Native advertising is the topic du jour in the online media world. The idea of taking sponsored content and dressing it up to look like real news offers no shortage of untapped revenue streams. And while a number of skeptics have written it off as the digital equivalent of lipstick on a pig, some proponents believe native advertising has the potential to solve one of the biggest quandaries in digital media: how to create a sustainable business model that can support online journalism. With such high stakes, it’s no wonder that both publishers and marketers are excited at the prospect.

But Google, apparently, is not sharing in that excitement. On Wednesday, the search giant sent a stern warning to online news publishers, essentially telling them not to get any cute ideas about passing off sponsored content as real news. In a post on the Google News blog, Richard Gingras, senior director of news and social products for Google Inc. (NASDAQ:GOOG), wrote that “credibility and trust” are crucial attributes of a great news site and that dissolving the wall of separation between editorial and advertising threatens to undermine those core journalistic values.

“It’s difficult to be trusted when one is being paid by the subject of an article or selling or monetizing links within an article,” Gingras wrote. “Google News is not a marketing service, and we consider articles that employ these types of promotional tactics to be in violation of our quality guidelines.”

Gingras added that there would be stiff consequences for publishers that did not comply with those guidelines. “Engagement in deceptive or promotional tactics such as those described above may result in the removal of articles, or even the entire publication, from Google News,” he wrote.

Losing placement on Google News, of course, would be devastating to most online publishers, which rely heavily on search traffic to draw eyeballs to their websites.

Google’s warning comes at a time when an increasing number of publishers are experimenting -- sometimes clumsily -- with different forms of sponsored content. Publishers say they want to create ads that work more seamlessly with their platforms and, theoretically, appear less obtrusive to the user, but the results have not always been pretty. In January, the Atlantic magazine was forced to issue an apology after a Scientology-sponsored blog post sparked outrage and ridicule across the Internet.

Despite such slipups, other big-name legacy publishers are getting in on the native-ad act as well, including the Washington Post. But perhaps the biggest native-ad news to make the rounds in recent weeks came from BuzzFeed, the rapidly growing viral news website that has been wooing investors and opening news bureaus faster than anyone can count. Earlier this month, Ad Age reported that BuzzFeed is experimenting with creating a sponsored content "network," which incentivizes outside websites such as the Awl and Cracked to publish content from Buzzfeed's sponsors. And in February, Forbes reported that the website partnered with Fark -- an aggregator of humorous news -- to generate a wider audience for its sponsored content.

That BuzzFeed would use its tremendous reach and resources to experiment with outside-the-box advertising methods is not surprising. This is a company, after all, that asks job applicants to nix cover letters and instead send instructions on how to make a peanut butter and jelly sandwich, as Poynter’s Andrew Beaujon reported on Thursday. But in light of all the native-ad buzz surrounding BuzzFeed, the Atlantic and the Post, it’s difficult to see Google’s recent blog post as a coincidence. The Internet behemoth, which generates more advertising revenue than the entire newspaper industry combined, wants to remind everyone who is in charge.

The concept of native advertising is not new. As the New York Times’ Bill Keller pointed out in an interview with Forbes earlier this month, sponsored articles -- or advertorials -- have been a part of the landscape of print media for ages. However, the idea of sponsored content online has gained significant traction over the last year, with many proponents trumpeting native advertising as a potential savior for online journalism, where profit margins are a fraction of what they were during the glory days of print. Last year, Solve Media, a digital marketing firm, even created an infographic (posted here by Mashable’s Todd Wasserman) explaining the concept of native advertising and its perceived benefits.

But the concept has its share of critics as well and not just among the upper ranks of Google. In January, Peter Kafka wrote that the only people fooled by sponsored content are advertisers. “[T]hese things never seem to work when it comes to print or print-like Web publications,” Kafka wrote. “Not because they fool readers into thinking they’re reading ‘real’ content, but because they seem like lousy imitations of ‘real’ content.”

At any rate, Google has recommendations for news sites that are experimenting with sponsored content: Publish that content on a different host or directory, block it from Google News Web crawlers or create a separate Google News site map for news articles. To help publishers along, Google posted detailed instructions on separating different types of content.

We stand warned.

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