Complex plastids

from Wikipedia, the free encyclopedia

Complex plastids are characterized by membrane shells that, unlike simple plastids, consist of three or more membranes. If the plastids contain chlorophyll , one also speaks of complex chloroplasts . In the space between the second and third membrane (from the inside) there is sometimes a rudimentary eukaryotic cell nucleus called a nucleomorph . This is the case , for example, with cryptomonads , chlorine arachniophytes and many apicomplexes .

Schematic representation of a complex chloroplast with four membranes and a nucleomorph .

The endosymbiont theory explains the simple chloroplasts (with their double membrane) as the result of a primary endosymbiosis, in which a heterotrophic eukaryotic cell ( eucyte ) has ingested a photosynthetically active ( autotrophic ) cyanobacterium with its double-layered gram-negative cell wall without 'digesting' ( lysing it) it ). The complex chloroplasts (possibly with their nucleomorph) are explained as the result of a secondary or endosymbiosis, in which a heterotrophic eukaryotic cell has taken up an autotrophic (also eukaryotic) algal cell (also eukaryotic) together with chloroplasts and retained it as an endosymbiont. In addition to the chloroplast (s), this endosymbiont initially had its own nucleus , its own eukaryotic ribosomes and mitochondria . In the course of a long partnership, however, a number of simplifications and the loss of redundant parts have occurred:

  • After various adaptations and endosymbiotic gene transfer (EGF) from genes in the nucleus of the phagocytosed alga to the outer host cell , its nucleus was reduced to a nucleomorph or disappeared completely, along with its own ribosomes,
  • The same thing happened to our own mitochondria.
  • Individual layers of the multi-layered membrane shell could also be lost.

The resulting secondary, complex chloroplasts can therefore have three or four membranes, the two cyanobacterial membranes, sometimes the cell membrane of the incorporated algae and the phagosome vacuole from the cell membrane of the host. Even the ability to photosynthesize could be lost if the endosymbiont was still of use to the host cell due to various other services, so that the endosymbiont then more generally represents a complex plastid.

Secondary and tertiary endosymbiosis

If such an alga with secondary plastids like matryoshka dolls was taken up again by a heterotrophic eucyte, a tertiary endosymbiosis developed. Depending on how many such nested endosymbioses have led to a complex chloroplast (or plastid), this is referred to as a secondary or tertiary chloroplast (plastid), the host corresponding to the secondary host . Chloroplasts and plastids are found today in an extremely wide range of organisms, some of which are not even directly related. This is explained as a consequence of many secondary and even tertiary endosymbiotic events.

Secondary endosymbiosis
A eukaryotic alga is incorporated into another eukaryote and thus becomes a chloroplast with three or four membranes.

All secondary chloroplasts are derived from green and red algae ; no secondary chloroplasts derived from glaucophytes have (so far) been found - probably because glaucophytes are relatively rare in nature and it is therefore unlikely that they were ingested by another eukaryote.

Cladogram of possible chloroplast evolution

a It has meanwhile been found that the chromalveolata are paraphyletic with respect to the rhizaria .

Chloroplasts derived from green algae

Green algae were ingested by the euglenids , chlorarachniophytes , a lineage of dinoflagellates and the possible common ancestor of the "CASH lineage" ( cryptophytes , alveolates , stramenopiles and haptophytes ) in three or four different endosymbiosis events. Many of the chloroplasts derived from green algae contain pyrenoids , but unlike chloroplasts in their green algae ancestors, the starch collects as a storage product outside the chloroplast as granules.

Euglenophytes

Euglena , a euglenid , contains secondary chloroplasts derived from green algae.

Euglenophytes ( Euglenida ) are a group of flagellates (taxonomically more precisely: Excavata or Discoba ) that contain secondary chloroplasts derived from green algae. The chloroplasts of the euglenophytes have three membranes. It is assumed that the membrane of the ingested green alga (that is the second from the outside) has been lost and the two membranes of the cyanobacteria on the inside and the phagosome membrane of the secondary host on the outside remain. Euglenophyte chloroplasts have a pyrenoid and thylakoids stacked in groups of three. The photosynthetic product is stored in the form of paramylon , which is contained in membrane-bound granules in the cytoplasm of the euglenophyte.

