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.
of the genus Nostoc
with prominent heterocysts
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
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.
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.
all "blue-green" bacteria are blue; some common forms are red or pink
from the pigment phycoerythrin. The
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.
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
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
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
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
Herald reports that a dark red algae bloom resembling angel's hair that
"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 [
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.
discover what's causing the bloom, researchers - including Valerie Paul of the
Smithsonian Marine Station in
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 (email@example.com). Henry Del
Campo, owner of H20 Scuba in
diver Ed Tichenor reported gobs of the stuff draped
on a popular dive spot off