UNRAVELING THE COMPLEX DYNAMICS BETWEEN WATER QUALITY AND THE MICROBIAL COMMUNITIES OF A SUBTROPICAL LAKE: A SEQUENCING APPROACH TO THE INSIGHTS OF THE BIOGEOCHEMICAL CYCLES.
Creators: name: Díaz-Torres, Osiris
Publisher: name: Zenodo
Year: 2023
Subjects: subtropical lake, hypereutrophic lake, fish kill, physicochemical and environmental parameters, 16S and 18s rRNA gene sequencing, shotgun sequencing, pathogenic bacteria, biogeochemical cycling.

description: SummaryThe municipality of Tlajomulco de Zúñiga, in the state of Jalisco, Mexico, is home to Lake Cajititlán, a shallow, endorheic, subtropical lake. Due mainly to untreated or partially treated wastewater discharged into the lake from municipal wastewater treatment plants, and the excessive use of fertilizers in agriculture in the basin this water body suffers under anthropogenic eutrophication problems. As a result of these sources of nutrient pollution, the lake's water quality and ecological integrity have been significantly affected, leading this reservoir to a hypereutrophic state, triggering a massive phytoplankton bloom that has generated several episodes of fish mortality during the rainy season due to oxygen depletion (hypoxia) at night caused by the respiration process by these primary consumers, from 2013 to date, and mainly during or immediately after the rainy season. Therefore, in this thesis work, we focused on characterizing the microbial communities of this lake during the rainy season using targeted metagenomic sequencing (16S and 18S rRNA) and shotgun sequencing to reveal the main physicochemical and environmental variables influencing the dynamics of microbial communities in response to anthropogenic pollutant inputs, to provide a scientific basis for protecting the aquatic ecosystem's stability. First, we describe the changes in the phytoplankton community in Lake Cajititlán during the rainy season by sequencing 16S and 18S rRNA amplicons, as well as the association between these changes and the lake basin's physicochemical water quality and environmental characteristics. The results indicate that nutrient carryover due to runoff during the rainy season casuses rapid changes in the physicochemical parameters, and important variations in the composition of the phytoplankton community. Planktothrix and Cylindrospermopsis (16S rRNA) were found to be the dominant genera in the cyanobacterial community, whereas the Chlorophyceae, Chrysophyceae, and Trebouxiophyceae (18S rRNA) classes were the most abundant in the microalgae community. Furthermore, possible relationships between the TN:TP ratio and the relative abundances of specific populations of cyanobacteria and microalgae were discovered. The evidence found showed that the death of fish in Lake Cajititlán could be related mainly by hypoxia, caused by the rapid depletion of DO levels as a result of phytoplankton blooms. These blooms were the result of nutrients entering the lake through direct urban wastewater discharges and agricultural runoff during the rainy season. Using high-throughput 16S rRNA amplicon sequencing and a functional inference approach (PICRUSt2), the temporal dynamics of bacterial communities within Lake Cajititlán and their associated genes with biogeochemical cycles of nitrogen, phosphorus, sulfur, and carbon during the rainy season were investigated. In addition, the influence of physicochemical and environmental variables on these dynamics and their microbial functional profiles were also analyzed. The main findings of this section of the thesis were that the nutrient load that reaches Lake Cajititlán modifies the physicochemical parameters, which in turn impact the taxonomic and functional structure of the bacterial communities. Genes associated with biogeochemical cycles were more abundant at the beginning of the rainy season and when the lowest levels of DO were detected. The last section of this thesis focuses on understanding Lake Cajititlán's water quality from physicochemical variables and shotgun metagenomic data. The most relevant results of this section indicate that most of the physicochemical variables analyzed (DO, pH, Secchi disk, NH4+, NO3-, BGA-PC, TP, and Chlorophyll-a) exceed recommended criteria for the protection of aquatic life. These parameters are altered by point and non-point pollutant sources reaching the lake during hot-dry and wet seasons. Likewise, genetic evidence from microbial communities reveals that anthropogenic sources of nutrients that reached the lake altered genes involved in the metabolism of nitrogen, phosphorus, sulfur, and carbon, resulting of excess of microorganisms that can thrive in environments with high nutrient content and low dissolved oxygen typically found in hypertrophic lakes.