How Did Amazing Tiny Structures Called Plastids Help Make Plants and People Possible?
By: Rich Sobel
Tags: Evolution, Science, Microbiology, Bacteria, Algae

How Did Amazing Tiny Structures Called Plastids Help Make Plants and People Possible?

Evolution is rarely straightforward and the selection for symbiotic plastids is a big part of the reason why plants and you exist. Rich Sobel Follow Dec 4, 2020 · 15 min read

This image was taken from the Wikipedia article about plastids and was photographed by Kristian Peters

What do you really know about how evolution produced creatures like the giant California Redwoods, or whales, elephants and humans?

What if I told you it all came about in no small part due to some tiny microscopic structures that are found inside one particular kind of cell?

And that these tiny structures are organelles called plastids.

All right, I can already hear you saying, why would I ever want to know about plastids?!

Simple. Without plastids, you probably wouldn’t be here to read this today.

Seriously. They’re. That. Important!

In this article, we’ll delve into how they came to be, where they’re found, what they do, how they evolved, how they are maintained and reproduced in cells, how they get passed to new cells and anything else about them we need to know!

So, let’s learn a bit about plastids and in particular, the ones found in red algae and see the critical role they played in the development of life on this planet.

What are plastids?

I know that you have heard of chloroplasts. These are the plastids in plants that convert sunlight and basic chemicals to nutrients and energy by the process we know as photosynthesis.

So you DO have some notion of what plastids are, at least one kind!

But that’s only one of several different kinds of plastids found in many different forms of life on our planet.

As I said, plastids are tiny organelles found in most plants and algae. You can also find them in ferns, mosses, sea slugs and some parasitic worms.

Derivation- The term plastid was derived from the Greek, πλαστός; plastós: formed, molded – plural plastids)

Plastids are where photosynthesis and many other biochemical pathways take place in these organisms. Plastids contain a small genome, but most of their proteins are formed from, and are encoded by, genes in the nucleus. Once the protein is made, it is transported by cellular “machinery” to the organelle.

The figure below shows the current thought of how plastids are related and evolved; it gives you an idea of several different types of plastids and the organisms they are found in. The plastids are shown as little ovals on the lines with a colour dot.

This figure was taken from this article. Credit: S Sibbald and J Archibald

A quick look shows there are several different kinds of plastids in lots of different organisms! (And classification names of beasties that I’m sure you’ve never heard of before, too. Just enjoy looking at all the pictures.)

In this article, we’ll be exploring and learning about the plastids found in the category positioned at about 9 o’clock and designated Archaeplastida.

The main structural feature that distinguishes plastids from other cellular organelles is that they have a double membrane. And like mitochondria (another tiny organelle similar to plastids and found in most cells) they also have a small amount of DNA with functional genes.

Plastids contribute to organisms by manufacturing and storing nutrients. Different plastids can make amino acids, lipids, and perform photosynthesis.

Let’s take a closer look at the plastid you’re most familiar with, a typical plant chloroplast.

This figure was adapted from the one shown on BioNinja’s page

As you can see, the structure is fairly complex.

Details, in case you're interested: The inside cavity is called the stroma and contains various biochemicals that enable metabolic processes to go forward. It also contains the plastid's DNA. A granum is a stack of thylakoids, the flattened discs that contain the pigments that absorb light and perform photosynthesis. The lamella connect and organize the stacks of thylakoids, or grana. And you can see the two membranes, inner and outer.

And that’s a quick overview of the chloroplast. Really quick!

Now you should have a basic idea of what a plastid is — a microscopic organelle that is made up of a double membrane, contains a small amount of genomic DNA, and has other structural components and biochemicals that it uses to synthesize food and energy.

(FYI, the feature image at the top of this article is a photomicrograph of green chloroplasts inside an array of plant cells)

The origin of plastids

So, how did plastids come to be in the first place? Did the first cells have them?

To answer that question we need to put a few more pieces of Biology in place.

So, back to your basic introductory Biology 101 course.

Do you remember that there are cells that package their DNA and genes in a membrane-enclosed organelle called a nucleus? And that all the organisms that are made up of cells with a nucleus are called Eukaryotes*?

