(Bloomberg News) The 20 wealthiest people on Earth lost a combined $9.1 billion this week as renewed concerns that Europe's debt crisis might worsen drove the Standard & Poor's 500 Index to its largest decline of 2012.

Mexican Carlos Slim's fortune fell by $1.5 billion during the week as shares of his telecom operator, America Movil SAB, dropped 2.2 percent through April 4. Mexican markets were closed yesterday for the Holy Thursday holiday. The 72-year-old remains the richest person in the world, with a net worth of $69.2 billion, according to the Bloomberg Billionaires Index.

"This is a little bit of a reality check," Leo Grohowski, chief investment officer for New York-based BNY Mellon Wealth Management, said in a telephone interview. "The super wealthy are among the most cautious investors in the world. Skepticism is still high, and they are feeling very, very nervous."

Global markets retreated this week as demand fell at a Spanish bond auction and minutes from the U.S. Federal Reserve's latest policy meeting indicated that it isn't ready to provide more monetary stimulus. The Standard & Poor's 500 Index fell 0.7 percent, to 1398.08, its third weekly decline of 2012. The STOXX Europe 600 lost 1.6 percent to close at 259.07.

Li Ka-Shing, Asia's second-richest person, lost $144 million during the week. Shares of his port operator, Hutchison Whampoa Ltd., dropped 1.1 percent in yesterday's Hong Kong trading, leading the Hang Seng Index to its fifth decline in six days. Li, 83, ranks 15th on the index with a $23.8 billion fortune.

Brazilian Eike Batista's net worth fell $574.4 million this week as shares of OGX Petroleo & Gas Participacoes SA dropped 3 percent during the week. On April 4, International Business Machines Corp., the world's largest computer-services provider, bought a 20 percent stake in Batista's technology unit, SIX Automacao. The two companies will set up a technology center to serve customers in Brazil, Chile, Colombia and Peru.

Batista, whose fortune rose $7.2 billion last week after the 55-year-old sold a 5.6 percent stake in his commodities empire, ranks 10th on the index. His net worth of $33.5 billion is up 49 percent year to date.

Spanish retail tycoon Amancio Ortega, 76, saw his fortune fall $1.2 billion to $39.4 billion as shares of his Industria de Diseno Textil SA, owner of the Zara fashion chain, lost 0.3 percent in Madrid trading during the week. Spain's Ibex stock index slid to a seven-month low in intraday trading yesterday amid concerns that the country may require international aid to meet deficit targets.

Stefan Persson, 64, chairman of Swedish clothing giant Hennes & Mauritz AB, fell four spots to rank 17th on the Bloomberg index as his fortune fell $1.4 billion to $23.2 billion. H&M shares declined by 3.8 percent during the four-day week shortened by the Good Friday holiday.

Microsoft Corp. co-founder Bill Gates, 56, is second on the index with a net worth of $63.2 billion, down $558.1 million for the week. Warren Buffett, 81, is third with $45.2 billion. The world's three richest people have gained a combined $17.2 billion year to date.