Reef tanks already have tremendous bacterial diversity.
Do they? On what evidence do you base this statement?
My surveys have led me to conclude that reef tanks vary about six-fold in their diversity. The diversity in reef tanks is comparable to the diversity in natural reef waters, but varies widely.
So it is hardly clear that having more is necessarily desirable.
More than what? More than a low diversity tank (~100) or a high diversity tank (>400)?
I absolutely agree, though, that we don't yet know what number is best, or how best to increase it if we wanted to (aside from live rock or similar materials).
Does it make your GI tract more productive, resilient, and stable to randomly consume bacteria? No.
No, but not because the gut microbiome doesn't matter. It's because the stomach is well adapted to destroy bacteria in food.
If you introduce bacteria from the other end it can have profound effects on human health...
Might some species be helpful? Certainly true. Are some species undesirable? That is true as well. Does having more as opposed to the "best" species help? That is far from clear
To be clear, I agree 100% with all of this. Perhaps the ideal state for an aquarium is not the most diverse community possible, but a particular community well suited for that environment. That seems possible and even likely as far as I'm concerned.
That's all consistent with diversity still being generally valuable and important for ecosystems (probably including our tanks), though. Which was my main point.
You hypothesized three possible benefits as if there were accepted facts.
To be clear, I was saying that these (and other) benefits of diversity are *generally* recognized by ecologists. Diversity is central to the field of ecology and is generally associated with these and other benefits at the ecosystem scale. Which is not to say that more diversity is always better, even in a general sense. Ecologists have been debating these subjects for longer than we've been alive, and I don't imagine they'll stop either studying the effects of diversity or debating its importance anytime soon.
It seems reasonable to question each of these in the aquarium setting, as you have done. I think we'd agree that the answer to almost all of your questions is "we don't have enough data on this yet", right?
My point was just that our starting point should probably be based on the longstanding recognition that diversity is generally valuable in microbial ecology [1-3]. Specifically, microbial diversity is associated with intact, healthy reef ecosystems in nature [e.g., 4]. This suggests diversity will also be valuable in a reef ecosystem. Its a reasonable starting assumption.
Since the starting diversity of every tank is essentially 0 until the reef keeper starts adding to it, it makes sense to me to simultaneously (1) value diversity as we establish and maintain our tanks and also (2) continue to ask questions about how much is enough and what kind of microbial community is ultimately best.
This is a tiny sampling of references on the subject, just making the point that diversity remains central to studies of ecosystems including reef ecosystems.
References:
1. Naeem, S., Thompson, L., Lawler, S. et al.
Declining biodiversity can alter the performance of ecosystems. Nature 368, 734–737 (1994)
2. Duffy JE, Godwin CM, Cardinale BJ. 2017.
Biodiversity effects in the wild
are common and as strong as key drivers of productivity. Nature 549:261–264.
3. Isbell F, Craven D, et al. (2015).
Biodiversity increases the resistance of ecosystem productivity
to climate extremes. Nature 526:574 –577.
4. Weber, L., González‐Díaz, P., et al. (2019),
Microbial signatures of protected and impacted Northern Caribbean reefs: changes from Cuba to the Florida Keys. Environ Microbiol.
