Sunday, April 11, 2010

The Evolution of Chloroplasts: endosymbiosis and horizontal gene transfer

Chloroplasts are the machines of life, upon which we heterotrophs (organisms which can't create their own food, but instead rely on eating other organisms) depend. Their evolution massively transformed the earth itself, changing the air by dramatically increasing atmospheric oxygen essential for animal life, and providing an energy source.

Micrograph of the cells of a moss species, Plagiomnium.
The round green organelles are chloroplasts. [Source]

All green plants contain chloroplasts, amazing molecular machines which use carbon dioxide, water and photons from sunlight to create sugar and oxygen. The oxygen, so precious to us, is in fact a mere waste product of the reaction. This is the process called photosynthesis:

The story of how plants evolved the ability to perform this chemical feat begins around 3.6 billion years ago. That was before multicellular organisms appeared, and indeed, before eukaryotic cells evolved. (A eukaryotic cell is one with a membrane-bounded nucleus (containing DNA) and various organelles performing specialised functions.) The only living organisms that existed were the prokaryotes, which were much less complex. Prokaryotes have a very different type of DNA structure. Instead of being wrapped around histones in the familiar X-shaped chromosomal form, they are circular. 

One type of prokaryote, cyanobacteria, developed the capacity to use energy from the sun to power its metabolic needs. We see the ancient remains of these early autotrophs today, in the form of stromatolites, layered, fossilised structures like these at Lake Thetis in Western Australia:


The ability to use sunlight as a means to make their own food provided photosynthesising prokaryotes with a huge competitive advantage, and they rapidly populated the earth's oceans and lakes. The oxygen they emitted as a waste product led to what's known as the Great Oxidation Event, around 2.4 billion years ago. We can see evidence of this event in rocks from the era. Here is sedimentary rock containing black-banded ironstone, showing the oxidation of iron.


About 2.7 billion years ago, another remarkable change was occurring: the evolution of eukaryotic cells. This entailed the process of endosymbiosis [Gk: endon "within", syn "together" and biosis "living".] In endosymbiosis, one organism engulfs another and incorporates it into its own body or cells. It's important to remember that this takes place by invagination: think of pushing your finger into the side of an inflated balloon. Your finger is surrounded by both its own external membrane (your skin) as well as the membrane of the balloon itself. Now imagine (and sorry, the metaphor gets a bit gross at this point!) that your finger falls off and the balloon seals itself up again. Now your finger is inside the balloon, wrapped in a double membrane. That endosymbionts evolved by this process is evidenced by the fact that they have a double membrane, including their own original form that resembles the ancestral bacterial surface. 

Below is a graphic I've put together to show the two crucial stages in the incorporation of photosynthesising prokaryotes into proto-eukaryotic cells, leading to a true eukaryotic cell with both a mitochondrion and a chloroplast. 

Although only green plants possess chloroplasts in their cells, all animal and plant cells have mitochondria, a specialised organelle that oxidises (burns) sugar and provides energy for cells to work.

Note that the DNA of the proteobacterium and the cyanobacterium are circular, compared with the condensed nucleus of the proto-eukaryotic cell. This structure continues down the lineage. Mitochondria and chloroplasts have their own, circular DNA that replicates independently from the DNA in the cell's nucleus. 

The first organisms resulting from the endosymbiosis of cyanobacteria were the green algae, the direct ancestors of all modern plants.

Chloroplasts, able to perform feats well beyond human technology--the efficient splitting of water into hydrogen and oxygen, and the synthesis of sugars from water and carbon dioxide to chemically store energy--are extremely complex structures. A crucial protein in the creation of sugar (during the Calvin Cycle phase of photosynthesis) is Ribulose-1,5-biphosphate carboxylase oxygenase, commonly known as RuBisCO.

