Is it worth noting that As and P are the same valence number?

That's exactly why they're able to substitute for each other; they have similar redox characteristics and similar atomic sizes. But, they're not 100% the same on either of those, which is why DNA prefers phosphorus strongly.

 

Carbon and Silicon have the same valence as well, but you're more familiar with how they behave; silicon tends to make rocks, carbon tends to make organic molecules.

So 26 posts and no ex-wife jokes to do with arsenic based lifeforms? For shame Soxtalk, for shame.

I'd bet there's a reasonable chance they actually do exist as As-rich bacteria in nature, if the chemist who presented yesterday is worth his salt. He argued quite convincingly that in a normal DNA chain, anything bonded with As instead of P is a particularly weak link and leaves the chain easily severed, requiring repair. The suggestion I got was that this substitution and weakening of P-rich structures was the main mechanism of As poisoning in normal life. Getting around this problem would probably require some mechanism by which either the chain is strengthened/protected by another cellular component, or a mechanism by which breaks are rapidly repaired so as to not become systemic and kill the organism. Both of those seem to me to be adaptations that would require significant expenditures of energy to maintain. As such, if the bugs weren't living as As-rich bacteria in nature, there's no reason for them to maintain this adaptation over multiple generations; it'd be a large selective disadvantage because it'd be such a large waste of energy.

 

A selective disadvantage perhaps, but in an extremophile environment with few or no competing species it is likely an energetic inefficiency the populations could tolerate. Based purely on energetic efficiency arguments obligate anaerobic metabolic pathways should not even persist, but they do because the anaerobes that occupy those niches don't have to compete with metabolically more efficient aerobes. That said, I also don't doubt that As-incorporating populations of these new bacteria exist in specific environmental conditions.

I always thought it was a world-is-flat sort of belief in the scientific community that all life must be carbon-based. Just because that is what we know, that doesn't mean a damn thing about what other things could exist somewhere else. It may make those things probable, but to write off anything else entirely just seemed ignorant to me.

 

My thoughts exactly.

 

That said, it's still important that those who matter in this field are willing to entertain that thought.

 

teehee