Google has acquired online ad optimization company Admeld for $400 million, according to a source close to the deal. The deal ads a missing piece to Google's online ad infrastructure that powers much of online display advertising today, but could bring more regulatory scrutiny.

AdMeld is one of several companies that help publishers decide which ad to accept from a host of buyers such as ad networks or buyers participating in exchanges. Others in the space include the L.A.-based Rubicon Project and Palo Alto-based Pubmatic.