The oceans constitute the world’s largest ecosystem. In this enormous and dynamic ecosystem, both micro and macro algae play a variety of roles that can be both positive and negative. Primary production by macro and micro algae forms the basis of the food web in most of the oceans and is also responsible for the phenomenon known as the “Biological Pump”—the biological pumping of carbon to the depths where it is either sequestered from the environment or respired back to the inorganic form. The Biological Pump transports carbon in the form of Particulate Organic Carbon (POC) as well as Particulate Inorganic Carbon (PIC) from the surface to the depths below. Not all algae however have positive effects upon the oceanic environment and its inhabitants. Some produce noxious blooms that by depriving the water column of Dissolved Oxygen (DO) kill all marine life whilst others produce potent toxins that reach humans as they make their way up the food chain. Still others include species that are non-toxic to humans but cause damage to fish and invertebrates especially by clogging or damaging their respiratory devices. Anthropogenic activities have radically altered the biogeochemical cycling of elements especially carbon and nitrogen and in their wake also altered the structure and functioning of numerous ecosystems. Marine ecosystems are no exception as human activities have also altered the abundance and distribution of several algae (both macro and micro algae).