Cyanobacteria
Characteristics:
Though
cyanobacteria do not have a great diversity of form,
and though they are microscopic, they are rich in chemical diversity. The
autotrophic cyanobacteria
were once classified as "blue green algae" because of their superficial
resemblance to eukaryotic green algae. Although
both groups are photosynthetic, they are only distantly related: cyanobacteria lack internal organelles, a discrete nucleus
and the histone proteins associated with eukaryotic chromosomes. Like all eubacteria,
their cell walls contain peptidoglycan. Studies
of metabolic similarities and ribosomal RNA sequence suggest that cyanobacteria form a good, monophyletic
taxon. Because motile species of cyanobacteria
utilize the same mysterious gliding locomotion as the gram-negative gliding
bacteria, some microbiologists suggest that cyanobacteria
should be classified together as a subgroup of gliding bacteria.
Although they are truly prokaryotic,
cyanobacteria have an elaborate and highly organized
system of internal membranes which function in photosynthesis. Chlorophyll A
and several accessory pigments (phycoerythrin and phycocyanin) are embedded in these photosynthetic lamellae,
the analogs of the eukaryotic thylakoid
membranes. The photosynthetic pigments impart a rainbow of possible
colors; yellow, red, violet, green, deep blue and blue-green cyanobacteria are known.
Filamentous cyanobacterium
of the genus Nostoc
with prominent heterocysts
Cyanobacteria may be single-celled or colonial.
The single celled forms are coccoid and are among the most abundant phytoplankton in the
middle of tropical oceans where nutrient levels are extremely low. In addition,
the coccoid form is prevalent among
Dividing coccoid cyanobacterial cell
certain invertebrates, especially sponges,
as symbionts.
Depending upon the species and environmental conditions, colonies may be
filamentous, forming individual chains or
complex mats. Filamentous cyanobacteria can
also form symbiotic relationships with a wide variety of plant hosts including
rice, ferns, diatoms, mosses and lichens. Some filamentous colonies show the
ability to differentiate into three different cell types. Vegetative
cells are the normal, photosynthetic cells formed under favorable growing
conditions. Climate-resistant spores may form when environmental conditions become
harsh. A third type of cell, a thick-walled heterocyst, contains the enzyme nitrogenase, vital for nitrogen fixation. They are one of
very few groups of organisms that can convert inert atmospheric nitrogen into
an oxidized form, such as nitrate (NO-3) or nitrite (NO--2)
or a reduced from as in ammonium (NH3). Nitrification cannot occur,
however, in the presence of oxygen, so heterocysts
are thick walled and anaerobic. Heterocysts are shown
as the larger, thick-walled cells in the filaments shown in Nostoc above.
Cyanobacterial cells
The membrane of the thylakoids
( blue-gray in diagram ) has the
electron transport system needed for the light reactions of photosynthesis.
This includes the important reaction center pigment, chlorophyll a , attached
to its membrane-bound proteins. This reaction center harvests light of a
broader range of wavelengths thanks to other membrane-bound accessory (aka
antenna) pigments . These
pigments include zeaxanthin, B-carotene, echinenone, canthaxanthin, and myxoxanthophyll bound to membrane proteins. Attached to the
cytosol face of the thylakoid
are phycobilisomes . These particles ( blue-gray specks along thylakoid in diagram ) also serve as a light-energy antenna
for photosynthesis. Extending into the cytosol, the phycobilisomes consist of a cluster of phycobilin pigments including phycocyanin (blue) and phycoerythrin (red) attached by their phycobiliproteins .
The cytosol
between the thylakoids is also loaded with
small grains (
silver-gray
in diagram ) of cyanophycean starch . This is
the polymerized carbohydrate product of photosynthesis. It is alpha-1,4-linked glucan, somewhat like
the amylopectin fraction of starch in higher plants.
