Barium Claude is a function of parting coefficients and the concentration in NSW at x, depth, temp and salinity. That means the concentration is directly proportional the concentration in seawater and has nothing to do with a "need". It acts a substitute ion just like Sr, as it fits into the Aragonite crystal lattice. If we pretend that a coral skeleton was 100 % pure Aragonite, guess what ? You will not find any Mg++ in the skeleton either as it can not fit into he Aragonite crystal lattice. Now lets look at some other form of marine life that is 100 % Calcite. Guess what, you will not find any Sr in its Skeleton as it does not fit into the Calcite crystal lattice. That is why when Aragonite gets altered, during geologic events, all the Aragonite gets converted to Calcite and kicks out the the Sr. That is why there is hardly any Sr in Limestone but it is loaded with Mg++, as Mg++ fits real nice into the Calcite crystal lattice. In short, you cannot stick a 1 cm ball into a 1.2 cm hole.
So, they do not need it but it is there for them to pick up. Many ions get tied to crystal lattices. As I said above so does Uranium. There are also other ions in coral "aragonitc" skeltons, that you may not known about, as chemically they are "drity".i.e, Cu, Zn, Si, Ti, Cr, Fe, Mn, Ni, Co, Ti, Pb, Sc and U. Just for example, Boron is 2-4 x as must as as Ba. And there is more Ti than Ba by 2x. When have you ever herad that corals need Boron in their skeltons other than if you read Randy's article on Boron in the reef tank ? The concentration of most of these is a function of temp, depth and salinity.
Back to Sr. For many years Paleoceanographers have studied core samples of ancient coral in the sea, like Chris Maupin. Do you know what ion they use to determine /reconstruct climate variability from past climates of the sea and earth.........Sr. Why ? Because its concentration in corals is a function of past cliamtes, as it is picked up by corals as a function of the concentration in seawater. You have to be pretty exact to do that and that is is what Sr does for them. Which means what ? It has no function in corals other than just being there.
Can either of these ions, Sr or Ba increase coral growth. Well, yes, maybe, as it is filling that "hole" before say a Ca++ is. But we can say the same for other ions also. That does not = Needed. I have no issue with one keeping ions the same or near NSW, the issue is being able to measure them. And if you know anything about Barium it is very toxic to life forms. Therefore why add it if it is not needed. Have you ever measured a running reef tank for Barium to see what it is at.
The sponges which get no barium was around 10 % less in grow
Ok that is called a test with data, where is the date to support it is really the Barium. One cannot assume it is Barium as it "seems" to be. How do you know it is not some other ion that is in the Barium Sup. Your Barium sup is not 100 % Pure Barium Chloride. Just because they grow faster, even if it was the Barium, does not mean it is needed for growth, anymore than Sr is. They grow just fine without them. And what sponges would those be ? Calcareous Sponges, they are no different than corals, mollusks, forams or other calcareous fauna or flora. What about Silica based sponges ?
A read on the issue of Sr/Ba ratio is seawater
Barium and strontium uptake into larval protoconchs and statoliths of the marine neogastropod Kelletia kelletii
http://biology.fullerton.edu/dzacherl/GCA67_p4091_4099.pdf