What it is
The giant brown seaweed behind dashi stock — real food, but the single most iodine-dense thing in the sea.
Why your biology objects
Kombu can carry thousands of micrograms of iodine per gram — enough to blow past the ~1,100µg/day adult ceiling and tip the thyroid either way; kelp *supplements* are especially unreliable on dose. A traditional broth ingredient used briefly and removed; the flag is concentration and frequency, not the plant.