The invention provides methods of transforming photosynthetic organisms, such as green algae. The methods involve methylating one or more DNA fragments of a DNA construct and transforming the one or more fragments into the photosynthetic organism. The DNA fragments can be the product of a DNA amplification procedure, such as PCR or a PCR-like procedure. In one embodiment the one or more fragments of DNA that comprise a DNA construct are dam methylated prior to being transformed into the photosynthetic organism.