this MIT study for ~100 years supply at present rates of consumptionP. 12 of that study says "most commentators conclude that a half century of unimpeded growth is possible", and the study appears to agree with that assessment. "Unimpeded growth" means a large increase in consumption rate over the present.
Also, that assessment is based on a once-through fuel cycle and mining as the only uranium source. Over a half century to a century time scale the first assumption is almost certainly invalid (breeder technology is advancing) and the second may be shaky as well. (Note, btw, that a key reason why the MIT study assumed a once-through fuel cycle was not that breeders were not available, but that they were uneconomic at current uranium prices; but obviously if uranium gets scarcer, uranium prices go up and breeders become economically viable.)
Finally, a half century is not 25 years. If we have a half century to a century of nuclear power even with a once-through fuel cycle, we have an obvious alternative to fossil fuels for that time frame. Your claim that we will hit an energy crisis in 25 years assumes that there is no such alternative.
Tell me how much algae fuel you want and what conversion pathways (biodiesel, ethanol, synthesis, solids) you're referring to. Then give me acreages and water estimates. Oh, and how you're going to fertilize all that goop and keep pests out of it.
All of which are issues that are going to be solved, if they're not solved already, probably well within your 25 years, certainly within a half century. Google turns up plenty of hits on "algae fuel" or "algae fuel study" that show plenty of progress in this area.
the foundation for your optimism
Human ingenuity plus profit motive plus an obvious market for cleaner forms of energy. What I noted above with regard to the assessment of breeders being viable is true more generally: a key limitation of most assessments of available energy resources is that they only look at what's economical given current prices, not what's possible. But the economics will change.
That's not to say that there are no obstacles; but as I said before, the main obstacles I see are political, not technical.