The present invention provides a method of producing a photosynthetic product, the method comprising maintaining a photosynthetic plant or algal cell suspension culture, in the presence of water, light and a carbonic acid-enriched growth medium. The carbonic acid may, for example be provided by feeding the photosynthetic plant cell suspension culture with a carbonic acid solution, a solid or liquid precursor thereof, or a gaseous mixture of carbon dioxide and one or more other gases. The invention also provides a method for producing a photosynthetic product, the method comprising maintaining a photosynthetic plant or algal cell suspension culture, in the presence of water, light and a carbon source selected from carbon dioxide and carbonic acid, wherein the culture is maintained at a pH of less than 7.0, preferably 4.5 to 5.5.