Aren't diatoms, (as is DIATOMaceous earth), protists? That puts them in the same kingdom as algae, and it is just as naturally occurring. As with the other kingdoms of living things, (animals, plants, fungi and moneran [bacteria]), there are good species that don't cause threats to humans, (like cats, flowers, morels, and the bacteria we eat in yogurt), and harmful species, (like poisonous snakes, poison ivy, poisonous mushrooms, and disease-causing bacteria). You can buy DE that is harmful to humans, among other animals, in the garden and the pool. When I first went to buy the stuff, I almost bought the wrong kind because many garden/feed stores don't generally know what it is and they especially don't know why chicken people would want to buy it. As the post before this one says, make sure you buy FOOD GRADE. Then it's safe for the chickies and you.
Food grade diatomaceous earth is available in 50 lb. bags from Amazon.com, the only issue is shipping. For my location, that was $35.00, which doubled the price per pound. I'll search around here for it, should be much cheaper locally.