When it Takes Guts to Take Up Nitrogen

One of the more stunning biological discoveries to date is that organisms like ants are not merely individual organisms, but also hosts to trillions of bacteria. Some biologists have increasingly focused on this “microbiome”, and naturally work in ants is no exception. A few months ago, a group of ant researchers discovered yet another cool thing about ants and their microbiome!

Cephalotes species. Images: Alex Wild


As reported in Nature Communications, Dr. Yi Hu and several collaborators found that a number of gut bacteria form a symbiosis with their hosts, Cephalotes turtle ants, taking up nitrogen and converting it to essential amino acids that the host ants require. In order to determine this, the researchers provided the ants with a special diet: an artificial form of urea (the key component in urine) that included an isotope of nitrogen, 15N, rather than the usual 14N. Then, they could determine how much of the nitrogen from the 15N-labeled urea was taken up by the ants, using gas chromatography–mass spectrometry. Hu and colleagues also tested nitrogen uptake in the absence of bacteria, using an antibiotic treatment, and showed a significant reduction in 15N uptake. So it seems that, indeed, the microbiome of turtle ants is critical to essential nitrogen uptake in these ants!

Why such a complicated set-up just to get nitrogen? Cephalotes ants are herbivorous, typically dieting on plant exudes, pollen, honeydew, or fungi. This type of diet is often considered challenging, as it is nitrogen-poor, and animals require nitrogen to live. So, turtle ants also consume vertebrate urine and feces, which are sources rich in nitrogen. But wait! This waste nitrogen is in a form inaccessible to animals on their own, and thus microbes are required to convert the nitrogen to a usable form. If turtle ants want nitrogen, they have only one place to turn: their friendly microbiome.

Accessing food resources like pollen likely gives turtle ants an advantage over competitor ant species that are unable to use such food resources in their environment. But in order to gain this advantage, Cephalotes ants had to first evolve a symbiosis with bacteria, which is likely not so easy. Perhaps this explains why Hu and colleagues also found that the turtle ants’ association with core nitrogen-converting bacteria has apparently remained largely unchanged over 46 million years of turtle ant evolution. If it ain’t broke, why fix it?