**1. Introduction**

Nowadays the majority of water bodies are with some degree of affectation, in Mexico City one of the main aquatic ecosystems are Xochimilco channels located in the political delegation of the same name, this zone have different appointments including that of Natural and Cultural Heritage of Humanity in 1987 by UNESCO, it was recognized as Protected Natural Area in 1992, also, is considered an ecological conservation area, it is part of the RAMSAR wetlands of global importance, and is cataloged as a GIAHS site [1].

Previously, Lake Xochimilco received water from various rivers and springs, which was mainly used for crop irrigation; besides being the habitat of native and migratory birds [2]; nevertheless, nowadays it is threatened by several problems, mainly by overexploitation of water and contamination by use of agrochemicals [3, 4].

Also, pollutant sources have been observed such as clandestine discharges of sewage from human settlements, and the deposit of solid waste [5]. Since the growth of the urban spot, there have been changes in the productive activities causing diverse problems in the area such as the excessive growth of introduced aquatic vegetation like water lilies. Water control structures have been affected in the area and it has been observed a decrease in the extension of the main and secondary channels. This has caused that in places with little or no movement of water, there is an increase in eutrophication levels and flooding during the rainy season [6].

Due to the above, the aquatic communities have been affected, one of these communities are the microalgae which, by having short life cycles, present changes in its structure and dynamics in short period of time. Also, microalgae have a great importance for Xochimilco channels, despite their small size and being unnoticed by most people, because they provide important environmental services, such as: CO2 capture, oxygen liberation from photosynthesis, natural cleaning of the water of the channels (deseutrophication), in addition to reduce the concentration of heavy metals, which are harmful to the ecosystem [7–11].

On the other hand, microalgae serve as food for other species present in water bodies, such as copepods, crustaceans, small fish, and some amphibians in juvenile stages, helping to the conservation of biodiversity. Also, can be used as biological indicators to monitor water quality [4].

Considering the above, the aim of this investigation was to know the role of microalgae in the purification of water from a Wastewater Treatment Plant, under field and laboratory conditions.
