Could you expand on what makes arsenic an essential micronutrient? What are the clinical signs and symptoms of severe arsenic deficiency? I've never heard of this before.
Humans get enough arsenic from water and other background sources that deficiency is virtually unknown. My understanding is that there was historical anecdotal evidence for rare arsenic deficiency from animal husbandry that caused it to be investigated.
These days they systematically test for the trace micronutrient status of e.g. heavy metals by inducing extreme deficiency using mammal models. Most of the time nothing happens but it is difficult to eliminate the possibility of contamination creating a null signal.
Probably the most surprising element for which they have suggestive evidence of biological necessity is lead.
Here is the first good reference I could find, which surveys some of the other literature. It mentions lead in rat models.
https://chatgpt.com/share/6a0d522f-5688-83eb-b80c-0f7b157fae... https://gemini.google.com/share/e2e52489a41e
The evidence for lead is much more sparse. It is interesting, and plausible, but without more evidence it is more of a curiosity. For better or worse, there has been zero interest in investigating that anomaly further. People should avoid lead exposure regardless.
Thankfully elements like mercury never show up in these studies. Nothing in my chemistry training suggests that mercury by nature could ever be anything but toxic to life and the evidence seems to support that. It would be weird to find out that was wrong, though I would accept it with sufficient evidence.
Also, just in general debate terms, the one who makes a claim is the one who has the burden of substantiating it.