German Chancellor Angela Merkel speaks during a meeting at the lower house of parliament Bundestag on 2017 budget in Berlin, Germany, November 23, 2016. REUTERS/Fabrizio Bensch

By Caroline Copley

BERLIN (Reuters) - Chancellor Angela Merkel's biggest contender in next year's German election campaign may not be a rival party leader but so-called "social bots" - software programmed to sway opinion on social media sites.

In her first speech to parliament since announcing plans on Sunday to seek a fourth term as chancellor, Merkel called for a debate on how fake news, bots and trolls can manipulate public opinion.

"In order to reach people, to inspire people, we need to deal with this phenomenon and - where necessary - regulate it," Merkel told lawmakers on Wednesday.

The role social networks Facebook and Twitter play in spreading false and malicious information has shot up Merkel's agenda after the shock election of U.S. Republican candidate Donald Trump as president.

Social bots are software programs that mimic human behaviour on social media sites, such as by publishing messages or liking posts. They can be used to spread erroneous information and muddy the debate. Some critics say the proliferation of fake news helped sway the U.S. election in Trump's favor.

With the far-right Alternative for Germany (AfD) having chipped away at support for Merkel's conservatives by attacking her open-door refugee policy, alarm is growing over the influence of bots and fake news on potential voters.

In a sign of how seriously Merkel is taking the matter, she invited Simon Hegelich, professor of political data science at the Technical University of Munich, to brief the executive committee of her Christian Democrats (CDU) on Monday.

"Merkel is really interested in the topic of bots and fake news and hate speech on the Internet and she's very well informed," said Hegelich.

While all of Germany's political parties have said they will not deploy bots in the campaign, the fact that they are mostly anonymous makes it hard to work out who is behind them. Hegelich does not rule out third parties using them to try to discredit parties or politicians.

The head of the domestic intelligence agency raised concern last week about potential Russian interference in Germany's election through the use of misleading media stories.

Moreover, fake news has become a lucrative business model. In the U.S. election campaign, bloggers without political affiliation realized they could earn money by spreading sensational and false stories.

"They act as if they were a news provider, but all the news is fake. It doesn't matter if it's true or not, it's just a business model to get more clicks on advertising," said Hegelich.

ECHO ROOMS

Germany's established parties, the CDU and the center-left Social Democrats (SPD), are lagging the AfD in engagement on social media. The AfD has racked up more than 300,000 likes on Facebook, while the CDU and SPD can barely muster 240,000 together.

While this may not seem many among Germany's 62 million voters, many Facebook users share posts with one another, increasing their reach.

Algorithms on Facebook also shape what content appears in a user's news feed, creating so-called "echo rooms" in which posts only reflect user's own views.

Facing a barrage of criticism for failing to stem a flood of phoney news articles in the run-up to the U.S. election, Facebook Chief Executive Mark Zuckerberg has disclosed steps to weed out hoaxes and other false information.

These include making it easier to report false content, working with third-party verification organizations and journalists on fact-checking efforts, and exploring posting warning labels on content that has been flagged as false.

Justice Minister Heiko Maas said last week Germany would closely monitor how Facebook responds to the challenge of fake news, but said legislative measures were limited. Merkel has raised the idea of a code of conduct for social networks.

Thomas Jarzombek, the CDU's spokesman for digital policy, says more transparency is needed and called on journalists at traditional media companies to debunk fake news and create a more informed debate.

He said with 10 months to go until the election, he believed Germany had time to raise awareness of the issue among voters.

(Additional reporting by Andreas Rinke; Editing by Janet Lawrence)