The Green Leopard Plague - Walter Jon Williams
Mobi
"It is based on the idea of a genetically engineered virus that allows people to photosynthesize food, leading the world to an agalmic society, where there are no more food shortages. It begins in the far future with a mermaid who makes her living by searching old archives. She is approached by a customer who wants her to find information on the man who founded the theoretical background on which their civilization is based, John Terzian. It is eventually revealed that he was involved in the release of the photosynthesis virus. The story then veers back and forward between his story and the mermaid's."
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One of my all time favorite short stories.