Simplify the following text for a Grade 10 native English-speaking person.
Original: They feed directly on minute phytoplankton , thereby using the primary production energy that the phytoplankton originally derived from the sun in order to sustain their pelagic ( open ocean ) life cycle .
Simplified: They feed directly on very small phytoplankton , so they can use the energy that the phytoplankton originally got from the sun in order to sustain their life in the open ocean .