Sand plankton is more than just wormgit. It has phytoplankton and zooplankton.
There are several mentions of “native plants” in Dune and Children of Dune, although these are sparse enough that the Fremen textile industry is built around the use of all-purpose ‘spice’ for essentially any non-metallic items, and while it isn’t stated explicitly, the sand plankton almost certainly includes phototropes which were adapted from the previously ocean-covered world as well as explicitly mentioning “fungusoid growths” emitted by the sandtrout which, when combined with water, produce large volumes of explosive gas. The (admittedly non-canonical) Dune Encyclopedia also notes that the sandtrout and worms are autotrophs with a metabolism driven by heat (presumably the difference between surface and subsurface) and static electricity stored in the vast expanses of sand. It is implied in the book there is a vast subsurface ecology utilizing the sequestered water, although the books focus on the worms-sandtrout cycle, of which even CHOAM and the Landsraad had the barest inkling because they were only interested in harvesting spice to maintain and expand their empire.
Herbert, of course, was not a systems ecologist or biologist, and didn’t attempt to create a planetary biome with explicit details because he was writing a fictional allegory and only presented tidbits of the diversity of the Arrakis ecosystem for the reader. But there are plenty of hints of the complexity of life on Arrakis, and the open question of where the worms came from originally as they do not appear to have evolved on Arrakis.
Stranger
The Moon is covered in a relatively black material, it reflects a fairly dim white color, and the sky stays black despite that light.
That’d probably be a fairly good approximation for a black sun.
(If we weren’t talking about space-faring people, you could even imagine a planet that is tidally locked to a star, and a people on the dark side of that planet who only see a Moon, and so think of it as their sun.)
Sunlight and moonlight are not that different, though the moon is maybe a little redder:
It’s darn hard to maintain a gaseous atmosphere on a tidally locked planet even given plenty of planetary gravity, robust magnetosphere, etc.
The dark side is the cold side where the atmosphere freezes and rather few inhabitants are walking around to confuse a moon w a sun.
Darn shame too, because you had a darn good story idea there.
Warmth from a cooling planet?