Chlorachniophytes

Chlorarchniophytes are a small group of organisms that are also secondary chloroplasts derived from green algae, although their history is more complicated than that of euglenophytes. It is believed that the ancestor of the Chlorarachniophyte was a eukaryote with an original chloroplast derived from red algae. This was later lost, whereupon a secondary endosymbiosis occurred again due to the ingestion of a green alga, from which today's complex chloroplast developed.

The (present-day) chloroplasts of the Chlorarachniophytes are surrounded by four membranes, except in the vicinity of the cell membrane, where they fuse to form a double membrane; their thylakoids are arranged in loose piles. Chlorarchniophytes produce a polysaccharide called chrysolaminarin , which they store in the cytoplasm, but often accumulates around the chloroplast pyrenoid that protrudes into the cytoplasm.

The chloroplasts of the Chlorarachniophytes are characterized by the fact that the green alga from which they originate has not yet been completely broken down - their core remains as a nucleomorph between the second and third chloroplast membrane. This corresponds to the cytoplasm of the green algae and Periplastidraum ( English periplastid space called).

Dinoflagellate chloroplasts derived from prasinophytes

Among the dinoflagellates (alias dinophytes) there is a group of close relatives of their member Lepidodinium viride (formerly part of the genus Gymnodinium ), which also lost their original chloroplast (containing the red pigment peridinin , see below for details) and by one of green algae (more precisely prasinophytes) ) derived chloroplasts.

With this replacement, Lepidodinium and its close relatives are the only dinoflagellates that have a chloroplast that is not of the rhodoplast lineage. This chloroplast is surrounded by two membranes and has no nucleomorph - all nucleomorph genes have been transferred to the dinophyte nucleus. It is more likely that the endosymbiotic event that led to today's chloroplast was such a serial secondary endosymbiosis rather than a tertiary endosymbiosis (see below); H. the endosymbiont was a green alga that contained a primary chloroplast.

Chloroplasts derived from red algae

Cryptophytes

Cryptophyceae (also called Cryptomonads) are a kindred group of algae that contain a chloroplast derived from red algae. The chloroplasts of the Cryptophyceae contain a nucleomorph that superficially resembles that of the Chlorarachniophytes . These chloroplasts have four membranes, the outermost of which is related to the rough endoplasmic reticulum . They synthesize common starch, which is found in granules in the periplastid space (the cytoplasm of the red algae) outside the original double membrane of the primary (derived from cyanobacteria) chloroplast. In the chloroplasts of the Cryptophyceae there is a pyrenoid and thylakoids in stacks of two each.

These chloroplasts do not have phycobilisomes , but they do have phycobilin pigments that they store in their thylakoid space instead of anchoring them to the outside of their thylakoid membrane.

Cryptophyceae could play a key role in the proliferation of chloroplasts derived from red algae.

The closest relatives of cryptophyceae are the most colorless and phagotroph living Katablepharidophyta (syn. Katablepharida). Together they form the taxon of the Cryptophyta . In contrast to the Cryptophyceae, the catable pharids have lost their red algae chloroplasts again.

Haptophytes

Haptophytes (alias Prymnesiophyta) are similar and probably closely related to the Cryptophyceen and Heterokonta . Their chloroplasts lack a nucleomorph, their thylakoids form stacks of three and they synthesize the polysaccharide chrysolaminarin , which they store completely outside the chloroplast in the cytoplasm (the haptophyte).

Heteroconta (stramenopile)

The Heterokonta (also called Heterokontophyta, Stramenopile, or Chromista) are a very large and diverse group of eukaryotes. The photoautotrophic lineage of the Ochrophyta with the following subgroups belongs to them :

  1. Diatoms (syn. Diatoms or Bacillariophyta)
  2. Golden algae (syn.Chrysophyceae or Chrysophyta s. S. )
  3. Yellow-green algae (syn.Xanthophyceae)
  4. Brown algae (syn.Phaeophyceae or Phaeophyta)

The first three groups are sometimes referred to as Chrysophyta in the broader sense ( see left ).

Common to these are the chloroplasts from red algae. The chloroplasts of the heteroconta are very similar to those of the haptophytes and contain a pyrenoid, triplet thylakoid. With a few exceptions, they have a four-layer covering, with the outermost epiplastid membrane connected to the endoplasmic reticulum . Like the haptophytes, the heteroconta store polysaccharide (sugar) as chrysolaminarin granules in the cytoplasm. The chloroplasts contain chlorophyll a and, with a few exceptions, chlorophyll c , but also carotenoids , which give them their various colors.

