to differentiate into several
different cell types: vegetative cells, the
normal, photosynthetic cells that are formed
under favorable growing conditions; akinetes,
the climate-resistant spores that may form
when environmental conditions become harsh;
and thick-walled heterocysts, which contain
the enzyme nitrogenase, vital for nitrogen
fixation. Heterocysts may also form under
the appropriate environmental conditions when
fixed nitrogen is scarce. Heterocyst-forming
species are specialized for nitrogen fixation
and are able to fix nitrogen gas into ammonia,
nitrites (NO−
2) or nitrates (NO−
3), which can be absorbed by plants and converted
to protein and nucleic acids.
Rice plantations utilize healthy populations
of nitrogen-fixing cyanobacteria for use as
rice paddy fertilizer.
Cyanobacteria are arguably the most successful
group of microorganisms on earth. They are
the most genetically diverse; they occupy
a broad range of habitats across all latitudes,
widespread in freshwater, marine and terrestrial
ecosystems, and they are found in the most
extreme niches such as hot springs, salt works,
and hypersaline bays. Photoautotrophic, oxygen-producing
cyanobacteria created the conditions in the
planet's early atmosphere that directed the
evolution of aerobic metabolism and eukaryotic
photosynthesis. Cyanobacteria fulfill vital
ecological functions in the world's oceans,
being important contributors to global carbon
and nitrogen budgets.
– Stewart and Falconer
Ecology
Many cyanobacteria form motile filaments of
cells, called hormogonia, that travel away
from the main biomass to bud and form new
colonies elsewhere. The cells in a hormogonium
are often thinner than in the vegetative state,
and the cells on either end of the motile
chain may be tapered. In order to break away
from the parent colony, a hormogonium often
must tear apart a weaker cell in a filament,
called a necridium.
Each individual cell of a cyanobacterium typically
has a thick, gelatinous cell wall. They lack
flagella, but hormogonia of some species can
move about by gliding along surfaces. Many
of the multi-cellular filamentous forms of
Oscillatoria are capable of a waving motion;
the filament oscillates back and forth. In
water columns some cyanobacteria float by
forming gas vesicles, as in archaea. These
vesicles are not organelles as such. They
are not bounded by lipid membranes but by
a protein sheath.
Some of these organisms contribute significantly
to global ecology and the oxygen cycle. The
tiny marine cyanobacterium Prochlorococcus
was discovered in 1986 and accounts for more
than half of the photosynthesis of the open
ocean. Many cyanobacteria even display the
circadian rhythms that were once thought to
exist only in eukaryotic cells.
Photosynthesis
Carbon fixation
Cyanobacteria use the energy of sunlight to
drive photosynthesis, a process where the
energy of light is used to split water molecules
into oxygen, protons, and electrons. While
most of the high-energy electrons derived
from water are used by the cyanobacterial
cells for their own needs, a fraction of these
electrons are donated to the external environment
via electrogenic activity. Cyanobacterial
electrogenic activity is an important microbiological
conduit of solar energy into the biosphere.
Metabolism and organelles
As with any prokaryotic organism, cyanobacteria
do not have nuclei or an internal membrane
system. However, many species of cyanobacteria
have folds on their external membranes that
function in photosynthesis. Cyanobacteria
get their colour from the bluish pigment phycocyanin,
which they use to capture light for photosynthesis.
In general, photosynthesis in cyanobacteria
uses water as an electron donor and produces
oxygen as a by-product, though some may also
use hydrogen sulfide a process which occurs
among other photosynthetic bacteria such as
the purple sulfur bacteria. Carbon dioxide
is reduced to form carbohydrates via the Calvin
cycle. In most forms, the photosynthetic machinery
is embedded into folds of the cell membrane,
called thylakoids. The large amounts of oxygen
in the atmosphere are considered to have been
first created by the activities of ancient
cyanobacteria. They are often found as symbionts
with a number of other groups of organisms
such as fungi, corals, pteridophytes, angiosperms
etc.
Many cyanobacteria are able to reduce nitrogen
and carbon dioxide under aerobic conditions,
a fact that may be responsible for their evolutionary
and ecological success. The water-oxidizing
photosynthesis is accomplished by coupling
the activity of photosystem II and I. In anaerobic
conditions, they are also able to use only
PS I—cyclic photophosphorylation—with
electron donors other than water just like
purple photosynthetic bacteria. Furthermore,
they share an archaeal property, the ability
to reduce elemental sulfur by anaerobic respiration
in the dark. Their photosynthetic electron
transport shares the same compartment as the
components of respiratory electron transport.
Their plasma membrane contains only components
of the respiratory chain, while the thylakoid
membrane hosts an interlinked respiratory
and photosynthetic electron transport chain.
