Story: humans trying to escape economic slavery by finding aliens' nuclear conversion method

Question

This was a short story or novella I read before 2010, likely in an anthology, and definitely printed.

Earth's economy had come to rely heavily on Mars for nuclear fuels that could be produced more cheaply there than on Earth. I think the Martian population were humans in a long-established colony.

Mars was contacted unexpectedly by an alien race from a remote star-system. The aliens had a technology that could transmit non-living matter over huge distances, and wanted to trade.

The aliens could easily convert uranium isotope 238 into 235, which provided Mars with an even cheaper way to produce their fuels. Eventually the economies of both Earth and Mars relied on the aliens, to the point that the aliens were calling the shots and humans felt they had been effectively conquered.

Two humans made the long journey to the alien home-worlds to try to learn how the conversion was done. They had to fly past at least one alien ship.

The aliens had a scanner that let them see and hear what was happening in the humans' ship, but the humans could tell when it was being used so they acted oddly to confuse the aliens. The scanner relied on whatever ambient light was in the area being scanned. At one point, knowing the aliens couldn't see red light, the humans switched their cabin lights to red and communicated in written notes while pretending to sleep.

They landed on an alien planet and found a huge, low, flat-roofed building with very square angles. One of them described it as a cubist's idea of a cornfield.

Eventually they

found a wild alien bacterium that naturally converted 238U to 235U, and were able to return it to Earth.

Answer 1

You're describing "Artnan Process" (1941) by Theodore Sturgeon.

Reference to "cubist":

"Huh!" Bell ran a thick forefinger up behind his ear. "We got a problem here, little man. We toss ourselves through nine-odd light-years of space and wind up flat-footed in front of a killer-wave thrown up around a cubist's idea of a beanfield. I sort of expected a city—machinery, people, maybe."

The bit about pretending to sleep and using red light to hide from the spy-eye:

Bellew motioned to Slimmy to duck the cellotab, winked, stretched and said, "You think we ought to grab some sleep?" in a voice dripping with exactly the opposite meaning.

Slimmy said, "Why, sure," with admirable promptness, considering that both of them had had the sleep-centers removed from their brains by outlaw Earth surgeons in preparation for the trip.

While Slimmy pulled off his shoes. Bell went to a locker and slid two pairs of thick spectacles under his tunic, along with two disks of the same material as the lenses. He switched off the lights, pulled his own bunk out from the bulkhead over Slimmy's, dropped a pair of spectacles and a disk on the little man's chest, and rolled into bed. Both men clipped the disks to their bunklights, switched them on, and donned the glasses. Martians, possessing vision far into the ultraviolet, are blind to the reds merging into the infrared which is so prevalent on their own planet. If the spy ray was functioning—and of course it was—all the screen showed was a lot of nothing on a background of the same, and all the amplifier picked up was the tiny whisper of a busy stylus.

The process of converting U238 to U235 is biological:

"It isn't an apparatus process, dope! The Artnans don't transmute '238 into '235 by electrochemistry or radiophysics or any other process we ever heard of! Those Artnans who work in the shed aren't scientists or even mechanics! They're gardeners!"

The Earthmen grab a sample of the spores:

Bellew let his ship settle even more, and dropped a tube of berlylusteel from the hull to a drift of spores. A few of them were drawn upward by the suction he set up; then, tube and all, he snapped the ship into space. Once out there, he experimented briefly and thoroughly with his prize. The mold certainly filled the bill. The cysts apparently could stay alive without nourishment indefinitely. They germinated readily at any temperature, as long as they were in the presence of uranium. Happily, Bellew slipped into hyperspace and drove back toward Artna.

You can read the story as originally published in Astounding, June 1941, at the Internet Archive.