The end of oil is imagined as a time of tightening — of air travel only for the rich and no more cheap bananas from Ecuador. An Ecotopia life with windmills and electric trucks, crowded buses and high-tax light rail. A lifestyle pinched.

I'm not eager to live in that world. Last week, I allowed myself to imagine another one, in which the rising price of petroleum calls forth human invention, and one kind of oil slickly replaces another.

This alternate dream came at a conference about algae. The dream is of oil wells replaced by green ponds of microscopic plants, each one genetically modified to excrete oil.

Plants that poop petroleum.

In September, $100 million was invested in a company that promises to do that. On its Web page, Sapphire Energy, a San Diego startup bankrolled partly by money from Bill Gates, promises "a completely new source of petroleum — domestically produced, 100 percent carbon neutral, and identical in composition to fossil fuels."

People promise a lot of things, and even Bill Gates makes mistakes, so don't count on this. But that sort of vision drew 600 people to Seattle last week to the Algae Biomass Summit. Drawn by the excitement of slime were university researchers, patent attorneys and entrepreneurs. Some came from Britain and Israel. Michael Weaver, 47, came from Redmond, promoting a company, Bionavitas, offering a new way to stir photons into ponds. Boeing was there as an observer. Venture capitalist Vinod Khosla was there.

One speaker said a billion dollars had been invested in algae companies over the past two years. None of the money is Khosla's. The famed Silicon Valley venture man has looked at dozens of algae deals and rejected every one. But he's interested.

Khosla is looking for the big enchilada: the replacement for petroleum all over the world, including China and India. This cannot be done with an incremental step like hybrids.

"I drive a hybrid," he said. "I love hybrids." But hybrids are not the long-term answer. He advised researchers to aim high.

Khosla wants a technology that can be scaled up. Corn ethanol can't because it eats up farmland. Algae can be grown in the desert, in ponds.

The fuels of the future must be unsubsidized, Khosla said, "because there is not enough money in all the world to subsidize them."

Algae's advantage is that it can be done on cheap land. Its disadvantage is that it's high-maintenance. The bottom line is whether it can compete with new offshore oil — a benchmark Khosla pegs conservatively at $50 a barrel. "It is not a test algae meets," he said. Others say a benchmark of $60 to $90, but are not claiming to have met that test, either.

Several are working on it. Robert Nelsen, managing director of ARCH Venture Partners, Seattle, and a board member at Sapphire Energy, said with some bravado, "Were talking about a million barrels a day or 5 million barrels a day...we assume no subsidy and no carbon tax."

I noted, however, that Sapphire Energy's Web page says its next step is a plant to produce 10,000 barrels a day — 1 percent of its director's dream — and that CEO Jason Pyle complains about the "limited incentives" available from the federal government.

At a news conference, Khosla said he was not sure that the replacement for petroleum would be algae. It would be one of several things now being investigated. "We need to try all of them," he said.

I asked Khosla (who called himself an "Obama Republican") if he thought people would live better in a material way 20 years from now, or worse. "Better," he said. "We'll have a higher standard of living 25 years from now. We will use much more energy than today, and it will have a much lower carbon footprint."

I'm not sure whether to believe this, but I like it.

Bruce Ramsey writes for the Seattle Times.