Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

plankton are drifting organisms that live in the surface layers of the ocean.
plankton

"god defend new zealand" is one of the national anthems of new zealand.
god defend new zealand

estaimpuis is a municipality in the belgian province of hainaut.

estaimpuis