Hi notahacker, the initial margin deposit is in bitcoin (apologies if we did not make this clear) since lender funds are also in bitcoin. Regarding fixed/floating: if you are borrowing on an exchange such as Bitfinex then for any one currency swap the interest rate is fixed, but these swaps are typically short term and so when you come to refinance a position you could find yourself facing significantly higher rates than you had previously. Since (at least in the short term) our earnings will be all be in bitcoin, a collapse in the price of bitcoin would be very bad news for us.