Global consulting company Mercer on Tuesday announced a significant expansion of its relationship with Benefitfocus Inc., including a strategic equity investment in the Charleston, S.C.-based company. Mercer’s private benefits exchange product, called Mercer Marketplace, is powered by Benefitfocus technology. Mercer said it will make an initial 9.9 percent investment in Benefitfocus, with the option to increase its ownership over time. Launched in 2013, Mercer Marketplace increased the number of participating employers, eligible employees and eligible lives by fivefold in 2014, the company said. Mercer has significant operations in Urbandale.