For those unfamiliar, at least here in the U.S. a “money market account” is sort of equivalent to a savings account, only usually with a minisculely higher interest rate. The accounts trade in T-bills and other “100% safe” government bonds to make their money.
For such an account to have a negative interest rate is nearly unprecedented. I don’t think I’ve ever heard of it happening in the last 30+ years. This WP article makes it sound like it’s happened just once before, and that wasn’t for retail investors.