Algae are responsible for approximately 50 % of global primary production. Algal production is thus essential for the global carbon cycle, ecosystem health and climate. The diversity of algae and the multitude of species provide a buffer for ecosystem functions in the face of changing environmental factors and climate change. Algae are, however, not a monophyletic group, but rather comprise many different organismal lineages which are not closely related to each other. The common characteristic of algae is that they are eukaryotic photosynthetic organisms with the exception of land plants. The capability for photosynthesis was gained independently in several lineages and even more frequently lost independently in different lineages. This article describes the diversity of algae, pattern of algal distribution and evolutionary scenarios triggering the differentiation of nutritional strategies, in particular the shift from phototrophic nutrition to heterotrophic nutrition. Specifically the availability of nutrients, light and inorganic carbon or of alternative carbon sources is key in triggering the multiple differentiation and specialization of nutritional modes in algae. Today, nutrients are usually removed biologically in waste water treatment facilities. The shortcoming of this practice, however, is the enormous energy consumption for aeration and the transformation of organically bound carbons into CO2. A sustainable alternative to remove nutrients is the use of micro algae. Different methods for the removal of nutrients by Scenedesmus species on a laboratory scale are described in the present article. Moreover, larger-scale tests were carried out at an urban wastewater treatment plant. The results suggest that the microalgae are effective in the removal of nitrogen, especially in a tubular reactor design. Furthermore, this article describes the degradation of diclofenac and sulfamethoxazole by the green algae Chlorella vulgaris on a laboratory scale. The results show that the microalgae can significantly reduce the concentration of the pharmaceuticals. Within 100 hours a removal efficiency of diclofenac by 65 % and of the antibiotic sulfamethoxazole by 40 % in an algae-bacteria mixture was observed.