Just as any shovelful of dirt may contain within it a thriving ecosystem of interdependent organisms, many animal species contain within their gut an equally active community of microorganisms, all collaborating to promote host survival. For the Formosan subterranean termite, Coptotermes formosanus, and its relatives, this means facilitating these insects’ destructive appetites and assisting them in extracting sustenance from wood—a challenging and relatively nutrient-poor food source.
Scientists have confidently fingered one protist, Pseudotrichonympha grassii, as this termite’s primary accomplice. “C. formosanus cannot survive without P. grassii, even when other protist species remain abundant,” explains Yuichi Hongoh of the RIKEN Advanced Science Institute in Wako. The picture gets more complicated, however, as this microbe is itself a haven for numerous tiny bacteria known as endosymbionts, which also make vital contributions to the gut ecosystem (Fig. 1).
Termite gut flora have proven difficult to cultivate, making it challenging to understand how they contribute to termite survival. However, powerful new DNA sequencing methods have now enabled Hongoh and his colleagues to fully decode the genome of an essential bacterium from C. formosanus, yielding valuable insights into the ‘black box’ of termite wood digestion (1).
They targeted the endosymbiont CfPt1-2, which resides within P. grassii and is the most abundant bacterium in the C. formosanus gut. From a single cell, they extracted thousands of CfPt1-2 bacteria, obtaining enough genetic material to derive its complete genomic sequence. These data provided valuable details about how CfPt1-2 facilitates termite survival, revealing genes that convert dinitrogen in the air into essential amino acids that are otherwise scarce in the termite diet.
Equally importantly, CfPt1-2 appears to assist P. grassii host cells by recycling nitrogenous wastes such as ammonia into more useful compounds for biosynthesis. This bacterium also has the apparent capacity to produce energy from hydrogen waste generated during the wood digestion process.
On the other hand, the evolutionary streamlining of this endosymbiont’s genome has resulted in the loss of other essential functions that make it equally dependent upon P. grassii for survival. “The CfPt1-2 bacterium has evolved like an organelle, which cannot live outside the host cell,” explains Hongoh. “Long-term evolution has established this elaborate, multi-layered symbiosis.”
Hongoh believes further exploration of the P. grassii-CfPt1-2 partnership will not only yield valuable insights about how they sustain their termite hosts, but may even facilitate development of effective strategies for extracting energy from wood-based biofuels.
1. Hongoh, Y., Sharma, V.K., Prakash, T., Noda, S., Toh, H., Taylor, T.D., Kudo, T., Sakaki, Y., Toyoda, A., Hattori, M. & Ohkuma, M. Genome of an endosymbiont coupling N2 fixation to cellulolysis within protist cells in termite gut. Science 322, 1108–1109 (2008).
The corresponding author for this highlight is based at the RIKEN Ecomolecular Biorecycling Science Research Team