The terminal oxidases in the thylakoid membrane
respiratory/photosynthetic electron transport
chain are essential for survival to rapid
light changes, although not for dark maintenance
under conditions where cells are not light
stressed.
Attached to thylakoid membrane, phycobilisomes
act as light harvesting antennae for the photosystems.
The phycobilisome components are responsible
for the blue-green pigmentation of most cyanobacteria.
The variations on this theme are due mainly
to carotenoids and phycoerythrins that give
the cells the red-brownish coloration. In
some cyanobacteria, the color of light influences
the composition of phycobilisomes. In green
light, the cells accumulate more phycoerythrin,
whereas in red light they produce more phycocyanin.
Thus the bacteria appear green in red light
and red in green light. This process is known
as complementary chromatic adaptation and
is a way for the cells to maximize the use
of available light for photosynthesis.
A few genera, however, lack phycobilisomes
and have chlorophyll b instead. These were
originally grouped together as the prochlorophytes
or chloroxybacteria, but appear to have developed
in several different lines of cyanobacteria.
For this reason they are now considered as
part of the cyanobacterial group.
Relationship to chloroplasts
Chloroplasts found in eukaryotes likely evolved
from an endosymbiotic relation with cyanobacteria.
This endosymbiotic theory is supported by
various structural and genetic similarities.
Primary chloroplasts are found among the "true
plants" or green plants – species ranging
from sea lettuce to evergreens and flowers
that contain chlorophyll b – as well as
among the red algae and glaucophytes, marine
species that contain phycobilins. It now appears
that these chloroplasts probably had a single
origin, in an ancestor of the clade called
Archaeplastida. Other algae likely took their
chloroplasts from these forms by secondary
endosymbiosis or ingestion.
Earth history
Classification
Historically, bacteria were first classified
as plants constituting the class Schizomycetes,
which along with the Schizophyceae formed
the phylum Schizophyta. then in the phylum
Monera in the kingdom Protista by Haeckel
in 1866, comprising Protogens, Protamaeba,
Vampyrella, Protomonae and Vibrio, but not
Nostoc and other cyanobacteria, which were
classified with algae later reclassified as
the Prokaryotes by Chatton.
The cyanobacteria were traditionally classified
by morphology into five sections, referred
to by the numerals I-V. The first three – Chroococcales,
Pleurocapsales, and Oscillatoriales – are
not supported by phylogenetic studies. However,
the latter two – Nostocales and Stigonematales
– are monophyletic, and make up the heterocystous
cyanobacteria. The members of Chroococales
are unicellular and usually aggregate in colonies.
The classic taxonomic criterion has been the
cell morphology and the plane of cell division.
In Pleurocapsales, the cells have the ability
to form internal spores. The rest of the sections
include filamentous species. In Oscillatoriales,
the cells are uniseriately arranged and do
not form specialized cells. In Nostocales
and Stigonematales the cells have the ability
to develop heterocysts in certain conditions.
Stigonematales, unlike Nostocales, includes
species with truly branched trichomes. Most
taxa included in the phylum or division Cyanobacteria
have not yet been validly published under
the Bacteriological Code. Except:
The classes Chroobacteria, Hormogoneae and
Gloeobacteria
The orders Chroococcales, Gloeobacterales,
Nostocales, Oscillatoriales, Pleurocapsales
and Stigonematales
The families Prochloraceae and Prochlorotrichaceae
The genera Halospirulina, Planktothricoides,
Prochlorococcus, Prochloron, Prochlorothrix.
Biotechnology and applications
The unicellular cyanobacterium Synechocystis
sp. PCC6803 was the third prokaryote and first
photosynthetic organism whose genome was completely
sequenced. It continues to be an important
model organism. The smallest genomes have
been found in Prochlorococcus spp. and the
largest in Nostoc punctiforme. Those of Calothrix
spp. are estimated at 12–15 Mb, as large
as yeast.
Recent research has suggested the potential
application of cyanobacteria to the generation
of renewable energy via converting sunlight
into electricity. Internal photosynthetic
pathways can be coupled to chemical mediators
that transfer electrons to external electrodes.
Currently efforts are underway to commercialize
algae-based fuels such as diesel, gasoline
and jet fuel.
Researchers from a company called Algenol
have cultured genetically modified cyanobacteria
in sea water inside a clear plastic enclosure
so that they first make sugar from CO2 and
the water via photosynthesis. Then, the bacteria
secrete ethanol from the cell into the salt
water. As the day progresses, and the solar
radiation intensifies, ethanol concentrations
build up and the ethanol itself evaporates
onto the roof of the enclosure. As the sun
recedes, evaporated ethanol and water condenses
into droplets, which run along the plastic
walls and into ethanol collectors, from where
it is extracted from the enclosure with the
water and ethanol separated outside the enclosure.
