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  • Alex Cloherty

Microbial Buddies: Part II

Welcome back to the Microbial Buddies series! As promised, in Part II of the series we will be exploring another beautiful phenomenon: the ability of flowers to grow.

Like in last week's post, today we'll be exploring a symbiotic relationship using a model organism. Today's model organism is the fragrant wild lilac.

My parents have one of these in their backyard!

We always called them "butterfly bushes".

Before we dive deeper, we must first understand the importance of nitrogen to plants. In other MM posts, we briefly discussed the importance of DNA and proteins. You might also have heard about chlorophyll, a pigment (yes, like in paint) that is absolutely necessary for plant photosynthesis. It turns out that nitrogen is absolutely necessary for the existence of all of these molecules! Because plants need DNA, protein, and photosynthesis, they also need nitrogen.

This comes along with a bit of a problem. Nitrogen is very abundant on Earth – in fact, it's one of the most abundant elements (those things on the periodic table which you probably had to memorize for science class as a teenager)! But, unfortunately for plants, nitrogen is mostly present as a gas, N2. This is unfortunate because plants can only use nitrogen that is attached to a hydrogen (like NH3, ammonia).

Have you ever wondered why fertilizer is such a big business? Well, this is why. Fertilizer with high amounts of reduced nitrogen (nitrogen that's attached to a hydrogen) is one way to give plants this necessary element. But, humans weren't always around to pour nitrogen over plants - so these smarty-plants found their own way. Like many other plants, the wild lilac got to know Frankia.

Frankia? Who's Frank, you say? The convivially named bacterium Frankia, a genus of Actinobacteria, takes its name from the German biologist Albert Bernhard Frank. As a side note, it wasn't actually Frank who discovered Frankia. The discoverer was Norwegian botanist, politician, and diplomat – all around busy guy – Jørgen Brunchorst. However, it was A. B. Frank who, according to the bacteriologist's bible, Bergey's Manual of Systematic Bacteriology, coined the term "Symbiosis", which I have been sprinkling through these last posts like salt.

Back to Frankia. Frankia bacteria are able to "fix" nitrogen, meaning that they have a protein that facilitates conversion of nitrogen gas (N2) to ammonia (NH3). That's the magic step!

As another side note for those interested in microbiological history, the process of Nitrogen fixation was actually discovered by the man known as the father of virology (the study of viruses): Dutch microbiologist Martinus Willem Beijerinck. Another busy guy!

Back to that magic step: nitrogen fixation, or the conversion of gaseous nitrogen to ammonia. Bacteria like Frankia are able to do this for themselves, so that the bacteria can make the ammonia into proteins, nucleic acids (like DNA), and other materials for themselves. But why would they want to share the nitrogenous magic?

You might have guessed from some of the other symbioses we've discussed that it's a give-and-take. As mentioned above, one of the reasons that plants need this nitrogen so badly in the first place is to make chlorophyll for photosynthesis. The product of photosynthesis is sugar, which the plant needs. It turns out that Frankia can also make good use of sugar: namely, to provide the energy needed to carry out nitrogen fixation! The deal is, the wild lilac provides the sugar for the duo, while Frankia provides the nitrogen. It's a winning combination!

But how exactly does this look, structurally? This actually requires infection of the wild lilac with Frankia! The bacteria have to infiltrate the roots of the plant, and end up actually growing inside the root cells. Like in the Hawaiian bobtail squid, the intimate relationship between the bacteria and the eukaryote (the wild lilac in our case) results in the creation of a whole new organ: the actinorhizal nodule! These are basically pods in which the cells of the wild lilac intermingle extremely closely with the cells of Frankia, allowing the exchange of sugar for fixed nitrogen.

Without this beautiful symbiosis, many different plants that grow all around the world could not exist! For instance, alders, cliff rose, and dryas all form symbioses with Frankia. A similar symbiosis with Rhizobium bacterium in place of Frankia exists in legumes such as clover, soybeans, rooibos, and peanuts. That’s right, there would be no peanut butter without bacteria!

With that, we come to the end of Part II of the Microbial Buddies series. Next week, we'll discuss a different kind of symbiosis: one in which not all parties benefit.

Until then, when you stop and smell the flowers, stop to appreciate the bacteria, too!

- Alex

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