Ciliate food vacuole content and bacterial community composition in the warm-monomictic crater Lake Alchichica, México

To evaluate trophic relationships between ciliates and bacterioplankton during the stratification period in a lake, samples from three different layers [the upper part of the metalimnion (UM), the base of the metalimnion (BM) and the hypolimnion] were studied. The autotrophic picoplankton numbers, phylogenetic composition of heterotrophic picoplankton (HPP), and HPP composition in ciliates’ food vacuoles were analyzed. Additionally, in situ incubations in dialysis bags were performed at the same selected depths to assess potential changes in picoplankton composition related to ciliates’ feeding activity. Among the in situ HPP assemblage, no phylogenetic group dominated in the selected layers within the course of the study. The ciliate assemblage was dominated by scuticociliates, haptorids, and hypotrichs. Comparing ciliates’ food vacuole content and in situ HPP composition, a high preference for A lphaproteobacteria was found at all three depths. P lanctomycetes , and D elta - and G ammaproteobacteria were selected at BM and hypolimnion, respectively. However, selection of a given phylogenetic group in the time course of this study was observed only for A lphaproteobacteria in the UM. Similar trends were found in the incubations, but no relationship was found between the vacuole content of the ciliates and changes in HPP composition.