Alveotalta (Apicomplexa, Chromerida and Dinoflagellata)

The alveolates are a large group of unicellular eukaryotes with autotrophic , heterotrophic, and mixotrophic members. The most notable common feature is the presence of cortical , i.e. H. Flat vacuoles or vesicles (sac-like structures), so-called alveoli, located under the cell membrane ( pellicula ) . These are packed in a continuous layer directly under the membrane and support it, typically forming a flexible pellicle (thin skin). However, in dinoflagellates they often form armor plate-like structures. Many members contain a plastid derived from red algae. A notable feature of this diverse group is the frequent loss of photosynthesis, with a majority of these heterotrophs still leaving a non-photosynthetic plastid.

Apicomplexa

The apicomplexa are a subgroup of the alveolata. Like the representatives of the genus Helicosporidium (see protothecosis ), they are parasitic and have a non-photosynthetic chloroplast. It was therefore previously assumed that they were related to Helicosporidium , but it is now known that Helicosporidium belongs to the green algae and is not part of the CASH line. The malaria parasite Plasmodium belongs to the Apicomplexa . Many apicomplexes inherited from their ancestors a complex plastid derived from red algae called an apicoplast . Other Apicomplexa like Cryptosporidium have completely lost this. The apicomplexa store their energy in amylopectin granules, which are located in their cytoplasm, even if they are not photosynthetically active.

Scheme drawing of the malaria pathogen Plasmodium

The apicoplasts have lost all photosynthetic functions and do not contain photosynthetic pigments or true thylakoids. They are bound to four membranes, but the membranes are not connected to the endoplasmic reticulum (ER). The fact that Apicomplexa still kept their non-photosynthetic chloroplasts testifies that the chloroplasts perform other important functions in addition to photosynthesis, as mentioned above - plant chloroplasts supply plant cells with many important substances in addition to sugar, and apicoplasts do not differ in this - they synthesize Fatty acids , isopentenyl pyrophosphate and iron-sulfur clusters . In addition, they perform part of the heme synthesis ( English heme pathway ). These make apicoplasts an interesting target for drugs to heal diseases that are caused by apicomplexa (such as the malaria pathogen). The most important function of the apicoplasts is the synthesis of isopentenyl pyrophosphate. In fact, if something prevents this function, apicomplexa die.

Chromerida

The Chromerida are a relatively newly discovered group of algae found in Australian corals. They are close, but photosynthetically active relatives of the Apicomplexa. The first member, Chromera velia , was discovered in 2001 and isolated for the first time. The discovery of Chromera velia with a similar structure as the Apicomplexa provided an important link in the evolutionary history of the Apicomplexa and Dinoflagellaten. The chloroplasts of the Chromerida, like the apicoplasts, have four membranes; there is no chlorophyll c . C .velia uses the RuBisCO type II, this form is homologous to that of dinoflagellates. Apparently this lineage obtained the RuBisCO type II by horizontal gene transfer from a proteobacterium .

Dinoflagellates

The dinoflagellates (syn. Dinophytes or armored flagellants) are another very large and diverse group of eukaryotes, about half of which are (at least partially) photosynthetically active.

The plastids of the dinoflagellates do not form a uniform lineage. Instead, the dinoflagellates can be classified as follows:

  1. Most of their chloroplasts are secondary chloroplasts derived from red algae.
  2. Many other dinophytes have lost their chloroplasts and therefore belong to the non-photosynthetic group of dinoflagellates.
  3. The secondary plastids have been partially replaced by a tertiary endosymbiosis. I.e. after the loss of the chloroplast, members of the second group above took up another eukaryotic alga which contained a chloroplast derived from red algae.
  4. Other dinoflagellates replaced their original chloroplast with one derived from green algae.

Most chloroplasts of dinoflagellates contain RuBisCO Type II, and possibly along with other photosynthetic pigments chlorophyll a, chlorophyll c 2 , beta-carotene , plus at least one Dinophytes-specific xanthophyll as peridinin , Dino xanthine or Diadinoxanthin , which many of her golden-brown color receive. All dinophytes store starch in the cytoplasm and their chloroplasts have most thylakoids that are arranged in stacks of three.


