Plastids, endosymbiosis and multiple histories of photosynthesis

Integration is more than temporary ingestion

A plastid is an organelle with an evolutionary history connected to a bacterial endosymbiont. In a photosynthetic plastid, reactions using light contribute to the production of organic material from inorganic carbon. Long-term integration also involves inheritance, exchange of metabolites and control by genes distributed between the organelle and the host nucleus.

A swallowed cell can be digested; an endosymbiont can persist within another cell; a deeply integrated organelle has a further history of dependence and transfer of functions. These are distinctions about biological organisation, not a sharp rule that a particular membrane count alone settles every case.

Primary endosymbiosis refers here to integration of a bacterial contributor into a eukaryotic host. The plastids shared by glaucophytes, red algae and the green lineage trace to a cyanobacterial contribution in their ancestry. Paulinella provides evidence of a separate acquisition of photosynthetic cyanobacterial machinery. It would therefore be misleading to declare that every photosynthetic organelle in every eukaryote comes from one universal acquisition.

An alga can become a later donor

In secondary endosymbiosis, the donor is already a eukaryotic cell containing a plastid. Its integration into a later eukaryotic host can carry the plastid across host-lineage boundaries. The nuclear ancestry of the later host and the ancestry of its plastid need not follow the same simple branching path.

The enclosure model assigns two original plastid bounding membranes, a donor-associated enclosing membrane and an additional outer bounding membrane in the later host. It is one retained-four-membrane teaching case. Some secondary plastids have three identifiable bounding membranes; others have more complicated histories involving losses, replacements or further endosymbiotic contributions. Count and origin must be investigated in the particular lineage.

One panel also assigns a retained donor-nucleus remnant, called a nucleomorph in real examples. It lies outside the inner plastid enclosure but inside the outer secondary enclosure. The host nucleus lies outside those plastid bounding membranes, within the larger host cell. Plastid DNA is a third, distinct location. These compartments must not be collapsed into one labelled nucleus.

Not every secondary plastid retains a nucleomorph. Nuclear genes can be transferred or lost during integration, and proteins made outside an organelle can be transported into it. The generic membrane diagram does not determine all protein routes, date the event or identify an actual species.

A plastid need not remain photosynthetic

A lineage can lose a function without losing every structure that once supported it. Some non-photosynthetic eukaryotes retain plastids with metabolic functions. In the original six-function model, P01 and P02 stand for photosynthetic functions while P03 through P06 stand for other metabolism. The assigned non-photosynthetic descendant loses the first two and retains the other four and the plastid.

These identifiers are invented. They are not six actual parasite genes, a measured genome reduction or a claim that exactly four functions are essential. They make one logical distinction visible: absence of photosynthesis does not establish absence of a plastid.

A real bounded example is the apicoplast of the malaria parasite Plasmodium falciparum. Research on its asexual blood-stage growth demonstrated the importance of isoprenoid-precursor production in a non-photosynthetic organelle. That result concerns the studied stage and conditions. It does not establish that every parasite has an apicoplast or that one function accounts for every plastid's role at every life stage.

Loss of an organelle is another possibility, requiring appropriate evidence. Failure to detect chlorophyll or photosynthesis is insufficient on its own. An investigator may examine organelle structures, genomes, protein targeting and metabolic activity to distinguish retained but changed compartments from complete loss.

Use more than one ancestry question

Ask separately about the organism's nuclear relatives, its plastid's relatives and the route through which the plastid arrived. A plastid sequence can resemble a donor lineage even though most of the host's nuclear history lies elsewhere. This is an example of an evolutionary history with contributions across branches rather than a failure of inheritance.

Temporary retention of plastids from ingested prey can also support photosynthetic activity in some organisms, but it is not automatically the same as a permanently inherited secondary plastid. Duration, replacement and dependence need investigation. A cell observed photosynthesising today need not establish the whole evolutionary route.

The lesson does not supply a complete reticulate history of all algae. It supplies a way to read evidence without turning one feature into an answer to every ancestry question. The later algal-lineage examples use that distinction to compare organisms with different host and plastid backgrounds.

Worked example

Check your understanding

1. What makes the assigned secondary contribution different from a primary one?

   Show reasoning

   Its donor is a eukaryotic cell already containing a plastid. A primary contribution in this lesson involves a bacterial donor. Secondary integration can carry plastids between host-lineage backgrounds.

2. In the four-membrane model, is the donor-nucleus remnant the host nucleus or plastid DNA?

   Show reasoning

   Neither. It is a separate assigned remnant outside the inner plastid enclosure and inside the outer secondary enclosure. The host nucleus is outside all four plastid bounds; plastid DNA occupies its own plastid compartment.

3. A cell does not photosynthesise. Does that prove it lacks a plastid?

   Show reasoning

   No. A retained plastid may perform other metabolic functions. The six-function model makes that distinction explicit, and the apicoplast is a bounded biological example. Actual organelle loss needs evidence beyond absence of photosynthesis.

What to carry forward

Follow host ancestry, plastid ancestry and retained functions separately. Endosymbiosis, replacement and loss can make their histories differ.