Large holes are where algae live. The chambers in larger foram tests often serve as a home for algae and cyanobacteria as well. Forams also eat bacteria, diatoms, other protozoa and even small animals such as copepods. In turn, forams are an important food source for snails, sand dollars, fish and other creatures. Some species are single-celled creatures that can't be seen without a microscope.
Yeast is an example.
Other species form the multi-celled structures that we know as mushrooms. Obviously, these can be seen perfectly fine without a microscope. So you might be thinking that there are lots of big fungi. But there's one that's bigger than all the rest, that forms structures that spread for miles and that kills large trees. Read about it in the News section of the site.