Dead wood has a very high carbon-to-nitrogen ratio: in addition to being difficult to break down, you need to eat an awful lot of it (and somehow eliminate the excess carbon) just to get enough nitrogen to digest it. Wood-decaying fungi deal with this by moving nitrogen acquired elsewhere into the wood (Watkinson et al. 2006), but what about wood-feeding termites?
It turns out that they can get at least some nitrogen from their environment (when fungi transport nitrogen into wood, termites can take advantage of that too, and they can seek out nitrogen-rich resources). Termites are also adapted to nitrogen scarcity: their preferred undertaking behaviour, cannibalism, allows them to recycle nutrients. Most intriguingly, termites also possess a bevy of nitrogen-fixing bacteria (diazotrophs) in their guts — symbionts that have long been assumed to account for termites’ ability to thrive on such a nutrient-poor diet. While there is evidence that these symbionts play some role in a colony’s nitrogen budget, what still isn’t clear is how much termites can rely on them in the absence of environmental nitrogen sources.
In a paper that came out last week, two University of Florida researchers, Aaron Mullins and Dr. Nan-Yao Su, investigated this question in the context of colony foundation. Wild-caught Formosan subterranean termite (Coptotermes formosanus) alates were housed in artificial habitats, each containing sand and a small (2 cm3) wood cube. Over the course of a year, ten established colonies were destructively sampled every two months to measure the amount of nitrogen present in each caste, as well as the amount of wood that had been consumed.
Mullins and Su predicted that prior to the production of the first workers, the royal couple would use previously-stored nitrogen to produce eggs and larvae. They were right: the king and queen transferred half of their nitrogen reserves to their brood within four months of colony foundation. Surprisingly, the couple’s nitrogen stores did not drop any further: they stayed more-or-less constant for the rest of the experiment.
Only after workers were produced, the authors reasoned, might we see the total nitrogen in the colony begin to increase: unlike larvae, workers have a full complement of gut symbionts, and they have a lot more diazotrophs than the royal pair. Surprisingly, this did not occur: the colony’s nitrogen content did not increase, and biomass remained unchanged after the initial parental investment. This suggests that at least during colony foundation, there is not enough nitrogen fixation in the gut to support colony growth.
The authors conclude that C. formosanus must rely on environmental nitrogen sources for colony foundation and early development, and they note that soil may be one such source: as I have also observed, subterranean termites do seem to do better when they have access to it.
I do however wonder whether this will apply to other species: if young C. formosanus colonies really do get the necessary nitrogen from soil (or soil fungi, as the case may be), how do drywood termites do it? The wood that they eat and nest in is not fungus-free, but it is typically soil-free, and we would expect far less microbial activity than in damp rotting wood on the forest floor. Do they have more efficient diazotrophs, do they conserve nitrogen better, do kings and queens just have larger nitrogen stores, or do they get it from some other source?
This paper provides evidence that in C. formosanus, at least, we have overestimated the importance of gut fauna in procuring nitrogen for colony growth. Do diazotrophs contribute to colony growth at some other stage of development, or in conjunction with dietary nitrogen sources? They might! But as is always the case with termite research, there is a lot of work left to be done.
Mullins, A. & Su, N.-Y. 2018 Parental Nitrogen Transfer and Apparent Absence of N2 Fixation during Colony Foundation in Coptotermes formosanus Shiraki. Insects 9, 37. (doi:10.3390/insects9020037)
Watkinson, S., Bebber, D., Darrah, P., Fricker, M., Tlalka, M., & Boddy, L. 2006 The role of wood decay fungi in the carbon and nitrogen dynamics of the forest floor. In G. Gadd (Ed.), Fungi in Biogeochemical Cycles (British Mycological Society Symposia, pp. 151-181). Cambridge: Cambridge University Press. (doi:10.1017/CBO9780511550522.008)
Formosan termite alates on a sticky trap photographed by Scott Bauer. https://www.ars.usda.gov/oc/images/photos/oct98/k8200-9/