Abstract
In deep lakes, many investigations highlighted the existence of exclusive groups of bacteria adapted to deep oxygenated and hypoxic and anoxic hypolimnia. Nevertheless, the extent of bacterial strain diversity has been much less scrutinized. This aspect is essential for an unbiased estimation of genetic variation, biodiversity, and population structure, which are essential for studying important research questions such as biogeographical patterns, temporal and spatial variability and the environmental factors affecting this variability. This study investigated the bacterioplankton community in the epilimnetic layers and in the oxygenated and hypoxic/anoxic hypolimnia of five large and deep lakes located at the southern border of the Alps using high throughput sequencing (HTS) analyses (16S rDNA) and identification of amplicon sequence variants (ASVs) resolving reads differing by as little as one nucleotide. The study sites, which included two oligomictic (Garda and Como) and three meromictic lakes (Iseo, Lugano, and Idro) with maximum depths spanning from 124 to 410 m, were chosen among large lakes to represent an oxic-hypoxic gradient. The analyses showed the existence of several unique ASVs in the three layers of the five lakes. In the case of cyanobacteria, this confirmed previous analyses made at the level of strains or based on oligotyping methods. As expected, the communities in the hypoxic/anoxic monimolimnia showed a strong differentiation from the oxygenated layer, with the exclusive presence in single lakes of several unique ASVs. In the meromictic lakes, results supported the hypothesis that the formation of isolated monimolimnia sustained the development of highly diversified bacterial communities through ecological selection, leading to the establishment of distinctive biodiversity zones. The genera identified in these layers are well-known to activate a wide range of redox reactions at low O2 conditions. As inferred from 16S rDNA data, the highly diversified and coupled processes sustained by the monimolimnetic microbiota are essential ecosystem services that enhance mineralization of organic matter and formation of reduced compounds, and also abatement of undesirable greenhouse gasses.
Highlights
In deep lakes, seasonal differences in vertical water density gradients driven by water temperature and salinity are a key feature in the control of stratification dynamics and patterns (Wetzel, 2001; Kallf, 2002)
This work demonstrated the existence of a clear link between the vertical partition of water layers and the ecological selection of different bacterial populations and groups adapted to different local habitats
Starting from the assumption that no barriers can prevent the migration of hypolimnetic inhabitants between the hypolimia of different lakes, the results of this study suggest that the exclusive presence of genera and species can be explained by ecological selection due to differences in environmental conditions in different lakes and strata
Summary
Seasonal differences in vertical water density gradients driven by water temperature and salinity are a key feature in the control of stratification dynamics and patterns (Wetzel, 2001; Kallf, 2002). The cooling of surface waters in the coldest months is sufficient to trigger the complete circulation and vertical homogenization of the water column every year. In meromictic lakes, circulation in the winter months affects only the upper portion (mixolimnion) of the water column, whereas the deeper stratum of water (monimolimnion) is perennially separated from the surface by a steep salinity gradient (the chemolimnion) (Wetzel, 2001). While reduced substances accumulate in the monimolimnion, biota in the surface illuminated layers and dark mixolimnion may be deprived of essential nutrients (Humayoun et al, 2003)
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