Biology of Australia
Notes on the Flora of Australia
Cyanobacteria (Blue-green algae)
These organisms are neither bacteria nor algae, but unlike the bacteria, and like algae, they photosynthesise. Cyanobacteria are believed to have arisen by the mutation of purple sulphur bacteria that possessed a photo-receptor, chlorophyll. The earliest signs of cyanobacteria have been found in stromatolites dating from about 3,500 Ma in Western Australia. The actual processes used by these earliest cyanobacteria are not known, though it has been assumed that they used a process of chemosynthesis or photosynthesis that didn't involve the splitting of water molecules to release oxygen. The release of oxygen into the atmosphere for the first time occurred when they adopted the process of photosynthesis that split water molecules, using the hydrogen and excreting the oxygen as a waste product.
The earliest beginnings of life on Earth are believed to have consisted of bacterial mats on the floor of shallow water, and on the surface of these mats, layers of increasingly aerobic cyanobacteria. The cyanobacteria being the only organisms that could tolerate oxygen, protecting the anaerobic bacteria below that were unable to tolerate oxygen, that was to them a toxic substance. Organisms with some level of oxygen tolerance, and able to tolerate little or no light would make up the middle layer of the mat, the lowest layers being the abode of non-photosynthesising, anaerobic organisms. The anaerobic component of these mats produced methane and the cyanobacteria produced oxygen, that began accumulating in the atmosphere once it had reached high enough levels in the water.
There are at least 10,000 species of cyanobacteria known at the present, though the full extent of their roles in the functioning of the environment is not well known, and their role is rarely acknowledged, important though it may be.
In bacteria the role of rigidifying and regulating the cell membrane is performed by bacteriohopaneplyols (BHP). BHP, the hydrocarbon skeleton of the cell membrane, is extremely resistant to degradation, being incorporated into kerogen, or bound in macromolecules that are sulphur linked. Cyanobacteria of the present, that is photosynthetic and oxygen producing, and extinct cyanobacteria both have a distinctive 2-methyl-BHP in their cell membranes. The presence of this marker in hydrocarbons from sedimentary deposits allows the determination that a particular hydrocarbon deposit was formed from these organisms. In eukaryotes it is sterols and methylated steranes that stabilise the cell membranes, so producing a different marker in hydrocarbons present in sedimentary deposits.
Based on these biomarkers, the earliest known occurrence of both oxygen-producing cyanobacteria and eukaryotes is in the kerogen-containing shales of the Mara Mamba Formation and the Roy Hill Formation, from the Pilbara Craton, Western Australia. These shales have been dated to 2.7 billion years ago. They were formed in a marine environment that is believed to be a continental slope. Records of cyanobacteria have been found that are much earlier, in stromatolites dated to 3.5 billion years ago, but it is the earliest record known of eukaryotes.
The ironstone deposited between 2.7 and 2.0 billion years were formed when the oxygen produced by cyanobacteria 'rusted' the iron dissolved in the oceans. The addition of oxygen to the atmosphere had to wait until most of the iron in the oceans had been precipitated out of the water.
At the present, cyanobacteria of many species occur in just about every environment on Earth, as single cells, colonies covered by gelatinous sheaths or filamentous. A feature that makes them unique is their ability to both photosynthesise and fix atmospheric nitrogen. As with other prokaryotes, their cells don't possess a nucleus or any other organelles. Photosynthesis is carried out in thylakoids, a membrane-bound structures in the cyanobacterial cytoplasm, either in or on the membrane.
Cyanobacteria are important components of the ecosystems in warm oceans, being able to fix nitrogen, as well as being a food source for other organisms. It has been found that a particular species of cyanobacteria, Synechococcus, has been credited with 25 %, at least, of the productivity of the oceans. Trichodesmium is another nitrogen fixing species of cyanobacteria that has been studied.
In terrestrial environments cyanobacteria are found free-living, in symbiotic relationships with plants and in soil communities and bacterial mats. see Biological Duricrust. In all these situations they are important in the cycling of nitrogen and carbon. They are the black stains on damp places such as paths. On bare sandy soil or bare friable loamy soil they can form felt-like mats. Some species can be symbiotic with the fungal portions of some lichens. Some, such as Scytonema and Calothrix, can be either free-living or symbiotic with lichens.
In crusty soils a cyanobacterium, Microcoleus vaginatus, forms clusters of finger-like filaments that are covered by a large sticky sheath. Mary White has compared their appearance to that of a tube worm. The sheath swells when wet, the filaments being forced out into the soil. The filaments produce a new sheath when the soil dries. The sheaths can eventually become so densely packed that the soil has the appearance of fibres in fibreglass. The sheath material is long lasting, surviving for many years, where it increases the organic content of the soil. It is a nutrient source for mites and termites.
Cyanobacteria in both forms, free-living and plant-symbiotic, have been found to contribute large amounts of nitrates to the soils in the arid zone of Central Australia, much of it finding its way into the groundwater.
Studies have been carried out in spinifex plains that are interspersed with mulga woodland of the Ti-Tree Basin. It was found that free-living cyanobacteria were widespread, fixing nitrogen. Common forms here were Scytionema and Ocscillatoria. Very low levels of nitrogen cycling by denitrification is believed to be the result of small amounts of organic carbon in the sandy soils. The result is the accumulation of nitrate in the soils and eventually into the groundwater.
These studies revealed just how successfully the cyanobacteria adapted to the heat, aridity and erratic climate of the central arid regions. They are capable of desiccating when there is no water available, reviving and becoming fully functional as soon as water returns in the infrequent, erratic rainfall. They are able to become the first colonisers on barren soils when conditions improve, as a result of metabolic strategies they have evolved. A novel water-stress protection mechanism has been found in Nostoc commune that has been found in these soils, that makes it drought resistant.
Cyanobacteria are very rare in the marine phytoplankton around Antarctica, but have been found in Antarctic soils, showing just how wide the temperature range is that various species of cyanobacteria can adapt to.
Allelopathic equivalents in cyanobacteria
Cyanobacteria have been found to produce many metabolites that have the effect of inhibiting oxygenic photosynthesis. Among the microbes, the ability to produce oxygen by photosynthesis is restricted to the cyanobacteria and algae. This might indicate that the metabolites are directed against possible competitors among the algae or other cyanobacteria. This is analogous to the allelopathic chemicals produced by the roots of plants to inhibit competition in the vicinity of the plant, in some cases inhibiting the germination of seeds from the same plant that fall to close to the stem.
Scytonema hofmanni, a slow-growing freshwater cyanobacterium, produces metabolites that are excreted into the environment that inhibits competitors to such an extent that it has been found that without them it could not compete successfully. Most of the metabolites of cyanobacteria that have been found to be algaecidal have been shown to act on the photosynthetic mechanism of the algae, making them natural herbicides. Electron transport is the process that is acted on in the cells they are aimed at. Some toxins inhibit the enzymes involved in phosphorus uptake. Still other metabolites attack the cell membrane, and all the processes occurring in them, of competing organisms.
|Author: M.H.Monroe firstname.lastname@example.org Sources & Further reading|