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Algaes, diverse group of simple, plant like organisms. Like plants, most algae use the energy of sunlight to make their own food, a process called photosynthesis However, algae lack the roots, leaves, and other structures typical of true plants. Algae are the most important photosynthesising organisms on Earth. They capture more of the sun's energy and produce more oxygen (a by-product of photosynthesis) than all plants combined. Algae form the foundation of most aquatic food webs, which support an abundance of animals.

Algae vary greatly in size and grow in many diverse habitats. Microscopic algae, called phytoplankton, float or swim in lakes and oceans. Phytoplankton are so small that 1000 individuals could fit on the head of a pin The largest forms of algae are seaweeds that stretch 100 m (300 ft) from the ocean bottom to the water's surface. Although most algae grow in fresh water or seawater, they also grow on soil, trees, and animals, and even under or inside porous rocks, such as sandstone and limestone. Algae tolerate a wide range of temperatures and can be found growing in hot springs, on snow banks, or deep within polar ice.

Algae also form mutually beneficial partnerships with other organisms For example, algae live with fungi to form lichens-plant like or branching growths that form on boulders, cliffs, and tree trunks. Algae called zooxanthellae live inside the cells of reef-building coral. In both cases, the algae provide oxygen and complex nutrients to their partner, and in return they receive protection and simple nutrients. This arrangement enables both partners to survive in conditions that they could not endure alone.

The earliest life-forms on this planet are thought to be early ancestors of cyanobacteria, a type of algae formerly called blue-green algae. Fossilized cyanobacteria have been found in rocks more than 3 billion years old. These early algae formed when there was no oxygen in the atmosphere, and scientists theorize that as the algae photosynthesised, they released oxygen as a by-product, which eventually accumulated in the atmosphere. Algae were probably the first organisms capable of photosynthesis and, until the appearance of plants on earth, the only photo synthesizers for billions of year.

LICHENS

New concrete roof tiles are too alkaline (about pH 11 or higher) for most living organisms to survive on them. It may take as long as five years for this alkalinity to drop down to a level (about pH 8) where colonisation can take place. It might take 12 years for 60% of the surface to be covered and as much as 20 years for this cover to become complete. In urban environments the higher levels of acid pollution may bring about this change more rapidly. At the same time, the number of species that can survive in this polluted situation is reduced Conversely, the neutralising effect of the very alkaline concrete reduces the acidity and it has been seen that, within four years in some urban areas, 80% of the surface of new concrete roofs were already covered with small lichens.

A roughish texture is best for colonisation as this enables air-borne dust and the small incipient lichens, algae and mosses that land on the surface to lodge there and not be immediately carried away again by the wind. On the damper northern side of a building the first plants to colonise the wall or roof are probably green algae. These may form a layer which holds the moisture (a serious problem) and dust particles enabling this algal layer to become rather thick and thus often preventing colonisation by lichens. If a soil, consisting of dust and broken down plant remains, builds up in the cracks this will enable mosses to grow. In time the soil in the cracks may well be sufficient for flowering plants. On the drier, better lit surfaces there will be fewer algae and it is the lichens that are more suited to this harsh regime of wetting and rapid drying out. Small, lichens are likely to be the first to appear, but by their great numbers they can quickly invade the whole roof area. These are normally followed by larger, grey lichens that will in time cover the complete roof. If the surface is, by some means, nutrient enriched this will encourage the large orange, leafy lichens (Xanthoria species) and also a number of grey species (eg Diploicia canescens). These orange patches will often be seen under television aerials, chimneys and overhanging branches.

The levels of air-borne sulphurous pollution have been dropping and this has enabled many species to return to areas where they have been absent for over 100 years. Indeed the appearance on concrete tiled roof in Middlesex of the grey-green lichen Lecanora muralis caused some consternation. It was not recognised as a lichen by many people and correspondence to the press suggested that it had come from outer space! As the pollution levels drop, growth of these larger, more sensitive lichens will continue to increase.

The microhabitat of the building will have a profound effect on which species will grow at a particular site. A low-pitched, north-facing roof will be colonised most readily by mosses. A steep-pitched, north-facing roof will frequently be covered in grey Physcia species as well as the mosses. A south-facing roof is better illuminated and also drier and would favour the yellow Xanthoria and other sun-loving species such as Lecanora muralis. Any shading from other buildings or trees. will also affect the range of species that colonise the site. Overhanging trees may increase the nutrient enrichment not only from bird droppings but also through pollen, dust and the "honey-dew" from aphids. Damp retained in cracks on roofs, or between tiles, will help the colonisation of mosses and lichens. If soft mortar is used in walls, this will retain the moisture for longer periods. Soft or cement mortars will, over a period, leach and make the surrounding brick-work more alkaline and therefore more suitable for the species that need an alkaline situation. Shaded damp sites will also encourage the growth of green algae.


