Because of their ability to convert dissolved organic matter into cell substances and to synthesize protoplasm below the depths to which radiant energy penetrates sea water, bacteria may play an important role in the nutrition of marine animals. It has been well established experimentally that bacteria serve as an important source of food supply for protozoa (Luck et al, 1931) but it is not definitely known to what extent they may play a role in the nutrition of higher forms of animal life....
Zobell, Claude E., and Catharine B. Feltham. 1938. "Bacteria as food for certain marine inverebrates." Journal of Marine Research 1, (4). https://elischolar.library.yale.edu/journal_of_marine_research/536