I have made dashi a couple of times now, starting soaking kombu in water. And always I found these little bubbles on the leaves, containing white slime:
I have never worried about them much and simply cut them away when eating the kombu later, but what are those bubbles? My hypotheses are: 1. just random bags of alginate slime, or 2. natural growth due to parasites or other incorporated matter.
According to the "Handbook of Kelp Farming", Sporangia are usually close to the centre of the blade, so it is highly unlikely to be that.
As seen here: https://www.ispotnature.org/communities/uk-and-ireland/view/observation/807675/rain-blister-39disease39
This appears to be what is called "Blister Disease", thought it is not really a disease but rather it "is caused by a sudden decrease in salinity due to mixing of rainwater with seawater. The disease often appears after heavy rainfalls and generally occurs in shallow bays which are vulnerable to salinity changes caused by freshwater run-off."
Source from FAO: https://www.fao.org/3/AB724E/AB724E09.htm