Most dinoflagellate chloroplasts are of the peridinin type, characterized by the carotenoid pigment peridinin together with chlorophyll a and chlorophyll c 2 . Peridinin is not found in any other group of chloroplasts. Peridinin chloroplasts have three membranes (sometimes only two), i.e. H. they have lost the original cell membrane of the red algae endosymbiont. The outermost membrane is not connected to the endoplasmic reticulum . They contain a pyrenoid and have thylakoids in a stack of three. The starch is found outside the chloroplast. An important feature of these chloroplasts is that their DNA is greatly reduced and fragmented into many small rings, a phenomenon similar to that observed in mitochondria of the human louse Pediculus humanus and other real animal lice (Anoplura), but not in the jaw lice (Mallophaga) . Most of the genome has migrated into the cell nucleus, and only critical genes related to photosynthesis remain in the chloroplast.

It is assumed that the peridinin chloroplasts represent the "original" chloroplasts of the dinoflagellates, and that they were either lost, reduced, replaced or “company” in several other dinoflagellate lineages (see below).

Fucoxanthin-containing chloroplasts of the dinoflagellates (derived from haptophytes)

In the fucoxanthin dinophyte lines (including Karlodinium and Karenia ), the dinoflagellates lost their original chloroplast derived from red algae and replaced it with a new chloroplast derived from haptophytes. Karlodinium and Karenia likely accepted different heteroconta. Since the (secondary) chloroplasts of the haptophytes have four membranes, it would actually be expected that this tertiary endosymbiosis would lead to a chloroplast consisting of six membranes, with the cell membrane of the haptophyte and the phagosomal vacuole of the dinophyte being added to the original four membranes. The haptophyte, however, has been greatly reduced and has lost some membranes and its core, so that only the chloroplast (with its original double membrane) and, depending on the case, one or two additional membranes around it remained.

Fucoxanthin-containing chloroplasts are characterized in that they contain the pigment fucoxanthin (more precisely 19'-hexanoyloxyfucoxanthin and / or 19'-butanoyloxyfucoxanthin), but no peridinin . Fucoxanthin is also found in chloroplasts of the haptophytes, which indicates the ancestry described.

Dinoflagellates with chloroplasts derived from diatoms

Dinophysis acuminata contains chloroplasts derived from Cryptomphyceen.

Some dinoflagellates such as Kryptoperidinium and Durinskia (both Peridiniaceae , also English dinotoms ) have a chloroplast derived from diatoms ( Heterokontophyta ). These chloroplasts are surrounded by up to five membranes, depending on whether the entire diatom endosymbiont is regarded as the chloroplast or only the red alga it contains is counted as the chloroplast. The diatom endosymbiont has been reduced relatively little - it still retains its original mitochondria and has endoplasmic reticulum , eukaryotic ribosomes , a cell nucleus and of course the complex (secondary) chloroplasts derived from red algae - practically a complete cell - all inside the host . However, the diatom endosymbiont cannot store its own reserves - its storage polysaccharide is instead located outside in granules in the cytoplasm of the dinophyte. The core of the diatom endosymbiont is - as already mentioned - present, but probably cannot even be called a nucleomorph , as it shows no signs of genome reduction and may even have been expanded. Diatoms were ingested at least three times by dinoflagellates as endosymbionts.

The diatom endosymbiont is limited by a single membrane. Inside there are chloroplasts with four membranes. Like the diatom endosymbiont ancestor, the chloroplasts have triple thylakoids and pyrenoids.

In some of the genera of this type, the chloroplasts of the diatom endosymbiont are not the only chloroplasts in the dinophyte. Your original three-membrane peridinin chloroplast was not lost at all, but was transformed into an eye spot .

See also

Web links

  • Wilfried Probst: Early Evolution and Symbiosis , Europa-Universität Flensburg, Institute for Biology and Science Education and its Didactics: §Secondary plastids, accessed on April 19, 2019

Individual evidence

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  23. ↑ External identifiers or database links for 19′-hexanoyloxyfucoxanthin : CAS number: 60147-85-5, PubChem : 6443044 , ChemSpider : 4947086 , Wikidata : Q85748832 .
  24. ↑ External identifiers or database links for 19′-butanoyloxyfucoxanthin : CAS number: 111234-30-1, PubChem : 14160128 , ChemSpider : 24823142 , Wikidata : Q90309501 .
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