The oil giant Exxon Mobil, whose chief executive once mocked alternative energy by referring to ethanol as “moonshine,” is about to venture into biofuels. Exxon Mobil Corp. said that it will make its first major investment in greenhouse-gas reducing biofuels in a $600 million partnership with biotech company Synthetic Genomics Inc. to develop transportation fuels from algae.

The agreement could plug a major gap in the strategy of Exxon, the world’s largest and richest publicly traded oil company, which has been criticized by environmental groups for dismissing concerns about global warming in the past and its reluctance to develop renewable fuels.

Despite the widely publicized “moonshine” remark a few years ago by Exxon’s chairman and chief executive, Rex W. Tillerson, the company has spent several years exploring various fuel alternatives, according to one of its top research officials.

“We literally looked at every option we could think of, with several key parameters in mind,” said Emil Jacobs, vice president for research and development at Exxon’s research and engineering unit. “Scale was the first. For transportation fuels, if you can’t see whether you can scale a technology up, then you have to question whether you need to be involved at all.”

He added, “I am not going to sugarcoat this — this is not going to be easy.” Any large-scale commercial plants to produce algae-based fuels are at least 5 to 10 years away, Dr. Jacobs said.

Exxon’s sincerity and commitment will almost certainly be questioned by its most galvanized environmentalist critics, especially when compared with the company’s extraordinary profits from petroleum in recent years.

“Research is great, but we need to see new products in the market,” Kert Davies, the research director at Greenpeace, said. “We’ve always said that major oil companies have to be involved. But the question is whether companies are simply paying lip service to something or whether they are putting their weight and power behind it.”