Final answer:
The cellulase-excreting protist found in a termite's gut that forms a significant part of its biomass is from the genus Trichonympha.
Step-by-step explanation:
The cellulase-excreting protist that can account for up to one-third of the biomass of a termite is a member of the genus Trichonympha. This organism resides in the termite's gut and plays a crucial role in the mutualistic relationship by digesting cellulose, which is a major component of the wood that termites consume. Not only do these protists help the termite to digest wood, but they also transform the cellulose into glucose monomers that termites can utilize as an energy source. This process is enabled by the presence of bacteria within the cells of the protists, which produce the necessary cellulase enzymes.