Longtime Mercury News columnist Mike Cassidy has frequently covered companies that are for sale, seeking mergers or on the verge of closing. Now he knows what it feels like to work at such a place. He writes in this morning’s (Feb. 21) edition that doom is in the air at the Mercury News now that the newspaper’s owner, Knight Ridder, is up for sale. “I suspect I speak for many with whom I share this building when I say that I like the paycheck, but that the Mercury News is more than a job. When we came to work here we came with a mission. We wanted to shine light in dark corners and make the world a better place,” Cassidy writes in this morning’s Merc.