The substance from the light-dependent reactions that is a source of energy for light-independent photosynthesis (also known as the Calvin cycle) is ATP (adenosine triphosphate).

Here's why:

* Light-dependent reactions: These reactions use light energy to create ATP and NADPH (nicotinamide adenine dinucleotide phosphate).

* Light-independent reactions: These reactions use ATP and NADPH, along with carbon dioxide, to produce glucose (sugar).

Therefore, the ATP generated in the light-dependent reactions acts as a direct energy source for the Calvin cycle to convert carbon dioxide into sugars.