It seems at first like an odd couple—the world’s biggest oil company jumping into a deal with a California inventor whose latest project is finding a way to replace petroleum with algae.

But that’s just what happened in July when Exxon Mobil announced a deal with Craig Venter’s Synthetic Genomics to develop a new biofuel. And while it may be the biggest such deal, at up to $600 million, between an oil company and an alternative fuel company, it’s far from the first.

Oil companies have long been at play in the alternative-energy field. They’re covering their bets, they say, preparing for a world that needs all sources of energy—including alternatives to oil. Virtually every major oil company has invested in biofuels, or solar, or using hydrogen as a fuel, or geothermal energy, or wind. Their critics, though—and there are many—point out that the oil majors’ investments in alternatives are dwarfed by their continuing spending on the quest to find more crude. They should, in short, act less like oil companies and more like energy companies.

Exxon’s initial investment in its joint venture with Venter’s company is $300 million for Synthetic Genomics and another $300 million within Exxon on the joint venture. The oil company will work with Venter’s company on growing an algae that can produce biofuel that could be substituted for petroleum—the holy grail of biofuel research. If the research pans out, the company could spend billions getting it into mainstream use.

“This is not going to be easy, and there are no guarantees of success, but we’re combining Exxon Mobil’s technical and financial strength with a leader in biosciences and genomics to take on this challenge, and we’re very excited about it,” Emil Jacobs, Exxon’s vice president of research and development, said at the July press conference announcing the joint venture.

Exxon made its move into green energy at a fortuitous time. Everywhere, it seems, are signs that public opinion and governmental attitudes about energy sources and their impact on the environment are changing. This week, the United Nations is hosting the highest-level conference yet on the topic of climate change, and President Obama was one of many to vow support for a global climate pact.

Among major oil companies, Chevron is already the world’s biggest producer of geothermal energy, has investments in biofuel research at universities, and has a division that works on finding energy efficiencies in buildings like schools.

“The big picture really is that longer term—where we see global energy demand—we’re pretty much going to need every energy molecule,” said Chevron spokesman Alex Yelland. “Given the growth in the developing world, we’re looking across all energy sources to where we can meet that demand. What we’re trying to do on the renewable side is build businesses that can scale up.” The company’s venture capital arm has invested $240 million since 2000, about a third of it in alternative energy.

BP, which for years marketed itself as Beyond Petroleum, has investments in solar, wind energy, and biofuel projects.

“We’ve taken a significant step into the alternative-energy business and are working hard on trying to grow that into a profitable business,” spokesman Tom Mueller said. “We see lots of potential for future income from energy projects. Otherwise we wouldn’t be investing in them, but it is a long road to profitability in the alternative-energy sphere.”

For all that, though, the oil companies are still, after all, oil companies. Critics say their investments in alternative energy are paltry compared to the amount they could be spending—and a fraction of a fraction of what they actually do spend looking for oil.

“They’re token at best, and they’re getting worse,” said Steve Kretzman of Oil Change International, a Washington, D.C.-based nonprofit advocacy organization. He estimated that Exxon spent $10 million last year on alternatives, Conoco Phillips spent $165 million, Shell spent $500 million, Chevron spent $1.25 billion, and BP spent $1.5 billion.