Glacial Influence Affects Modularity in Bacterial Community Structure in Three Deep Andean North-Patagonian Lakes.

We analyze the bacteria community composition and the ecological processes structuring these communities in three deep lakes that receive meltwater from the glaciers of Mount Tronador (North-Patagonia, Argentina). Lakes differ in their glacial connectivity and in their turbidity due to glacial particles. Lake Ventisquero Negro is a recently formed proglacial lake and it is still in contact with the glacier. Lakes Mascardi and Frías lost their glacial connectivity during the Pleistocene-Holocene transition. Total dissolved solid concentration has a significant contribution to the environmental gradient determining the segregation of the three lakes. The newly formed lake Ventisquero Negro conformed a particular bacterial community that seemed to be more related to the microorganisms coming from glacier melting than to the other lakes of the basin. The net relatedness index (NRI) showed that the bacterial community of lake Ventisquero Negro is determined by environmental filtering, while in the other lakes, species interaction would be a more important driver. The co-occurrence network analysis showed an increase in modularity and in the number of modules when comparing Lake Ventisquero Negro with the two large glacier-fed lakes suggesting an increase in heterogeneity. At the same time, the presence of modules with phototrophic bacteria (Cyanobium strains) in lakes Frías and Mascardi would reflect the increase of this functional photosynthetic association. Overall, our results showed that the reduction in ice masses in Patagonia will affect downstream large deep Piedmont lakes losing the glacial influence in their bacterial communities.

Increased similarity of aquatic bacterial communities of different origin after antibiotic disturbance.

Stochastic or deterministic processes control the bacterial community assembly in waters and their understanding is a fundamental question to correctly manage aquatic environments exposed to the release of antibiotics from anthropogenic sources. It has been suggested that microdiversity (i.e. the rare biosphere) convers freshwater communities with stability, meaning that previously rare taxa bloom when the community is disturbed. Since there might be a seed bank of similar, but not abundant, bacterial taxa in different waters, we tested whether a disturbance by an antibiotic cocktail would increase similarity in bacterial communities from different freshwater systems (a wastewater effluent and two lakes). In a continuous culture set-up in chemostats, we show that disturbance with antibiotics causes communities from different environments to become more similar. Once the antibiotic pressure is released the communities tend to become more dissimilar again. This shows that there is a similar shift in community composition even in waters from very different origins when they are disturbed by antibiotics, even at low concentrations. Antibiotics impact the bacterial communities at the cell and the community level, independently by the original degree of anthropogenic stress they are adapted to, altering the original phenotypes, genotypes, and the relations between bacteria.

A georeferenced rRNA amplicon database of aquatic microbiomes from South America.

The biogeography of bacterial communities is a key topic in Microbial Ecology. Regarding continental water, most studies are carried out in the northern hemisphere, leaving a gap on microorganism's diversity patterns on a global scale. South America harbours approximately one third of the world's total freshwater resources, and is one of these understudied regions. To fill this gap, we compiled 16S rRNA amplicon sequencing data of microbial communities across South America continental water ecosystems, presenting the first database µSudAqua[db]. The database contains over 866 georeferenced samples from 9 different ecoregions with contextual environmental information. For its integration and validation we constructed a curated database (µSudAqua[db.sp]) using samples sequenced by Illumina MiSeq platform with commonly used prokaryote universal primers. This comprised ~60% of the total georeferenced samples of the µSudAqua[db]. This compilation was carried out in the scope of the µSudAqua collaborative network and represents one of the most complete databases of continental water microbial communities from South America.

Bacterial community structure in patagonian Andean Lakes above and below timberline: from community composition to community function.

Lakes located above the timberline are remote systems with a number of extreme environmental conditions, becoming physically harsh ecosystems, and sensors of global change. We analyze bacterial community composition and community-level physiological profiles in mountain lakes located in an altitude gradient in North Patagonian Andes below and above the timberline, together with dissolved organic carbon (DOC) characterization and consumption. Our results indicated a decrease in 71 % of DOC and 65 % in total dissolved phosphorus (TDP) concentration as well as in bacteria abundances along the altitude range (1,380 to 1,950 m a.s.l.). Dissolved organic matter (DOM) fluorescence analysis revealed a low global variability composed by two humic-like components (allochthonous substances) and a single protein-like component (autochthonous substances). Lakes below the timberline showed the presence of all the three components, while lakes above the timberline the protein-like compound constituted the main DOC component. Furthermore, bacterial community composition similarity and ordination analysis showed that altitude and resource concentration (DOC and TDP) were the main variables determining the ordination of groups. Community-level physiological profiles showed a mismatch with bacteria community composition (BCC), indicating the absence of a relationship between genetic and functional diversity in the altitude gradient. However, carbon utilization efficiencies varied according to the presence of different compounds in DOM bulk. The obtained results suggest that the different bacterial communities in these mountain lakes seem to have similar metabolic pathways in order to be able to exploit the available DOC molecules.

Glacier melting and stoichiometric implications for lake community structure: zooplankton species distributions across a natural light gradient.

Glaciers around the globe are melting rapidly, threatening the receiving environments of the world's fresh water reservoirs with significant changes. The meltwater, carried by rivers, contains large amounts of suspended sediment particles, producing longitudinal gradients in the receiving lakes. These gradients may result in changes in the light : nutrient ratio that affect grazer performance by altering elemental food quality. Thus, glacial melting may induce a shift in the phytoplankton carbon : nutrient ratio and hence influence the dominance of herbivorous zooplankton through stoichiometric mechanisms. To test this hypothesis, we combined field and experimental data, taking advantage of a natural light intensity gradient caused by glacial clay input in a deep oligotrophic Patagonian lake. Across this gradient, we evaluated the abundances of two consumer taxa with different phosphorus requirements, the copepod Boeckella gracilipes and the cladoceran Daphnia commutata, using a six-station transect along the lake. We found significant differences in light : nutrient ratio and stoichiometric food quality of the seston, together with a switch from dominance of P-rich Daphnia in low carbon : nutrient stations to dominance of low-P copepods in high carbon : nutrient stations. The laboratory experiments confirmed that the difference in the carbon : nutrient ratio across the gradient is sufficient to impair Daphnia growth. The overall patterns are consistent with our prediction that shifts in the environmental light : nutrient ratio as a result of glacial melting would contribute to shifts in the dominance of stoichiometrically contrasting taxa in consumer guilds.