Plankton loaf was a type of Human bread made from tiny creatures that lived in the oceans of Earth.
When she was growing up, Keiko O'Brien regularly ate plankton loaf, kelp buds, and sea berries, a typical Japanese meal. However, her husband Miles did not share her affinity for seafood, preferring foods such as bacon and eggs and potato casserole. (TNG: "The Wounded")