TAMPA — SoHo Capital, which is developing The Heights mixed-use project just north of downtown Tampa, has bought the Mariner Square office complex near the Howard Frankland Bridge for $7.5 million. The 5.4 acre complex, sold by the L.M. Hughey Company, consists of five low-rise buildings with a total of 73,000 square feet in the booming West Shore Business District. SoHo executives could not be reached for comment Monday but selling agent Ward Viator said the private equity firm apparently plans to renovate the aging buildings at 200 S. Hoover Blvd.SoHo Capital spent $20 million converting the old Armature Works warehouse on the Hillsborough River into the Heights Public Market, a dining and commercial emporium whose food hall recently opened. It is one piece of a plan that SoHo has for the 43 acres known as the Heights. The Pearl, a 314-apartment complex with ground-floor retail, is almost finished and SoHo plans to start construction in coming months on a boutique hotel and a $41 million, six-story office building.