Final answer:
Lichen retains the least chemical energy fixed by photosynthesis.
Step-by-step explanation:
The organism in the arctic tundra ecosystem that retains the least chemical energy fixed by photosynthesis is lichen.
Lichens are a symbiotic organism consisting of a fungus and an alga or cyanobacterium. They have a low net primary productivity and do not retain as much chemical energy as other organisms in the ecosystem.
On the other hand, the arctic fox, caribou, and snow goose rely on consuming other organisms for energy and are higher up in the food chain, thus retaining more chemical energy.