The phrase "certain fungi" refers to specific types of fungi. It means that not all fungi are being referred to, but only particular ones that meet a specific condition or characteristic. Full definition
Poisonous and cancer - causing, aflatoxins are a type of mycotoxins produced by certain fungi (Aspergillus flavus, for example) growing on and in certain foods. (sciencedaily.com)
Plants and algae, as well as certain fungi and bacteria, also synthesize carotenoids, and in all of these organisms the pigments form part of the photosynthetic machinery. (scientificamerican.com)
Certain fungi form specialised cellular structures or «traps» to feed on nematodes (a type of worm). (natureasia.com)