Over the course of plant evolution since the original endosymbiotic events, interactions have occurred between genes in the chloroplasts, nucleii and mitochondria. Some genes have disappeared because they are redundant to the organism (both doing the same work in synthesising proteins), and others have physically moved from one organelle to another. The latter process is known as horizontal (or lateral) gene transfer (HGT). Thus, in RuBisCO, some of the protein chains are synthesised by the DNA in the nucleus (nDNA), while others are synthesised by the DNA in the chloroplasts (cDNA). In an astoundingly intricate dance, the cellular machines work together to create what is one of the most important organic molecules on earth. Without it, we and all the other animals on earth wouldn't be here.

Before I finish this rather long post (and if you're still reading, thank you!), I just wanted to mention the mysterious case of the klepto-plast slug.


This gorgeous creature is Elysia chlorotica, the Green Sea Slug. And it photosynthesises! Just think about that for a moment. An animal capable of making its own food by using light from the sun. Wouldn't you love to be able to do that? It'd certainly solve the global energy crisis, but perhaps a green hue wouldn't suit everyone's taste.

The Green Sea Slug eats algae, and incorporates the algal chloroplasts into the spaces between its own cells. It differs, however, from the endosymbiosis of plant chloroplasts, because in this case there is no horizontal gene transfer. Once the algae in the slug die, that's it. The slug needs to eat more to keep photosynthesising. It's tempting to think that this might be an evolutionary stage towards a truly autotrophic animal, but apparently too much damage is done to the chloroplasts as they travels through the gut of the slug, and the chloroplasts can no longer replicate themselves. 


This post was adapted from a genetics seminar I recently gave as part of my studies. If you'd like to read more on the topics raised, here's some further reading:

Archibald, J.M. & Keeling, P.J. (2002) Recycled plastids: a 'green revolution' in eukaryotic evolution. Trends Genet 18:11:577-584.

Delwiche, C.F. (1999) Tracing the thread of plastid diversity through the tapestry of life. The American Naturalist 154 Supplement:S165-S177.

Gould, S.B., Waller, R.F. & McFadden (2008) Plastid Evolution. Annu. Rev. Plant Biol. 59:491-517.

Keeling, P.J. & Palmer, J.D. (2008) Horizontal gene transfer in eukaryotic evolution. Nature Reviews: Genetics 9:606-618.

Keeling, P.J. (2009) Role of horizontal gene transfer in the evolution of photosynthetic eukaryotes and their plastids, in Gogarten, M.B. et al. (eds.) Horizontal Gene Transfer: Genomics in Flux, vol. 532: 501-515.

Martin, W. & Schnarrenberger, C. (1997) The evolution of the Calvin cycle from prokaryotic to eukaryotic chromosomes: a case study of functional redundancy in ancient pathways through endosymbiosis. Curr Genet 32:1-18

Smith, A.M. et al.(2010) Plant Biology, Garland Science, New York.

Taiz, L. & Zeiger, E. (2006) Plant Physiology, Sinauer Associates, Sunderland, MA U.S.A.

UPDATE: This post was featured in the May 2010 edition of Blog Carnival of Evolution, this month hosted by the official blog for Springer Verlag's journal, Evolution: Education and Outreach! I feel chuffed. 

FURTHER UPDATE: This post also won the 3 Quarks Daily 2010 Prize in Science "Charmed Quark" award. Which makes me exceeding happy.


greenspace said...


great post, great images.

I'd never heard of RuBisCO before, but now I know it's awesome!

I first heard about prokaryotes and eukaryotes some years ago, but this is the first time I've really got it, due to your clear explanation and particularly your graphic showing the endosymbiosis that results in a cell with nucleus, mitochondria, and chloroblast.

and I don't really want to look like that (admittedly very pretty) slug, but green skin would be just fine, if it meant I could photosynthesise.
I'd even try to get over my dislike of the taste of spirulina, if humans could use chloroblasts :-)

Margaret said...

Thanks! I am so pleased I explained it clearly. That's the trick with writing about science, as I increasingly discover. But so much in biology is just gobsmackingly wonderful, it just aches to shared with non-biologists.

