By Tetsushi Kajimoto and Francesca Landini

OSAKA, Japan (Reuters) - The weak U.S. dollar and oil speculators took centre stage as Group of Eight finance ministers gathered in Japan on Friday to grapple with surging inflation and a slowing global economy.

As soaring energy costs stirred protests from Malaysia to Spain, the world's most powerful governments talked up a link between a dollar slide and a doubling of oil prices in 12 months.

Their communique at the end of their meeting on Saturday will describe commodity prices as a "serious challenge" to economic growth, a G8 source told Reuters.

The G8 countries, mostly importers of crude, wield little influence over oil markets that are driven by demand from India and China and concerns about supplies. But they can try to arrest a slide in the U.S. currency that has prompted investors to buy oil futures and other commodities to hedge dollar risks.

"On top of the (oil) barrel there is a magnum of speculative champagne," Italian Economy Minister Giulio Tremonti said, floating a plan to make speculation in oil futures more costly. "There are more contracts than barrels," he told reporters.

The G8 will ask the International Monetary Fund to study the rise in commodity prices, Tremonti said.

Other officials put the focus on currency markets.

A Japanese official called for a defence of the dollar to contain commodity prices. France welcomed the U.S. currency's rebound of the past week, after Washington signaled it was worried enough about the dollar's long decline to raise the prospect of intervening in markets.