Why did Bassanio look so pale after reading Antonio's letter? What fearful tidings did it contain?

Bassanio turns pale at the news that Antonio has lost his fortune and his ships, and he asks Salerio if it is true that all of Antonio's ventures have failed. Salerio tells him it is true, and that Shylock is so excited about getting his pound of flesh that even if Antonio could repay him he would likely refuse it.