As of March 2013, Algenol was claiming to
have tested its technology in Florida and
to have achieved yields of 9,000 US gallons
per acre per year. This could potentially
meet US demands for ethanol in gasoline in
2025, assuming a B30 blend, from an area of
around half the size of California’s San
Bernardino County, requiring less than one
tenth of the area than ethanol from other
biomass, such as corn, and only very limited
amounts of fresh water.
Cyanobacteria may possess the ability to produce
substances that could one day serve as anti-inflammatory
agents and combat bacterial infections in
humans.
Spirulina's extracted blue color is used as
a natural food coloring in gum and candy.
Health risks
Cyanobacteria can produce neurotoxins, cytotoxins,
endotoxins and hepatotoxins, and are called
cyanotoxins.
Specific toxins include, anatoxin-a, anatoxin-as,
aplysiatoxin, cyanopeptolin, cylindrospermopsin,
domoic acid, nodularin R, neosaxitoxin and
saxitoxin. Cyanobacteria reproduce explosively
under certain conditions. This results in
algal blooms, which can become harmful to
other species, and pose a danger to humans
and animals, if the cyanobacteria involved
produce toxins. Several cases of human poisoning
have been documented but a lack of knowledge
prevents an accurate assessment of the risks.
Recent studies suggest that significant exposure
to high levels of some species of cyanobacteria
producing toxins such as BMAA can cause amyotrophic
lateral sclerosis. The Lake Mascoma ALS cluster
and Gulf War veteran's cluster are two notable
examples.
Dietary supplementation
Some cyanobacteria are sold as food, notably
Aphanizomenon flos-aquae and Arthrospira platensis.
Microalgae contain substances of high biological
value, such as polyunsaturated fatty acids,
amino acids, pigments, antioxidants, vitamins
and minerals. Edible blue-green algae reduce
the production of pro-inflammatory cytokines
by inhibiting NF-κB pathway in macrophages
and splenocytes. Consumption of edible blue
green algae may also reduce risks of cataracts
and age related macular degeneration. It has
also shown mitigative effects in animal models
of non-alcohol related liver disease, such
as steatohepatitis, and Parkinson's disease.
Sulfate polysaccharides exhibit immunomodulatory,
antitumor, antithrombotic, anticoagulant,
anti-mutagenic, anti-inflammatory, antimicrobial,
and even antiviral activity against HIV, herpes,
and hepatitis. One pilot study concluded that
Spirulina supplements may reduce insulin resistance
caused by anti-HIV medications. Some studies
have found that Spirulina mitigates aflatoxin
and cisplatin chemotherapy induced liver damage
in rodents. These positive health benefits
must be distinguished from non-edible species
of algae, which are detrimental to health.
See also
Anatoxin
Archean Eon of Earth's prehistory
Bacterial phyla, the other major lineages
of domain Bacteria
Biofertilizer
Cyanobiont
Geological history of oxygen
The Great Oxygenation Event
Green algae
Hypolith
Microbial mats
Microalgae
Phoslock
Phytoplankton
Proterozoic Eon of Earth's prehistory
Sippewissett Microbial Mat
Stromatolite
References
Further reading
Gillian Cribbs, Nature's Superfood: the Blue-Green
Algae Revolution, Newleaf, ISBN 0-7522-0569-2.
Marshall Savage, The Millennial Project: Colonizing
the Galaxy in Eight Easy Steps, Little, Brown,
ISBN 0-316-77163-5.
Fogg, G.E., Stewart, W.D.P., Fay, P. and Walsby,
A.E., The Blue-green Algae, Academic Press,
London and New York, ISBN 0-12-261650-2.
"Architects of the earth's atmosphere", Introduction
to the Cyanobacteria, University of California,
Berkeley, 3 February 2006.
Whitton, B. A., Phylum Cyanophyta, in The
Freshwater Algal Flora of the British Isles,
Cambridge, Cambridge University Press, ISBN
0-521-77051-3.
Pentecost A., Franke U.. "Photosynthesis and
calcification of the stromatolitic freshwater
cyanobacterium Rivularia". Eur. J. Phycol
45: 345–353. doi:10.1080/09670262.2010.492914. .
Whitton, B. A. and Potts, M., The Ecology
of Cyanobacteria: their Diversity in Time
and Space, Springer, ISBN 0-7923-4735-8.
"From Micro-Algae to Blue Oil", ParisTech
Review, December 2011.
External links
What Are Cyanobacteria And What Are Its Types?
Overview of cyanobacteria
Webserver for Cyanobacteria Research
CyanoBase
Growth Model for the Blue-Green Alga Anabaena
catenula Wolfram Demonstrations Project—requires
CDF player
Diving an Antarctic Time Capsule Filled With
Primordial Life
 This article incorporates text from this
source, which is licensed under CC-BY 2.5.
