Think about it Living organisms are composed mostly of four elements: oxygen, carbon,hydrogen,and nitrogen. These four elements (and a few others, such as sulfur and phosphorus) are the basis of life’s most important compounds: water, carbohydrates, lipids, nucleic acids, and proteins. In short, a handful of elements combine to rom the building blocks of all known organisms. And yet, organisms cannot manufacture these elements and do not “use them up. “so, where do essential elements come from? How does their availability affect ecosystems ?