This part of plant starch is not detectible with iodine, so glycogen and cyanophycean starch similarly do not respond to iodine. The
grains are small and the failure to react to iodine makes them not visible to
light microscopy. Indeed the central region features the naked (no histone proteins), circular DNA genome in the nucleoplasm. Upon exposure to chromosome stains, stained
bodies appear in this region of the cell ( lavender in diagram ). These
have been interpreted as "chromosomes." Indeed cyanobacteria
are host to a range of small, circular DNA molecules called plasmids . As with
any DNA, transcription and translation is expected and achieved in a living
cell. For cyanobacteria the process appears to be
identical to that in other Eubacteria. The
translation process is accomplished by the assistance of abundant 70S ribosomes in the centroplasm ( brown in diagram ). These
are smaller than their counterparts in the eukaryotic
cytoplasm, but are typical of all prokaryotes, mitochondria, and chloroplasts.
Another
interesting protein that accumulates in sufficient abundance to form a visible
structure is called cyanophycin. The 500 nm cyanophycin granules ( green in diagram ) are
large enough to be observed sometimes with light microscopy! This protein is a
polymer composed of just two amino acids: arginine
and asparagine. These granules are considered to be
an adaptation for accumulating and sequestering nitrogen for future use. They
explain why these organisms can often grow nicely in areas with
nitrogen-depleted waters. Some of the proteins translated by these ribosomes are of interest. Some of course are the carbon
fixation enzyme, Rubisco, produced in sufficient
abundance to form the polyhedral bodies
( yellow in diagram ). Of
course other soluble enzymes of carbon fixation and membrane proteins of
electron transport are translated by these ribosomes
in the cytoplasm.
Cyanobacteria are among the easiest microfossils
to recognize. They are larger than other bacteria, and morphologies in
the group have remained much the same for billions of years.
Trichodesmium -Not
all "blue-green" bacteria are blue; some common forms are red or pink
from the pigment phycoerythrin. The
Habitats:
Cyanobacteria are found in almost every conceivable habitat, from oceans to fresh water to bare rock to soil. Cyanobacteria produce the compounds responsible for "earthy" odors we detect in soil and some bodies of water (such as those being cyanobacterially cleaned at water treatment plants). The greenish slime on the side of your damp flower pot, the wall of your house or the trunk of that big tree is more likely to be cyanobacteria than anything else. Cyanobacteria have even been found on the fur of polar bears, to which they impart a greenish tinge. In short, Cyanobacteria have no one habitat because you can find them almost anywhere in the world.
Cyanobacteria are able to reproduce through a
variety of methods: binary fission, budding, or fragmentation. These
forms of reproduction explain to a great extent the various appearances that Cyanobacteria take on: patches, slimy masses, strings,
filaments, and branched filaments, for example.
Binary
fission is reproduction that involves merely duplicating the DNA and dividing
in half. It does not involve a complex cell cycle like that shown by eukaryotic cells. Budding, or cell fission, involves the
formation of smaller cells from larger ones.
Fragmentation
involves breaking into fragments, each of which then regenerates into a
complete organism.
History:
Cyanobacteria - the earliest reef builders (>3.5 billion years ago), are still around at the present time. During the Precambrian (>80% of geological time), the only reef-building organisms were mat-forming cyanobacteria.
Cyanobacterial mats trap fine-grained
carbonate sediment. Each filament forms an external
gelatinous sheath containing sticky organic matter (mucillage),
to which sediment sticks. When the mat becomes covered in sediment, it grows
another layer of filaments through and over the sediment, so it can continue to
photosynthesize.
Then another layer of sediment is trapped, building
up in layers which are alternately organic-rich and
sediment-rich.
This combination of cyanobacterial
mat and sediment is a stromatolite. Microbial mats allow fine-grained sediments to be deposited in
environments whose energy would normally be too high for such deposition.
Stromatolite close-up with characteristic banded
structure
Stromatolite reef Lower Jurassic,
southern France
Modern stromatolites are found mainly in warm climates and very shallow water. They
occur in shallow subtidal, intertidal
environments, and may extend into the supratidal
environment where the climate is humid. They also occur in freshwater.
Stromatolites show a variety of forms, related
to environmental conditions, such as energy level and rate of sedimentation.
This is a simple example of more or less parallel-laminated stromatolites.
More complex forms develop when energy and
sedimentation rate are high.
