Mr. Ricketts wrote that he founded DNAinfo in 2009 “because I believe people care deeply about the things that happen where they live and work,” and he thought he could build “a large and loyal audience that advertisers would want to reach.” DNAinfo and Gothamist, which Mr. Ricketts bought in the spring, attracted more than 9 million readers a month, in New York and other cities where they operate satellite sites, DNAinfo said.

But in the financially daunting era of digital journalism, there has been no tougher nut to crack than making local news profitable, a lesson Mr. Ricketts, who lost money every month of DNAinfo’s existence, is just the latest to learn. In New York City, the nation’s biggest media market, established organizations such as The Village Voice, The Wall Street Journal and The Daily News have slashed staff or withdrawn from street-level reporting. The Voice stopped publishing its print edition in September.

For DNAinfo and Gothamist, the staff’s vote to join the Writers Guild of America East was just part of the decision to close the company. A spokeswoman for DNAinfo said in a statement, “The decision by the editorial team to unionize is simply another competitive obstacle making it harder for the business to be financially successful.”

The decision puts 115 people out of work, both at the New York operations that unionized and at those in Chicago, Los Angeles, San Francisco and Washington that did not. They are getting three months of paid “administrative leave” at full salary, plus four weeks of severance, DNAinfo said.

The news took the newsroom by surprise. David Colon, a reporter for Gothamist, said that a lawyer for DNAinfo was present when the staff was told, but that he “didn’t really” take questions.