Microbial Communities

We have discovered a new symbiosis, based on nutrients exchange,
between a plant-like microbe –Chlamydomonas- and
a plant growth-promoting bacterium –Methylobacterium-.

Chlamy-Methylo Cartoon 2

“The work started off with what was perhaps a rather fortuitous observation than then lead to a wonderful piece of scientific detective work”
– Anonymous peer reviewer

You must have heard about it: World population is expected to reach 9.8 billion in 2050, according to the last United Nations report. By that time, we need not only to at least double crop production, but to do it in a sustainable way if we want to feed humanity. For that, research in agriculture becomes crucial. The main focus in our research group is to study the metabolism of the most important nutrient in quantity for plant growth: nitrogen. This knowledge can be highly helpful to develop more efficient and sustainable crops.

Instead of working with land plants, we use a plant model organism that is unicellular, fast-growing, easier to handle and to genetically modify, and thus, easier to study: the green microalga Chlamydomonas. This alga has been extensively studied for more than 50 years helping researchers to better understand the physiology of not only microalgae, but also of higher organisms including land plants.

“The most exciting phrase to here in science, the one that heralds new discoveries,
is not ‘Eureka! (I’ve found it)’ but ‘That’s funny…’
– Isaac Asimov

During our study in nitrogen metabolism we discovered that, like plants, Chlamydomonas cannot assimilate some organic nitrogen compounds (e.g. some amino acids and peptides). Unexpectedly, one of our algal cultures was accidentally contaminated. Even though contaminations are in most cases truly annoying, I had a different feeling that time, that was funny: that contamination allowed the alga to grow on one of those organic compounds it was not able to use by itself. We found this fact definitely interesting and worthy to further investigate.

However, we were not familiar with symbioses so we decided to contact Erik in the Hom Lab, who demonstrated a symbiosis between this alga –Chlamydomonas– and a yeast -like the one is used for making bread or beer-. In his work at Harvard University (Cambridge, MA) he showed that both microbes exchanged nutrients creating a stable symbiosis. At that point, we started an exciting collaboration to figure out who was allowing Chlamydomonas to grow and how. I spent 4 fun months in the Hom lab –such an adventure!-.

It turned out that the contamination was a Methylobacterium species. These bacteria are present on the surface of many land plants. What do they do there? They are not pathogenic, but right the opposite. In a somehow similar way our gut bacteria play a fundamental role in human lives keeping us healthy, plants have established as well mutualistic relationships with bacteria. Thus, plants are surrounded with bacteria that benefit them, and so it is said that ‘plants wear their guts in the outside’^[1]. Among these let’s call them ‘plants-gut bacteria’ there is an important group of bacteria named methylobacteria, like the one we hunted in our algal culture. These are pink bacteria well known to improve plant growth in different ways such as inhibiting plant pathogens growth, degrading toxic compounds or breaking down complex nutrients transforming them into simpler molecules that plants can assimilate.

Then, as methylobacteria benefit plants, it was not surprising that these bacteria were helping this plant-like microbe Chlamydomonas as well. But, how was it allowing algal growth? Since the alga was not able to grow by itself on these complex compounds such as the amino acid proline, we assumed the alga could not use it as a nutrient for growth. This amino acid was the only carbon and nitrogen source added to the medium, both essential nutrients for growth. Therefore we assumed the alga must have been lacking at least one of them.

What nutrient was this alga missing? Since algae are photosynthetic organisms, they can use atmospheric carbon dioxide. Then, carbon did not look like the limiting nutrient. Therefore, nitrogen seemed to be the missing nutrient in the culture, and thus, the most likely nutrient bacteria was providing to the alga. As expected, we discovered that bacteria were releasing ammonium, an inorganic form of nitrogen algae can very efficiently grow on. Hence, these bacteria were breaking down complex nitrogen compounds feeding algae with simpler forms of nitrogen allowing its growth.

But, part of the mystery was still unsolved: were bacteria getting something back from algae? And if so, what was that? As most symbioses examples in nature, it seems likely that not just one but both partners get benefit from the symbiosis. Then, what could this alga provide to bacteria? Well, we know that as a consequence of photosynthetic metabolism in algae, organic carbon compounds are generated and can be released to the medium as waste products. Then, just like bacteria were making nitrogen accessible to algae, algae might be making carbon available for bacteria, we thought. After looking into the algal media we found out Chlamydomonas was releasing glycerol, a carbon compound that these bacteria can use as a carbon source to grow.

Thus, we discovered a new mutualism between green microalgae and pink bacteria in which a carbon-for-nitrogen exchange may take place in natural environments where inorganic forms of nitrogen are limiting but more complex nitrogenous compounds are present. Thus, we have found that these bacteria that are well-established benefiting plants are also benefiting green algae, which are the land plant ancestors. This information has a substantial potential in agronomic and evolutionary applications. Not only that, but also this new algal-bacterial system has a great potential in biotechnology since both organisms are already used separately in different applications that may lead now to new applications. In conclusion, the study of this interaction may reveal new keys about the rules that govern natural systems that can be used in our own benefit.