LIVING STROMATOLITE REEFS
Modern
stromatolite reefs are very uncommon, but there are
two examples:
They look like rocky lumps strewn around the
beach but are actually built by living cyanobacteria that use sediment and
organic material to build stromatolites up to 1.5
meters high. Because they grow very slowly, a meter-high stromatolite
could be millions of years old. When the
conditions from the microbial structures
(modern or fossilized) which thrive in a given location. Increasingly our
observations suggest that the activities and locations of various cyanobacterial species also contribute greatly to the
localization of new mineral
precipitation through a variety of processes, including mineral deposited by
photosynthesis as well as trapping of sediment. The process of lithification is not completely understood, but appears to
involve several cyanobacterial genera with large
filament sizes. The dominant one was Schizothrix sp.as
well as unicellular cyanobacteria. Studies have shown
that a complex succession occurs with these cyanaobacteria
so that as the laminations of stromatolites are not
produced in a simple and regular manner.
Schizothrix filaments with CaCO3 deposited outside the mucilaginous sheath
From: mgg.rsmas.miami.edu/.../STROMATOLITE/ autoR3.HTML
Cyanobacterial filaments (arrow) and coccoid cyanos bind as well as
precipitate sediments on surface
of living stromatolite.
Scale
= 100 µm
http://www.home.duq.edu/~stolz/
RIBS/lab/rolecicling/role.html
OTHER CYANOBACTERIA AND THEIR ROLES ON REEFS
Cyanobacterial mats are
common in many environments. The mats themselves are not just composed of cyanobacteria, but are complex communities that co-exist
with other bacteria, both photosynthetic and non-photosynthetic. The consortium
of organisms that constitutes the mats represent an important micro-community. They not only produce
stromatolites, but because the cyanobacteria
are often motile they can move across surfaces including those of living
corals. Black band disease is the
result of one such mat community composed primarily of filaments of the genus Phormidium. It was once thought that it was the cyanobacteria
that causes black band disease, but it is more likely the associated
bacteria, especially the sulfate reducers that form toxic hydrogen sulfide (H2S).
Read the pdf
on black band disease to get an idea of the scope of this disease on coral
reefs today. Pay particular attention to sections 7-10 and the photos at the
end.
Fluorescence microscopy deep
in cyanobacterial mat TEM
of cyanobacterial
mat with showing
sulfate reducing bacteria
mucilage sheaths
(yellow). in sheath. Scale = 5 µm
Scale = 0.1 mm
www.mbl.ku.dk/mkuhl/pages/PDF/Fenchel&Kuhl2000.pdf
BLACK BAND DISEASE OF CORALS IS GLOBAL IN SCOPE
Red filamentous cyanobacteria
are also known to grow in different form on sea whips and sea fans smothering
them, as in this outbreak off
Scientists worried about continued winter spread of nutrient
pollution-fueled cyanobacterial algae bloom that is
killing soft corals such as sea whips and sea fans in southeast
The Miami
Herald reports that a dark red algae bloom resembling angel's hair that
covered some
"It usually eases off in the
winter, but it's not doing that," said Ken Banks, manager of marine
resources programs for Broward's Department of Planning and Environmental
Protection. "It got cropped down by the weather, but it didn't go away. We
went diving [February 2], and there was a rope [
After
testing large samples collected by Broward divers in September 2003, the
filamentous algae was been identified by
A proliferation of cyanobacteria
is a sign of environmental deterioration because the algae
feeds on nutrients in the water. However, Banks said, no one has been
able to pinpoint the source of the high nutrient levels. Many factors can
contribute to nutrient loads, such as deep-ocean upwellings,
improperly treated sewage, urban and agricultural run-off, and contaminated
groundwater bubbling up from the ocean floor.
To
discover what's causing the bloom, researchers - including Valerie Paul of the
Smithsonian Marine Station in
Banks
is drafting a map depicting areas where the cyanobacteria
growth is heaviest and where there is none. He is asking divers who spot heavy
concentrations to e-mail the GPS coordinates (kbanks@broward.org). Henry Del
Campo, owner of H20 Scuba in
Recreational
diver Ed Tichenor reported gobs of the stuff draped
on a popular dive spot off