this specification discloses a process and a culture composition for growth of an alga and synthesis of biopolymer by the alga . the process involves growth of the alga and concomitant synthesis of the biopolymer in an aqueous culture containing as a source of phosphate for said alga dibasic sodium or dibasic potassium phosphate . the nitrogen source can be sodium nitrate or urea . control of the ph of the culture is effected by injecting a mixture of carbon dioxide and air into the culture during growth of the alga and synthesis of the biopolymer . the carbon dioxide also serves as a source of carbon for growth of the alga and synthesis of the biopolymer and provides agitation / mixing within the culture chamber . preferably , the mixture of carbon dioxide and air is injected continuously into the culture during the entire growth period of the alga .