Vertical and seasonal distribution of picoplankton and functional nitrogen genes in a high‐altitude warm‐monomictic tropical lake

Abstract

Abstract The nitrogen (N) cycle is highly dependent on microbial processes. Distribution of these communities is one of the most important factors in the variation of the N cycling in warm‐monomictic lakes. However, the effects of the alternation of water stratification and mixing periods on the ecology of microbial communities involved in these processes are rarely studied in this type of aquatic ecosystem. We explored vertical and seasonal patterns of picoplankton and the genetic potential for ammonium oxidation (amoA gene for bacteria and archaea), denitrification (nirS and nirK), anammox (hzsA), and DNRA (nrfA) and their relationships with the main limnological variables in Lake Alchichica (Central Mexican Plateau) to provide insight into the distribution and importance of these planktonic communities in warm‐monomictic tropical lakes. Ten depths were sampled during late stratification (November 2015) and mixing (February 2016) periods, covering the epilimnion, metalimnion (oxycline), and hypolimnion layers in the first case. We showed that temperature and oxygen stratification shaped the distribution of picoplankton and functional N genes in this lake. These communities also varied in relation to nutrient availability and underwent temporal changes throughout the water column. The amoA genes, along with autotrophic picoplankton, were more abundant during the stratification, indicating that nitrification could be potentially more important during this period, mainly at the oxycline layer. Denitrifying genes showed strong variations during the stratification period, with highest gene copy numbers at the oxycline and hypolimnion layers. Anoxic conditions were characterised by a relative increase in the abundance of the nrfA gene with depth, which was positively correlated with NH4+ concentration. On the other hand, the hzsA gene was not detected in any sample. Our findings highlight the importance of thermal stratification as one of the main factors influencing the genetic potential for N transformations within the water column in warm‐monomictic tropical lakes.