Based on the information provided, the organism most likely feeding on the rotting log sample found by Nathan in the wildlife sanctuary would be a fungus-like protist. Here's why:
1. The organism is eukaryotic, which means its cells have a nucleus and membrane-bound organelles. This characteristic is shared by both fungi and protists.
2. The organism cannot photosynthesize, indicating that it does not use sunlight to make its own food through photosynthesis like plants do.
3. The presence of a cell wall is a common feature of fungi, which have cell walls made of chitin, a structural polysaccharide.
4. Fungi-like protists exhibit similar characteristics to true fungi but belong to the protist kingdom due to some genetic and structural differences.
Therefore, based on the given information, the most likely type of organism feeding on the rotting log sample is a fungus-like protist.