The present invention provides methods and compositions for sustained methane production from atmospheric CO2 and solar energy from the sun. In general the methods involve culturing cyanobacteria in a first culture vessel and collecting and diverting the photosynthesis products, including glucose or acetic acid, to a second culture vessel including methanogenic bacteria. The photosynthesis products are then used as nutrients by the methanogenic bacteria in the second culture vessel in the production of methane. The methane produced is subsequently collected and used as a clean energy source. The invention also features compositions, including genetically modified cyanobacteria and systems for use in the methods of the invention.