Investors feared a sharp decline in US demand in case one of the big automobile producers goes bankrupt.*

    - "in case" sounds a little informal to me, but i can't think of an alternative!

On Wednesday, the Dow Jones Index declined by 427,47 points or 5,07 percent to 7,997.28 numerators for thirty leading industrial securities. This is the lowest level [seen] since March 2003. The NASDAQ stock market index closed with a minus of 96,85 points and 1,386.42 numerators.

The strong exchange losses in Japan and in Wall Street also pushed the German stock market sharply into deficit. A few minutes after the opening bell on Thursday, the DAX quoted 2,5 percent less at 4246 points, after it had closed already 4,9 percent less on Wednesday. No DAX figure was among the winners. The stocks of Hypo Real Estate were among the losers, losing thirteen percent.

[text] = added text
text = text i am referring to

i am by no means familiar with terminology related to the stock market, so i did my best. i hope someone else will offer another correction! in general, most of my corrections aren't corrections at all, and only suggestions or clarifying remarks. your english is amazing!

Strong Losses on Stock Markets Worldwide

On Thursday, export figures were negatively affected by the strong yen and the fading prospect of a fast rescue of[for] the US automobile giant. The bleak prospects had [previously] sent the US stock markets on a downward trend before and forced them on to an all-time low since March 2003.
    - the before>previously is purely stylistic; "had... before" sounds more colloquial to me, so i feel like previously is a better fit.
    - "The bleak prospects... forced them to an all-time low" is grammatically sound, but i'm not sure about "since March 2003". this seems to imply that the stock markets were forced to an all-time low in March 2003 and have stayed at that low ever since. is my understanding of this sentence correct? 

In Tokyo, many financial securities belonged to the losers. Traders are very concerned that the stocks of Citygroup in New York had [have] dropped by almost one quarter[25 percent] to the lowest level since[seen in] thirteen years.
    - the phrase "almost one quarter" is a little ambiguous, so i changed it to simply directly say "25 percent".
    - "since 2004" sounds correct to me, but not "since thirteen years". Cambridge dictionary offers this explanation: http://dictionary.cambridge.org/us/grammar/british-grammar/linking-words-and-expressions/since

“All US-securities[US securities] have quite low quotations – this gives me an extremely bad feeling. I wonder what else will happen,” said Takashi Ushio, head analyst at Marusan Securities.

The Mitsubishi UJF shares declined by sicsix percent, [and] those of Sumitomo Mitsui Financial by more than ten percent. Export figures were also on traders' sales lists. The stocks of the camera producer Canon dropped by a good[?] seven percent and those of chip producer Tokyo Electron dropped by 11.4 percent.
    - in comparison to the rest of your article, "good" here seems too informal. maybe "dropped a significant seven percent" instead?  
    - you probably know, but just in case: the function of commas and decimals in english is the opposite of german. i left the rest of the numbers untouched in case it was just a stylistic choice.