I got a question for you: Cyanobacteria / sanobktri /, also known as Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis, and are the only photosynthetic prokaryotes able to produce oxygen. The name "cyanobacteria" comes from the color of the bacteria (Greek: ( kyanos) = blue ). Cyanobacteria (which are prokaryotes) used to be called "blue-green algae". They have been renamed' cyanobacteria' in order to avoid the term "algae", which in modern usage is restricted to eukaryotes.

Based on this passage, where do blue-green algae get their energy from?
Let's think first:
The relevant information to answer the above question is: Cyanobacteria / sanobktri /, also known as Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis, and are the only photosynthetic prokaryotes able to produce oxygen.
Therefore, the answer is through photosynthesis.