Evolution of Chloroplasts from Energy Providers to Carbon Fixers

Endosymbiosis, a process where one organism engulfs another and integrates its DNA and functions instead of digesting it, is one of the most significant events in evolutionary history.

Scientists agree this process occurred twice, leading to the development of mitochondria—organelles that produce energy—and later, plastids, including chloroplasts, which enable photosynthesis.

A recent study published in Nature Communications examines the origin of chloroplasts, the plastids that allow plants to absorb atmospheric carbon and build their structures. The researchers focused on a plastid-specific energy-transport molecule and found evidence suggesting that early chloroplasts initially served as chemical energy generators for host cells. Over time, their function shifted to using the energy they produced primarily for carbon assimilation.

Angad Mehta, a Chemistry Professor at the University of Illinois Urbana-Champaign and the study's lead author, explained, "Chloroplasts are thought to have evolved from photosynthetic cyanobacteria, but it is unclear what roles the cyanobacteria initially served for the cells that ingested them."

We asked the question: What chemical role did the primitive symbiont that led to chloroplasts perform for the host cell? Was it carbon assimilation or ATP synthesis or both?”

Angad Mehta, Professor, Chemistry, University of Illinois Urbana-Champaign

Evidence suggests that the plastids in red algae and glaucophytes, another class of photosynthetic organisms, represent more ancient evolutionary stages than the chloroplasts found in land plants. However, according to Angad Mehta, a Chemistry Professor at the University of Illinois Urbana-Champaign, current bioinformatics techniques have limitations in advancing this research further.

Mitochondria and plastids rely on their ability to generate energy, a critical aspect of their functional evolution. Both organelles produce ATP, the molecule that powers most chemical reactions in living cells. They also use membrane-bound ADP/ATP carrier translocases to exchange ATP for its precursor, ADP.

To explore how differences in translocase activity might shed light on chloroplast evolution, Mehta and his team studied the translocases in plastids from land plants, red algae, and glaucophytes. In their experiments, the researchers modified cyanobacteria to express each of the three types of translocases and created a synthetic endosymbiosis by integrating the modified cyanobacteria into yeast cells.

The lab environment was manipulated to make the yeast cells entirely dependent on the cyanobacterial endosymbionts for energy. This approach built on a method Mehta's lab developed in 2022 to induce yeast cells to internalize cyanobacterial endosymbionts. The experiments revealed significant differences in translocase activity among the plastid types.

Most notably, we saw that the endosymbionts expressing translocases from the plastids of red algae and glaucophytes were able to export ATP to support endosymbiosis, whereas those from chloroplasts imported ATP and were unable to support the energy needs of the endosymbiotic cells.”

Angad Mehta, Professor, Chemistry, University of Illinois Urbana-Champaign

ADP was being expelled and the chloroplast translocases of land plants were importing ATP.

The new findings imply that chloroplasts once shared their primary function of supplying energy to the larger cell because the plastids of glaucophytes and red algae seem to resemble a more ancient form of the photosynthetic organelles.

However, it appears that at some point during their evolutionary history, land plants' chloroplasts changed to power their carbon-assimilation processes using the ATP they generated through photosynthesis. According to Mehta, chloroplasts even steal some of the ATP produced by mitochondria.

According to Mehta, the new research provides evidence in favor of this theory, even though it does not prove that this is how chloroplasts evolved.

The proposal is that the initial interaction between the endosymbiont and cell was based on ATP production and ATP supply. Now, you can imagine a scenario in which, as these organisms go on to become land plants, they grow in oxygen-rich conditions. This allows the mitochondria to become specialized in ATP synthesis and chloroplasts to focus and become an engine that drives carbon assimilation.” 

Angad Mehta, Professor, Chemistry, University of Illinois Urbana-Champaign