Merrill Lynch & Co., the investment brokerage and financial services firm, is expected to announce an added loss of more than $2 billion. That's on top of $5 billion of write-downs announced earlier this month. It's the biggest loss for any Wall Street firm, and the reason is the collapse of the credit market. When the market first fell apart in July, many Wall Street banks were caught with debt they couldn't resell. But Merrill Lynch is suffering more than the other banks, because of more investments in subprime mortgages. Now it's left with a big stockpile of debt that investors don't want to touch.