The study is based on computer models of the entire production process and reveals that current algae production takes six times the energy of growing crops and emits more greenhouse gases. The problem is apparently the amount of fertilizer required to grow the algae. Thankfully, the fertilizer requirements are dramatically reduced if the algae is fed with sewage waste water which contains nutrients that help it grow. Getting the waste water to the the algae ponds or reactors does require some infrastructure investment, though.
[Source: New Scientist | Image: Jurvetson - C.C. License 2.0]