Other people are dismissing this idea right off the cuff because of the arsenic concentration but that's a good thing. Now we use it as a feed stock for arsenic loving single celled creature. Then we concentrate it further. And again and again until you have arsenic pure enough for use. Then you can add that arsenic back into the pesticide and herbicide production processes that helped it get into the seawater instead of mining it on land.

As Close04 mentioned, sargassum accumulates arsenic. You can't eat it or use it as fertilizer without poisoning.

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