This is a lovely bit of science writing. Thank you for posting.
Ed Yong is the best science journalist of our time.
Symbiotic relationships are well known already?
It's unclear if it's symbiotic (sea grapes without the bacteria do fine). It's very odd!
Well I would say that it's highly likely that the presence of the bacteria is either slightly positive (symbiotic) or slightly negative (parasitic).
As been discussed numerous times on HN our own guts are teeming with bacteria that we can live without but may sometimes derive some benefit or harm from.