A TV ad for ExxonMobil featured a scientist talking about researching algae as a source of biofuel.
In the advert, a scientist said that "in using algae to form biofuels, we're not competing with the food supply, and they absorb CO2, so they help solve the greenhouse problem as well".
But a complainant, who noted that any carbon dioxide absorbed by algae would be re-released back into the atmosphere when it was burned as fuel, objected that the ad misleadingly implied that the technology would reduce CO2 levels.
ExxonMobil said that one of the advantages associated with second-generation biofuels like algae was their potential to reduce greenhouse gas emissions by partially replacing conventional transport fuels derived from hydrocarbons.
The company said this was because biofuel feedstocks absorbed CO2 from the atmosphere and therefore the gases emitted during their combustion did not contribute to additional emissions.
ExxonMobil also cited a study by the European Commission's in-house research body, the Joint Research Centre, which they claimed proved that second-generation biofuels achieved greenhouse gas reductions on a comparative basis.
However, the UK Advertising Standards Authority (ASA) considered that the ad's claim "went beyond stating the mitigation benefit".
According to the authority, the scientist's statement may mislead viewers to conclude that it was due to the absorption of CO2 from the atmosphere that using algae to form biofuels helped "solve the greenhouse problem," by acting as a carbon sink.
The ASA said it understood that any CO2 absorbed by algae would eventually be re-released into the atmosphere and concluded that the ad was misleading because it "overstated the technology's total environmental impact".
The UK authority previously upheld a complaint against an advert by Shell, another oil giant. It judged that the ad, which showed refinery chimneys sprouting flowers, misled viewers into thinking that the company uses all of its waste CO2 to grow flowers.