A biotech company plans to announce Tuesday that it has won a patent on a genetically altered bacterium that converts sunlight and carbon dioxide into ingredients of diesel fuel, a step that could provide a new pathway for making ethanol or a diesel replacement that skips several cumbersome and expensive steps in existing methods. The bacterium’s product, which it secretes like sweat, is a class of hydrocarbon molecules called alkanes that are chemically indistinguishable from the ones made in oil refineries. The organism can grow in bodies of water unfit for drinking or on land that is useless for farming,...