Final answer:
The cellulase-excreting protist responsible for a significant portion of a termite's biomass is from the genus Trichonympha. They assist in digesting cellulose, creating a mutualistic relationship with the termite.
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. These anaerobic species exist in the digestive tracts of termites and are crucial for digesting cellulose ingested by the insects as they bore through wood. The enzyme cellulase, produced by bacteria within the protist cells, breaks down the cellulose into glucose monomers that can be used as an energy source by the termite. This mutualistic relationship between the termites and Trichonympha is an example of how these protists contribute to the nutritional needs of non-marine organisms as well.