After years of skepticism about the promise of biofuels, ExxonMobil has decided to make one of the biggest biofuel bets so far. Last week, the world's second largest company announced that it will spend up to $600 million over 5 to 6 years to produce biofuels from algae. Half the money will go to Synthetic Genomics Inc., a San Diego, California-based start-up run by genomics pioneer J.