Diatoms are just one example of algae, and this fact doesn’t change what we think about algae in general. This doesn't tell us anything more about the connection between ferrous material and algae.

If anything, this supports the scientist’s argument by reinforcing the causal connection between ferrous material and algae. We want to weaken this connection, not strengthen it.

Did these other minerals cause algae growth? Did they have some other effect? We don’t know enough to say whether these minerals are relevant at all, much less whether they weaken the argument.

In other words, at least one population of algae didn’t seem to increase at all in Antarctica during the period in question. This suggests the ferrous material wasn’t causing algae growth, thus undermining the argument.

Ferrous material doesn't harm algae. This doesn’t weaken the scientist's claim, and in fact defends against a possible problem, that too much ferrous material is harmful to algae.