By Letícia Klein/Ambiental Media TAPAJÓS NATIONAL FOREST, Brazil — We have to walk fast inside the forest to keep up with the scientists and their local guides. It’s the dry season, and the heat in the Amazon Rainforest is suffocating. An hour and a half later, after going up and down several slopes, some so steep that a helping hand is needed from the colleague ahead, we arrive at the stream. Relief. The pure and crystal-clear water of this small river branch isn’t just perfect for quenching our thirst after the walk; it’s the very subject of the scientists’ research. In Brazil’s Tapajós Nati…