Phototroph

Phototrophy or Fototrophie ( from Ancient Greek φῶς phos = light τροφή trophé = food ) refers to the use of light as a source of energy by living organisms. The light is used to synthesize the energy-rich chemical compound adenosine triphosphate ( ATP) as an energy carrier and short-term energy storage. This ATP synthesis creatures convert the light energy into chemical energy.

Only certain organisms can use light energy for their metabolism directly. They are called phototrophic organisms or Phototrophic. Phototrophy is widespread both among prokaryotes ( organisms with cells without a nucleus ) and from eukaryotes ( organisms with nucleated cells ). The phototrophic prokaryotes are metabolically physiologically divided into two groups. The first group operates photosynthesis. They use light energy using chlorophyll pigments ( chlorophylls or bacteriochlorophylls ). The second group does not use chlorophyll pigments. It operates accordingly, no photosynthesis. Such prokaryotes use instead of light energy with the help of different type rhodopsin pigments bacteriorhodopsin, proteorhodopsin or Xanthorhodopsin. The phototrophic eukaryotes show similar metabolic physiological diversity. All phototrophic eukaryotes rely for their phototrophy on chlorophyll pigments. Is, using sämtlichst photosynthesis.

• 2.1 Phototrophic prokaryotes
• Phototrophic 2.2 Eukaryotes 2.2.1 algae and land plants
• 2.2.2 Phototrophic protists without chloroplasts
• 2.2.3 Phototrophic Opisthokonta

Forms of phototrophy

To perform phototrophy, organisms require specific dyes ( pigments). These special pigments sit in biomembranes. There they absorb light and make the radiation energy contained in it usable. In phototrophic organisms so far two different classes of such pigments were detected: chlorophylls ( chlorophylls, bacteriochlorophylls ) and rhodopsins ( bacteriorhodopsin, proteorhodopsin, Xanthorhodopsin ).

Prototrophy with chlorophylls: light dependent reaction of photosynthesis

During photosynthesis, chlorophyll is displaced (or bacteriochlorophyll ) by light energy from its chemical ground state to a high-energy ( " excited ") state. In the excited state of a chlorophyll molecule are slightly from a high-energy electron.

The electron is about some of the molecules passed away ( electron transport chain ) also sit in the biomembrane. In the course of the electron transport hydrogen nuclei (protons, H ) to be scooped from one side to the other of the biomembrane. Therefore their concentration increases on the one side of the membrane to the same time on the other side from. In this way, a high H concentration gradient between the two sides of the membrane is formed ( proton gradient ).

The proton gradient is used to build ATP: Set into the biomembrane is the ATP synthase. This is an enzyme that catalyzes the synthesis of ATP from adenosine diphosphate (ADP ) and phosphate (Pi). Within the ATP synthase extends a passage which connects both sides of the membrane. Through the channel protons to flow along its concentration gradient (→ diffusion). The kinetic energy of flowing through the hydrogen nuclei is used by the ATP synthase for ATP synthesis, so converted into chemical energy (→ Chemiosmose ).

Light energy is used to establish a proton gradient. The proton gradient is used to synthesize ATP. This phototrophic operation belongs to the so called light-dependent reaction of photosynthesis.

The binding of Pi to other substances is called phosphorylation. While the light-dependent reaction ADP is phosphorylated using ATP to light. Consequently, the process is called photophosphorylation.

We distinguish between different forms of photosynthesis. In oxygenic photosynthesis water molecules are split. The splitting of water molecules also requires light and chlorophyll (→ photolysis ). From the split water molecules of the electron supply for the electron transport chain is obtained. In addition, oxygen is released. In anoxygenic photosynthesis other organic or inorganic substances are used for the electron supply and provide no water. For this purpose, light is not required and there is no oxygen.

Prototrophy with rhodopsins

The phototrophic ATP synthesis ( photophosphorylation ) using bacteriorhodopsin, proteorhodopsin or Xanthorhodopsin also extends chemiosmotisch. A Rhodopsin consists of a protein that spans an entire biological membrane ( transmembrane protein). In the protein substance is a molecule called retinal.

When light of a certain wavelength on the retinal, the molecule changes its shape. There are a proton to the outside of the biological membrane. The retinal is then fed from the membrane inner side of a new hydrogen nucleus. The new proton, the molecule falls back to its original shape - until it is taken back by light, it changes its shape again and again discharges a proton to the outside of the membrane. In this way, a high H concentration gradient between the two sides of the membrane occurs at a rhodopsin -based phototrophy ( proton gradient ). The proton gradient is reduced by flow through a channel of the ATP synthase proton along its concentration gradient on the membrane inside back. The kinetic energy of the flowing through, hydrogen nuclei will be used for ATP synthesis (→ function of bacteriorhodopsin ).

Phototrophic organisms

Phototrophic prokaryotes

Various prokaryotes have evolved various forms of prototrophy. On the one hand, the ATP synthesis developed with the help of pigments bacteriorhodopsin, proteorhodopsin or Xanthorhodopsin. On the other hand, evolved regardless of the phototrophy with the help of chlorophyll pigments, photosynthesis. For many of the phototrophic prokaryotes phototrophy is not the only way of energy metabolism. Especially in the dark, they can resort to different ways of chemotrophischen energy deployment policy.

( a α - proteobacteria )

( an extremely halophilic bacterium)

Phototrophic eukaryotes

Eukaryotes were not originally phototroph. However, some eukaryotes have obtained the ability to prototrophy by addressing communities for mutual benefit ( mutualisms ) with phototrophic organisms. Such mutualisms formed repeatedly at different times and independently. Plastids represent the most advanced form of phototrophic mutualism is (→ endosymbiont theory ).