There's a science fiction book called "The Child Garden" by Geoff Ryman in which humans have been genetically modified to be able to photosynthesise. It was written in 1989, well before the green sea slug's amazing capacity was discovered. I imagine, though, that if we could photosynthesise, it would play havoc with our insulin/sugar regulation processes. But I'm not into human biology, so I'm only guessing there!

Cherokee Hill said...

Back in 2003 I sent a message to Bill Rudman of the Australian Museum regarding the likelyhood of photosyhthetic ability in an animal very similar to a sea slug; a sea hare (I didn't know it was a sea hare at the time). You can see my question and the photo I sent in here near the bottom of the page: . Bill's reply stated he wasn't aware of photosythetic plastids being incorporated into the tissue of the sea hare as it is in some sea slugs, however, he found my observations about it changing colour interesting and he hypothesised it was incorporating algal pigments into its tissue to assist in camoflague.

In his reply he linked to a fascinating article called 'Solar Powered Seas Slugs' ( In this he discusses how some sea slugs are able to consume and partly digest various types of algae, releasing the chloroplasts into a modified digestive tract. These structures are actually capable of keeping the photosynthetic plastids alive. They even remained photosynthetically functional thus setting up a symbiotic relationship between the sea slug and the chloroplast.

Then there is the case of coral having live zooxanthellae living within their tissue. This relationship is a mutually beneficial one because the zooxanthellae are able to share photosyhthetic products and by-products with the coral and in return the coral provides a stable environment in which the algae can live and at the same time maximise their exposure to the sun without being washed away into potentially less productive and less habitable waters. It is this relationship which is in jeopardy when people talk about coral bleaching. Water temperatures rise resulting in a mass exodus of the algae from the coral's tissue. Much of the pigment in coral comes from the zooxanthellae and in their absence they look like they have been bleached. The corals starve, as they obtain up to 90% of their resources from the zooxanthellae, revealing their skeletons as they die. If temperatures drop before the coral dies they are able to replenish their supply of zooxanthellae and recover. However, with global warming marching on with such momentum this is not happening and we are losing large chunks of coral reef in affect areas.

You are right! Biology is gobsmackingly fascinating! Have you done any research yet into the theories of how prokaryotes are thought to have evolved from micelles? That's another amazing concept! Thanks for such a wonderful post! It's nice to read well written and interesting science... the kids in my science classes would find it most interesting!



Margaret said...

Hi Simon,

That link to your photo isn't working for me. I get the top of the page, but no real content. The response is really interesting. I believe someone at Macquarie Uni is doing her doctorate on green sea slugs. I must see what she's up to!

Micelles are deeply cool, aren't they? I haven't looked into them in any depth at this point, but they are such a crucial element in the evolution of life, I certain will at some point.

Happy teaching!



Margaret said...

(Er... "certainly", not "certain". :)

Cherokee Hill said...

That's strange... I copied and pasted the link above and it worked for me. This is it here:

Margaret said...

Okay, worked for me that time. How cool that you use algae-eating invertebrates to clean your aquarium! Very impressive. I've never owned a marine aquarium. It's one of those "one day..." things. :)

Margaret said...

I don't know why, Simon, but my blogger account wouldn't let me approve your post! Anyway, here it is:

"I only have freshwater aquariums since moving down to Tas. I'm in the process of planning two new ones though. The first is a temperate seagrass tank for seahorses and the other is an anemone tank for clownfish. Still a while off as there is much to learn about seagrass biology before jumping in and trying to cultivtae them... another fascinating thing from nature :) Did you know that some of the seagrass beds in northern Tas have been shown to reproduce asexually (preferentially), and single colonies have been dated at up to 4000 years old making them one of the oldest plants on the planet! Amazing!!! I teach marine science (mainly) now and aquariums have always been 'my thing'. Once I did my B.Ed and B.Sc I started a hort degree by correspondance but never completed it (time poor) as I'm into all things plants too... but the sea is still my first love. For a scientist such as yourself there is a wealth of science you can experience with low-tech marine aquariums... don't be sucked in by all the technology... success through natural science systems is still the best way to tackle marine aqariums by far!"

One day I'll definitely do it. And I'll be hunting you down for advice!

