Unity College specializes in environmental studies and is known as one of the “greenest” schools in the country, so it bothered college officials that the stock portfolio in which the school’s $14.5 million endowment was invested included shares in some of the world’s largest polluters. Three years ago, the private college in Waldo County joined other institutional and individual investors that have eliminated the biggest contributors to greenhouse gas emissions – particularly Big Oil and coal companies – from their portfolios. “The college made an ethical decision that we’re going to walk the walk,” said Melik Khoury, Unity’s executive vice president and chief financial officer. Working with its investment firm, Portland-based Spinnaker Trust, the college developed a new portfolio that largely excluded the 200 businesses identified by climate change activists as the top fossil fuel companies based on their existing reserves.