France’s new Prime Minister, a moderate Socialist with an affinity for Germany who will no doubt be quickly pressed into service to tend to the nation’s all-important relationship with Berlin, took office Wednesday.

Jean-Marc Ayrault was welcomed at the 18th century mansion in central Paris that serves as the prime minister’s office by his predecessor Francois Fillon. The two men chatted for half an hour before emerging. Mr. Fillon, a conservative and staunch ally of former President Nicolas Sarkozy, was driven away to applause by onlookers gathered in the building’s courtyard.

Mr. Ayrault waved his predecessor off and then it was time to get to work.

“The essential thing ... is that we get to work very quickly in the service of the French people,” Mr. Ayrault said later in quick remarks to reporters.

The 62-year-old has led the country’s Socialists in the lower house of Parliament for more than a decade, but it is his knowledge of Germany and German that has attracted the most attention.

All eyes are trained on how President Francois Hollande, who was sworn in Tuesday, and German Chancellor Angela Merkel will get along, since that relationship is at the core of how Europe tackles its debt crisis. Mr. Sarkozy and Ms. Merkel were said to be so close they were sometimes referred to as one person, Merkozy. Franco-German proposals usually carry the day in Brussels as European leaders try to contain a debt crisis that has dragged several countries into recession and ensure that it never happens again.

Just hours after being sworn in, Mr. Hollande flew to Berlin to meet Ms. Merkel. The German chancellor said their differences had been overstated, and the two committed Tuesday to finding ways to encourage growth in a continent where many countries are beset by recession.

But observers wonder how they’ll reconcile the French leader’s insistence that growth measures be added to a European treaty aimed at limiting overspending, and the German leader’s demand for budget discipline. The conservative Merkel has balked at reopening negotiations of the fiscal compact that brought at least an uneasy calm to markets when it was signed earlier this year. Mr. Hollande says imposing drastic cuts on countries that aren’t growing is counterproductive and will only further impair their ability to pay off debts.

Mr. Ayrault, a former German teacher, will be central to that discussion.

Mr. Ayrault has served as a deputy in that lower house since 1986. He is also mayor of Nantes, a city on the Atlantic coast.

The rest of the government ministers will be announced later Wednesday. They will hold their first official meeting on Thursday.

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