Answer
Since the best lipid production for biofuels involves a combination of heterotrophic and autotrophic nutritional activity, along with stress, genetic engineering for optimal biofuel production should aim to upregulate the genes for both heterotrophy and autotrophy as well as genes associated with stress responses.
Work Step by Step
Just look at the text and see what factors (p. 342) lead to the greatest lipid production in microalgae. Upregulating these processes by upregulating expression of the genes involved should lead to miximal lipid/biofuel production.