The organisms that have their DNA kind of floating around inside the cell as a nucleoid with no defined membrane or specific location are called Prokaryotes* and are usually single-celled beasties.

karyo- is a word-forming element used since c. 1874 in biological terms referring to cell nuclei, from Greek karyon "nut, kernel," possibly from PIE root *kar- "hard," but Beekes leans toward the notion that it is a Pre-Greek word. Taken from - is a word-forming element used since c. 1874 in biological terms referring to cell nuclei, from Greek"nut, kernel," possibly from PIE root *kar- "hard," but Beekes leans toward the notion that it is a Pre-Greek word. Taken from here Eu- means "true" and the prefix Pro- means "before" or "no", hence Eukaryotes are creatures with a true cell nucleus and Prokaryotes are those that came before and don't have a true cell nucleus. Taken from The prefixmeans "true" and the prefixmeans "before" or "no", hence Eukaryotes are creatures with a true cell nucleus and Prokaryotes are those that came before and don't have a true cell nucleus. Taken from here

Without getting too technical the simplified cartoon below shows some of the differences and similarities between eukaryotic and prokaryotic cells.

Image is taken from this article.

One important thing to note is that the Prokaryotic cell has a double membrane made up of an external cell wall and an internal cell membrane.

Ok, back to the origin of plastids.

Oops. Before we can do that, we need another quick side trip. You need to know about another organism, the cyanobacteria.

Cyanobacteria are also commonly known as blue-green algae but in fact, they are, as their name indicates, a category of bacteria. They are found in all types of water — marine, brackish, fresh.

Like plants, they use sunlight to make their own food. But unlike plants, their photosynthesizing pigment is not green, hence their blue-green or cyan colour.

For hundreds of great pictures of cyanobacteria, check out this page.

A species of cyanobacteria, oscillatoria taken from here and provided by the University of Wisconsin Botanical Images Collection.

They are also the oldest known fossilized creatures, some fossils being over 3.5 billion years old. And they are one of the largest and most important groups of bacteria.

So as you can see, they have been around for a really long time!

Also, they can be toxic to other organisms in the environment they are growing if they get too prolific.

Here’s where, finally, we get back to the origin of plastids!

The way the thinking goes is that back in the distant past, a single-celled eukaryote ingested a prokaryotic cyanobacterium. And instead of simply digesting the cyanobacterium and going along on its merry way, it formed a mutually beneficial symbiotic relationship.

A study in 2017 suggests that the closest relative to plastids among cyanobacteria that are living today is the recently discovered freshwater-dwelling Gloeomargarita lithophora.

This image is taken from this article.

Of course, that took a few millions of years but, hey, we’re not counting, are we?

For the record, this particular relationship is called endosymbiosis (the prefix “endo” means inside of and “symbiosis” means living together).

If you want to play really loosey-goosey with the terminology, roommates could be said to be in an endosymbiotic relationship, right? Or any partners that live together in the same dwelling.

But I digress.

Why would that original eukaryotic cell not just get rid of the cyanobacterium as waste if it couldn’t eat and digest it?

Remember what chloroplasts do? Their green chlorophyll uses sunlight to convert water and carbon dioxide into carbohydrates.

Carbohydrates are food! And cells need to eat to stay alive.

And remember that above, I said that cyanobacteria can do that too; photosynthesize to convert sunlight, carbon dioxide and water into carbohydrates for food and energy.

The eukaryotic host could use those carbohydrates to generate the energy it needed and in return, protect the cyanobacterium from being eaten by other predatory cells.

Hmmm, that sounds like a pretty good arrangement to me. A eukaryotic cell “eats” the prokaryotic cyanobacterium, and gets all kinds of nutrients and energy without having to really do anything else!

And if the prokaryotic cyanobacteria can live inside the eukaryotic cell, it doesn’t have to worry about getting eaten by any other creatures.

Who said there’s no such thing as a free lunch! And poof! the first photosynthetic eukaryotes were formed.

