What are rhizobia?
Rhizobia are legume root-nodule bacteria. They are soil bacteria that induce the formation of special structures (nodules) on the roots of their host plants. Inside these nodules, the rhizobia fix nitrogen. This means that they convert dinitrogen (the nitrogen gas that makes up 80% of the air you breathe) into ammonia. Ammonia is toxic, so it is rapidly assimilated into organic compounds, most of which the bacteria pass to the plant to fulfil its nutritional need for nitrogen. Nitrogen fixation is an energy-intensive process, and the bacteria take carbon compounds from the plant to fuel it. It is a symbiosis, because the bacteria live in intimate association with the plant, and a mutualism, because both partners gain.
That’s it, basically. Of course, there is much more to say about the diversity of both partners, the way in which they choose each other, the biochemistry of the interaction, the great importance of this nitrogen fixation for ecosystems and agriculture, and so on. However, it is not my purpose to write a textbook. There are many thousands of papers, reviews and books on this symbiosis.
If you want to know more, you could start by looking up “rhizobia” on Wikipedia. The article seems reasonably accurate, though it is very brief and has few references. Most of these relate to the “sanctions” hypothesis for the evolution of the mutualism, an interesting but tiny corner of the vast literature on rhizobia. I wonder who wrote the article. Anyone can write or rewrite a Wikipedia article, so if you want to do the community a service, why not flesh this article out a bit? Of course, there are other Wikipedia articles that have some relevant information. In fact, there is also one on “Rhizobium“.
When is a rhizobium not a Rhizobium?
Rhizobia are legume root nodule bacteria. A rhizobium is a legume root nodule bacterium. One rhizobium, many rhizobia – just like one bacterium, many bacteria. You would never make the mistake of writing “a bacteria”, would you? (I just wish the journalists at New Scientist were as well educated!) Words like “bacterium” and “rhizobium” are common nouns, like “bread” and “butter”, so they do not have to start with a capital letter. You can make adjectives from them: “bacterial”, “rhizobial”. By the way, it’s “rhizobia” not “rhizobiums” because the word is pretending to be Latin. It is only pretending, because it is actually derived from two Greek words that convey the meaning “root-dweller”. If it were really derived from Latin words it would not be “rhizobium” but something like “radicola”.
However, “Rhizobium” is a very different thing from “rhizobium”. It is the formal taxonomic name of a bacterial genus, and you can’t mess with it. It is always written just like that, in italics with a capital “R“. You certainly can’t write Rhizobia, because a genus name has no plural form.
Which bacteria are rhizobia?
Back at the beginning of time (or at the beginning of bacterial taxonomy, at any rate), all legume root-nodule bacteria were placed in the genus Rhizobium. Gradually, it was realised that they were rather diverse and, more importantly, some were more closely related to other, nonrhizobial bacteria. Some slow-growing rhizobia were split off into a new genus called Bradyrhizobium, and after that the number of bacterial genera that include rhizobia grew rapidly. Root-nodulating bacteria are increasingly turning up in genera that have already been created to describe other, non-nodulating bacteria, so the genus name is no longer a good guide to whether a bacterium will be a rhizobium.
If you want a recent list of rhizobial species, I recommend the web page maintained by Bevan Weir. He started it when he was a PhD student (setting up a useful web site is great way to get yourself noticed at the start of your career!). He’s now a postdoc working on fungi, but still maintains the list of rhizobia. Thanks, Bevan! For an authoritative list of all valid bacterial names, not just rhizobia, you should consult the List of Prokaryotic names with Standing in Nomenclature. There are several long-standing disagreements concerning the taxonomy of rhizobia and their relatives, though, and I shall probably write something about them in future posts.