Most of the phototrophic eukaryotes are photohydroautotroph. This means that they synthesize carbohydrates exclusively with light, water and carbon dioxide. These phototrophic eukaryotes carry out oxygenic photosynthesis with the help of their mutualistic partners. In this case, the phototrophy for some phototrophic eukaryotes not the only source of nutrients; they can moreover chemoorganoheterotroph still feed by thus supplied with nutrients that they eat other organisms in whole or in part. Derlei both autotrophic and heterotrophic nourishing life forms operate mixotrophy.

Algae and land plants

The best known phototrophic eukaryotes harboring in their cells phototrophic plastids, chloroplasts. The chloroplast organelle arose a single time and the front about 1.6 billion years ago. At that time, to keep a cyanobacterium permanently within the eukaryotic cell succeeded. It emerged called primary chloroplasts.

Later, other eukaryotes have taken such eukaryotes together with their primary chloroplasts in itself. The eukaryotes recorded were gradually reduced to almost solely on their primary chloroplasts. In this way arose secondary, complex chloroplasts. Highly versatile ran the chloroplast uptake in the tank flagellants. In this group, chloroplasts seem to have been several times and obtained independently from each other. Some tanks have Geissler secondary chloroplasts, derived from the drawn in green algae. Many others have even tertiary chloroplasts. They came from recorded diatoms, throat flagellants or coralline algae, which in turn had secondary chloroplasts themselves.

If share the phototrophic eukaryotic cells multiply in parallel, to them and their chloroplasts. Eukaryotes with such chloroplasts are very roughly summarized under the name of algae. Most algae are among the microalgae. They remain microscopically small, often live as unicellular or form cell colonies or coenobia of very limited number of cells. A few groups of algae have developed multicellular forms. Such macro-algae are found exclusively among the Rhodophyta ( red algae ), the Phaeophyta ( brown algae ) and the Chloroplastida ( green algae, etc.).

A single group of algae succeeded the permanent settlement of the country. They were among the Streptophytina, which are among the Charophyta, which in turn are assigned to the Chloroplastida. The chloroplasts of all Chloroplastida appear green. The oldest fossils of green land plants possess an age of 475 million years ago. It is Spores of plants that are likely to have counted among the liverworts. Today's land plants can be divided into mosses and in Tracheophyta ( vascular plants ).

Phototrophic protists without chloroplasts

In addition to the phototrophic protists with chloroplasts exist many other phototrophic unicellular, but do not have chloroplasts. Protists this gain the ability to prototrophy in other ways. It is often done by other phototrophic unicellular - absorb them - or cyanobacteria Zoochlorellen.

A notable exception to this Oxyrrhis marina dar. The unicellular used to prototrophy a rhodopsin pigment. However, that form of phototrophy is widespread among prokaryotes, eukaryotes probably under very rare. Although rhodopsins are widely before taking eukaryotes. However, they are not commonly thought of phototrophy here, but the front desk.

Or another form of phototrophy

Phototrophic Opisthokonta

Not only algae, land plants and certain protists operate phototrophy. Also among the Opisthokonta - ie under fungi and animals - were able to go to some groups to phototrophic lifestyle. The phototrophy was achieved in very different ways.

• Prototrophy with photobionts: lichens ( lichens) are mutualisms between fungi and phototrophic unicellular organisms that are collectively referred to as photobiont. Many photobionts are on or wenigzellige Chlorophyta ( green algae ). The remaining photobionts come from the group of cyanobacteria.

• Prototrophy with other green algae: In addition to the frequently occurring sporadically went Zoochlorellen other unicellular Chlorophyta ( green algae ), a phototrophic mutualisms with animals. The green alga Tetraselmis convolutae lives between the cells of green Acoelomorphen Symsagittifera roscoffensis. And Oophila amblystomatis lives in eggs and embryos of Ambystoma maculatum (patch - tooth cross- newt ). This pig is the only known example of a phototrophes vertebrate.

• Prototrophy with zooxanthellae: Even more common than Zoochlorellen zooxanthellae be used as phototrophic mutualists of animals. The most common are zooxanthellae of the genus Symbiodinium, which belong to the tank flagellants. Many cnidarians harboring zooxanthellae, above all the various corals. Zooxanthellae are also utilized in many sea anemones, for example, in the guard Rose ( Anemonia sulcata ), in the sun Rose ( Cereus pedunculatus ) and - in addition Zoochlorellen - in the anemone Anthopleura elegantissima and green giant Anthopleura xanthogrammica. Zooxanthellae are still found in some jellyfish. For example, in the screen jellyfish Mastigias papua and Linuche unguiculata and in the mangrove jellyfish Cassiopea xamachana. Outside the cnidarians only a few cases of animal mutualisms with zooxanthellae were found so far. But at least they occur in the Acoelomorpha genus Waminoa and Tridacnidae, the giant clams.

• Prototrophy with Kleptoplastiden: Representatives of Sacoglossa ( throat sac snails) rob chloroplasts of green algae. They operate Kleptoplastie. These cells they eat certain green algae. In the intestine, the chloroplasts are removed from the cell contents eaten. You will be transported by the screw body. Finally, the chloroplasts are stored in vacuoles in skin cells. These chloroplasts were originally stolen by the eaten green algae are called Klepto (chloro ) plast (id) s. The best known phototrophic representative of the pharyngeal pouch snails are the green Samtschnecke ( Elysia viridis ) and their close relatives Elysia chlorotica.

• Prototrophy with xanthopterin: The Oriental hornet (Vespa orientalis) appears to have evolved an entirely new form of prototrophy. She uses the yellow dye xanthopterin in the cuticle of their exoskeleton.