Rhodoplast

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The special chloroplasts of the red algae (Rhodophyceae) are called rhodoplasts . They differ from the chloroplasts of the Viridiplantae or Chloroplastida in some features, but, like these, serve the photoautotrophic nutrition of the alga with the help of sunlight. It is assumed that the rhodoplasts, the chloroplasts and the muroplasts called organelles of the glaucophyta have the same evolutionary origin, i.e. are homologous to one another. The differences between them therefore emerged secondary in the course of evolution. Many systematists and phycologists have therefore started to refer to the plastids of red algae as chloroplasts, so that today two names are used side by side for them.

features

Rhodoplasts are usually lenticular, disc or egg-shaped, but can also be lobed in structure (for example in the genera Porphyridium and Rhodella ) to star-shaped ( Porphyra variegata ) or cup-shaped. They are rarely bilobed ( Rhodosorus ) or ribbon-shaped ( Audouinella ). They contain chlorophyll a as photosynthetically active pigment , while chlorophyll b (typical of the green Chlorophyta and Charophyta , including land plants) and chlorophyll c (typical of the photosynthetically active representatives of the Chromista or Stramenopilen , often grouped together as Chromophyta, including brown algae , for example ) absence. Until about 10 years ago it was thought that another, dark-adapted form of chlorophyll, chlorophyll d, was typical for red algae, until it turned out that it was an impurity that originates from growing cyanobacteria; in fact, chlorophyll d probably comes from red algae not before. The characteristic red to purple color is due to accessory pigments from the group of phycobilins and phycoerythrins . With these pigments you can also use yellow-green light beyond the absorption maximum of the chlorophyll and transfer the photons obtained to it (so-called antenna pigments ). The phycobilins form complex compounds with proteins that build up the so-called phycobilisomes . These occur in the rhodoplasts of the red algae and the cyanobacteria (also in the muroplasts), but are absent in the chloroplasts of the viridiplantae, so they are a plesiomorphism in the evolution of the algae . The phycobilins serve as accessory pigments of the photosystem II . Also, the photosystem I of rhodoplasts has accessory pigments to the xanthophylls belonging to Zeaxanthine . The pigment complex of the rhodoplasts is superior to that of the chloroplasts of the viridiplantae at low light intensities. Some red algae can still be found at sea depths of almost 270 meters, where the relative exposure strength is only 0.0005 percent of that at the water surface. But at high light intensities it is destroyed by the light itself.

The enzyme ribulose-1,5-bisphosphate-carboxylase / -oxygenase (abbreviated RuBisCO ), a key enzyme in photosynthesis, consists, as with the viridiplantae, of eight large and eight small subunits, but unlike these it is derived exclusively from its own genome of the plastid. A special form of starch , the so-called Florideen starch , is produced as a storage material .

Like all primary chloroplasts, rhodoplasts of red algae are only surrounded by two cell membranes . The membrane systems in which the light reaction takes place, the thylakoids , are structured differently than in the chloroplasts of the Viridiplantae: they do not form roll-like stacks of disk-like structures, but are individually, evenly spaced from one another. In parasitic lines of red algae, which are colorless, the thylakoids are largely reduced, but the rhodoplasts themselves still occur.

evolution

The similarity of the plastids to the free-living cyanobacteria led the botanist Konstantin Sergejewitsch Mereschkowski to the hypothesis that the plastids were due to the uptake of a cyanobacterium in a eukaryotic cell, presumably through phagocytosis , in which the cyanobacterium was retained. In the course of time, the community has solidified for mutual benefit until the cyanobacterium, as an organelle, became part of its host cell. This so-called endosymbiotic theory has been confirmed by research. It turned out, unexpectedly, that the primary recording was apparently only once, 1.5 billion years ago in the Archean . The plastids of the red algae, the green plants and the glaukophyta probably all go back to the same endosymbionts. The uptake was evidently an evolutionarily unlikely event that could only be successful in the event of a synchronous failure of several genes. The relationship of the cyanobacterium, which became plastids, is still controversial today, possibly it was a relative of the recent genus Gloeomargarita , which lives in fresh water. In the course of evolution, the plastids have lost most of their original genes; in the event of copying errors, these were integrated into the nuclear genome as mutations, so that today's plastids only have about 10 percent of their original genes. By comparing the genes in the nuclear genome, however, it is sometimes possible to elucidate relationships between organisms that have plastids and even with those in which these have been lost secondarily in the course of evolution.

Such comparisons showed that the plastids of the chromista or stramenopil groups, including those of the brown algae, are derived from the rhodoplasts of the red algae, not from the chloroplasts of the viridiplantae. These have four enveloping membranes instead of two; it is assumed that they are due to the uptake not of a single plastid, but of a complete red alga, including rhodoplasts, which then (leaving behind some genes) almost completely, except for the rhodoplast itself , has been regressed. Some lines, such as the dinoflagellates, have completely regressed it and later, independently of it, presumably accepted new endosymbionts, resulting in extremely complicated relationships.

The relationship between the three groups with primary plastids is unclear. Often the red algae and the glaucophytes are combined in a taxon Biliphyta, but that is not certain.

swell

  • Dinabandhu Sahoo, Joseph Seckbach (editors): The Algae World. Springer Verlag, Dordrecht etc., 2015. ISBN 978-94-017-7320-1 .
  • Robert R. Wise: The Diversity of Plastid Form and Function. Chapter 1 in Robert R. Wise and J. Kenneth Hoober (editors): The Structure and Function of Plastids. Springer Verlag, Dordrecht etc., 2006. ISBN 978-1-4020-4060-3

Individual evidence

  1. ↑ Akio Murakami, Hideaki Miyashita, Mineo Iseki, Kyoko Adachi, Mamoru Mimuro (2004): Chlorophyll d in an Epiphytic Cyanobacterium of Red Algae. Science 303: 1633. doi: 10.1126 / science.1095459
  2. ↑ Exception: The wired amoeba Paulinella chromatophora , in which it occurred a second time convergent. ECM Nowack, M. Melkonian, G. Glöckner (2008): Chromatophore genome sequence of Paulinella sheds light on acquisition of photosynthesis by eukaryotes. Current Biology 18: 410-418.
  3. ^ Wolfgangöffelhardt: The Single Primary Endosymbiotic Event. In: Woffgang Löffelhardt (editor): Endosymbiosis. Springer Verlag, Vienna etc., 2014. ISBN 978-3-7091-1302-8 .
  4. ↑ Jan de Vries, Sven B. Gould (2018): The monoplastidic bottleneck in algae and plant evolution. Journal of Cell Science 131: Article jcs203414 doi: 10.1242 / jcs.203414
  5. ↑ Rafael I. Ponce-Toledo, Philippe Deschamps, Purificacion Lopez-Garca, Yvan Zivanovic, Karim Benzerara, David Moreira (2017): An Early-Branching Freshwater Cyanobacterium at the Origin of Plastids. Current Biology 27: 386-391. doi: 10.1016 / j.cub.2016.11.056
  6. ↑ Thomas Cavalier-Smith (2017): Kingdom Chromista and its eight phyla: a new synthesis emphasizing periplastid protein targeting, cytoskeletal and periplastid evolution, and ancient divergences. Protoplasm 255 (1): 297-357. doi: 10.1007 / s00709-017-1147-3