In evolutionary terms, endosymbiotic plastids gave their hosts a selective advantage over similar organisms that lacked them. At least that’s the supposition.

Over the millennia, most of the initially ingested cyanobacterium’s internal structures disappeared. What remained was its membranes, a bit of DNA and the photosynthetic or chemosynthetic “machinery” parts.

And that is how plastids came to be, why they were “accepted” or tolerated as a friendly symbiote, and eventually evolved and adapted.

For some people, that would be enough and the end of the story.

But if like me, you’re a curious person, you gotta know that other curious people were driven to find out way more intricate details, like which cyanobacteria, which eukaryotic cell, when this actually happened, and so forth.

Nowadays, with modern DNA sequencing technology, we can delve deeper into some of those questions and hopefully get a few answers.

And when scientists did, that’s where the picture started to include red algae and its evolution.

One type of red algae or Rhodophyta. The image is taken from here.

Red algae aka Rhodophyta

The red algae started to branch off from the other groups on the tree of life about 1.2 billion years ago. They are thought to be the oldest eukaryotic algae and today, are one of the largest groups of algae with over 7000 identified species!

“Once considered ‘primitive’ eukaryotes, red algae come in all shapes and sizes, from morphologically complex multicellular forms (including the nori used to wrap sushi) to single-celled acid- and heat-loving extremophiles.” from Colp & Archibald

In the evolutionary “Tree of Life” cartoon below, you can see that the bacteria are at the bottom while the next higher evolutionary branch is on the left and contains both the red and brown algae.

A simple version of the Tree of Life

The red algae also have the greatest diversity of plastids and were the source of plastids for many other groups, including some ecologically important diatoms, haptophytes and dinoflagellates found in our modern-day oceans, that are also fairly low down on the tree.

Remember I mentioned that plastids contain DNA?

Scientists interested in plastid origins and evolution started to look at that DNA and see what genes it contained.

What they found was in other organisms that contain plastids, the number of genes present in the plastid’s DNA tended to shrink over the millennia. However, in red algae, once it reached a certain level, it seems to have remained relatively constant. And compared to other algae, the actual number of genes in the red algae’s nuclear DNA has undergone a reduction.

And here’s where it gets interesting.

The ancient cyanobacteria are thought to have had about 2–3 thousand genes while today’s endosymbiotic plastids typically contain up to about 200 or so genes. So 90% of the original genome has disappeared.

How could this happen?

Endosymbiotic cyanobacteria could have lost their genes by 2 different processes. Many of the genes were likely “transferred” into the host's genome and others were simply no longer necessary and were “lost”.

Back in 2017, the scientists Huan Qui at Rutgers University in New Jersey and Jun Mo Lee at Sungkyunkwan University in Suwon, Korea hypothesized that the reason the red algae plastids contained more genes than plastids in other organisms was that the red algae lost parts of their genomic DNA.

That loss would be ok if the genes their plastid’s genes compensated for the loss. And if that was the case, losing plastid genes was not a good option as that would reduce chances for survival of both the plastid and the red algae.

They admit that the evidence they have accumulated was not sufficient to declare this to be the sole reason for this situation and that other “contributing factors …remain to be fully investigated”.

And that was where things stood in 2017.

Enter Rhodelphidia

Fast forward another couple of years to 2019 and a paper by Ryan Gawryluk and Denis Tikhonenkov where they describe some interesting new findings.

First, they discovered two previously undescribed species that constituted a new phylum, Rhodelphidia. When they sequenced parts of the genomes of these two species, they found that they are highly related to red algae.

That was very interesting because other than having closely related genomic sequences they didn’t resemble other red algae in any obvious way.

Typical red algae are multicellular, photosynthetic, and lack any structures that would confer motility, such as a flagellum. And as we mentioned, they have relatively small genomes and plastids with more genes than are usually found in green algae or the Glaucophyta.

In contrast, the two Rhodelphidia species, Rhodelphis marinus and Rhodelphis limneticus are single-celled, non-photosynthetic, flagellate predators. And get this. They have extensive genomes without any gene loss while the plastid they contain is DNA deficient and may even be a relic plastid.