Dave said...

I have to share this with you Margaret. Apart from discovering your wonderful blog, which reanimates my long-held fascination with biology, I should say how I got here.

I was commenting on Time magazine's inclusion of Sarah Palin as one of the 100 most influential people. I used the analogy of a sea slug to suggest the regression in U.S. political thought (as in 'back to sea slug thinking'). When someone misused my analogy to praise SP, I wrote that on reflection, I should have used a leech analogy.

I thought I'd check the connection between sea slugs and leeches and searched on 'sea slug leech evolution' and found your page!

Looking at that great picture of the green sea slug I can see that I was right to change my tack :-)

Margaret said...

Dave, that is a magnificent tale. Thanks for sharing it here. Mind you, although leeches are not my favourite form of inverts, and even though I am allergic to their bites, I think they're a far more sophisticated and valuable lifeform than Sarah Palin. ;)

Dave said...

Haha, thanks for responding. Hmmm, you're into endosymbiosis and I'm into autopoiesis (in the work of Varela, Maturana, and Antonio Damasio in complex biological systems relating to consciousness). Wonder if the two terms can be linked :-)

outerhoard said...

As I write this, I'm in the process of reading the nominations for the 2010 3QD Prize in Science. This article is one of the better nominations, bearing in mind that I'm not even halfway through the list.

My main reservation is with this sentence: "Instead of being wrapped around histones in the familiar double helix shape, they are circular in form." That sounds as though you're saying that prokaryote DNA not only isn't wrapped around histones, but also doesn't have a double helix, when in fact you meant only the former.

It's at times like this - when your articles are nominated for a blogging competition - that small editorial lapses can come back and bite you. :-)

Margaret said...

You're quite right, Outerboard--thanks for the heads-up!

I'll edit it, making clear what the original text was.



outerhoard said...

I'll forgive the misspelling of "outerhoard", which has a "h", not a "b". :-)

It's my blog's name rather than mine (after all, a blog is a hoard of thoughts, ideas, etc). But the convenience of signing with OpenID overrules the weirdness of using my blog's name to identify me. Pros and cons.

Margaret said...

Sorry about that. I did think outerboard was a rather odd name, but hey, this is the Internet. :)

Lee said...

Just found this article/post. This is terrific! Crystal clear explanation w/great illustrations.

Is the green sea slug the only example of an animal capturing functioning chloroplasts? Are there any observed cases of recent horizontal gene transfer? Is there speculation of what the threshold/conditions need to be for it to occur?

Thanks for such a stimulating post.

Lee said...

Just found this article/post. This is terrific! Crystal clear explanation w/great illustrations.

Is the green sea slug the only example of an animal capturing functioning chloroplasts? Are there any observed cases of recent horizontal gene transfer? Is there speculation of what the threshold/conditions need to be for it to occur?

Thanks for such a stimulating post.

Margaret said...

Hi Lee--thanks, and welcome! I'm glad you enjoyed the post.

I'm not aware of any other examples of chloroplast capture, but I'm told that one of the genetics PhD students in my biology department is doing research into the green sea slug, so I might follow up with her. If I discover more, I'll post it here.

Horizontal gene transfer is a pretty common process, especially in bacteria which frequently exchange genetic material. It's one of the means by which they develop immunity to antibiotics. A resistant bacterium can "share" the genes conferring that resistance to others. Check out this paper by some people in my department on "gene cassettes":

It also occurs in multicellular species. Lots of the human genome is in fact viral. See for example:

Bjørn Østman said...

I just found right now that Larry Moran has something to say about the idea that the double membranes were the result of endocytosis:

The original bacteria had a double membrane and that double membrane was an integral part of the energy producing pathway that became so important for the eukaryotic cell. It's simply not true that the double membranes of bacteria and chloroplasts were the result of endocytosis.

Margaret said...

Bjørn, that's very interesting, thanks. I'll follow up what he has to say. Wonder what Margulis thinks!

Margaret said...

Sorry, Ray, no.

Deepak Malik said...