			ALGAE and LICHENS a background Use the arrows to scroll up and down Algaes, diverse group of simple, plant like organisms. Like plants, most algae use the energy of sunlight to make their own food, a process called photosynthesis However, algae lack the roots, leaves, and other structures typical of true plants. Algae are the most important photosynthesising organisms on Earth. They capture more of the sun's energy and produce more oxygen (a by-product of photosynthesis) than all plants combined. Algae form the foundation of most aquatic food webs, which support an abundance of animals. Algae vary greatly in size and grow in many diverse habitats. Microscopic algae, called phytoplankton, float or swim in lakes and oceans. Phytoplankton are so small that 1000 individuals could fit on the head of a pin The largest forms of algae are seaweeds that stretch 100 m (300 ft) from the ocean bottom to the water's surface. Although most algae grow in fresh water or seawater, they also grow on soil, trees, and animals, and even under or inside porous rocks, such as sandstone and limestone. Algae tolerate a wide range of temperatures and can be found growing in hot springs, on snow banks, or deep within polar ice. Algae also form mutually beneficial partnerships with other organisms For example, algae live with fungi to form lichens-plant like or branching growths that form on boulders, cliffs, and tree trunks. Algae called zooxanthellae live inside the cells of reef-building coral. In both cases, the algae provide oxygen and complex nutrients to their partner, and in return they receive protection and simple nutrients. This arrangement enables both partners to survive in conditions that they could not endure alone. The earliest life-forms on this planet are thought to be early ancestors of cyanobacteria, a type of algae formerly called blue-green algae. Fossilized cyanobacteria have been found in rocks more than 3 billion years old. These early algae formed when there was no oxygen in the atmosphere, and scientists theorize that as the algae photosynthesised, they released oxygen as a by-product, which eventually accumulated in the atmosphere. Algae were probably the first organisms capable of photosynthesis and, until the appearance of plants on earth, the only photo synthesizers for billions of year. LICHENS New concrete roof tiles are too alkaline (about pH 11 or higher) for most living organisms to survive on them. It may take as long as five years for this alkalinity to drop down to a level (about pH 8) where colonisation can take place. It might take 12 years for 60% of the surface to be covered and as much as 20 years for this cover to become complete. In urban environments the higher levels of acid pollution may bring about this change more rapidly. At the same time, the number of species that can survive in this polluted situation is reduced Conversely, the neutralising effect of the very alkaline concrete reduces the acidity and it has been seen that, within four years in some urban areas, 80% of the surface of new concrete roofs were already covered with small lichens. A roughish texture is best for colonisation as this enables air-borne dust and the small incipient lichens, algae and mosses that land on the surface to lodge there and not be immediately carried away again by the wind. On the damper northern side of a building the first plants to colonise the wall or roof are probably green algae. These may form a layer which holds the moisture (a serious problem) and dust particles enabling this algal layer to become rather thick and thus often preventing colonisation by lichens. If a soil, consisting of dust and broken down plant remains, builds up in the cracks this will enable mosses to grow. In time the soil in the cracks may well be sufficient for flowering plants. On the drier, better lit surfaces there will be fewer algae and it is the lichens that are more suited to this harsh regime of wetting and rapid drying out. Small, lichens are likely to be the first to appear, but by their great numbers they can quickly invade the whole roof area. These are normally followed by larger, grey lichens that will in time cover the complete roof. If the surface is, by some means, nutrient enriched this will encourage the large orange, leafy lichens (Xanthoria species) and also a number of grey species (eg Diploicia canescens). These orange patches will often be seen under television aerials, chimneys and overhanging branches. The levels of air-borne sulphurous pollution have been dropping and this has enabled many species to return to areas where they have been absent for over 100 years. Indeed the appearance on concrete tiled roof in Middlesex of the grey-green lichen Lecanora muralis caused some consternation. It was not recognised as a lichen by many people and correspondence to the press suggested that it had come from outer space! As the pollution levels drop, growth of these larger, more sensitive lichens will continue to increase. The microhabitat of the building will have a profound effect on which species will grow at a particular site. A low-pitched, north-facing roof will be colonised most readily by mosses. A steep-pitched, north-facing roof will frequently be covered in grey Physcia species as well as the mosses. A south-facing roof is better illuminated and also drier and would favour the yellow Xanthoria and other sun-loving species such as Lecanora muralis. Any shading from other buildings or trees. will also affect the range of species that colonise the site. Overhanging trees may increase the nutrient enrichment not only from bird droppings but also through pollen, dust and the "honey-dew" from aphids. Damp retained in cracks on roofs, or between tiles, will help the colonisation of mosses and lichens. If soft mortar is used in walls, this will retain the moisture for longer periods. Soft or cement mortars will, over a period, leach and make the surrounding brick-work more alkaline and therefore more suitable for the species that need an alkaline situation. Shaded damp sites will also encourage the growth of green algae.