Resumen de Understanding stochastic and deterministic microbial assembly processes in ephemeral saline lakes
Understanding the mechanisms and processes driving biological communities assembly, i.e. determining why a specific community is found under certain circumstances, is one of the fundamental questions in ecology. The potential effect of the different assembly processes (selection, dispersal and ecological drift, together with speciation which operates at evolutionary timescales) has been studied over multiple systems and types of organisms, proposing a complex equilibrium with a variable dominance of either stochastic or deterministic assembly processes. In this PhD thesis, we aimed to reach a deep understanding on the processes and mechanisms driving microbial communities assembly using a highly dynamic system of ephemeral saline lakes potentially interconnected by dispersal processes as study model. Ephemeral saline lakes are a perfect scenario to assess the effect of variable selective pressure (promoted by wide salinity fluctuations) and the potential effect on complementary assembly processes as dispersal and ecological drift. The study of these processes across environmental, spatial and temporal scales may offer a mechanistic understanding on how assembly processes interact to determine communities diversity.
Along three consecutive hydrological cycles, we characterized the monthly dynamics of the aquatic microbiome inhabiting 14 ephemeral salt lakes in the Monegros Desert (NE Spain). These systems experienced severe variations in water volume along the hydrological cycle, which promote wide salinity fluctuations from freshwater to saltsaturation.
The inhabiting bacterial, archaeal and microeukaryal communities were characterized by high-throughput sequencing of the 16S and 18S rRNA genes, respectively. We assessed the effect of increasing salinity concentrations in the diversity and composition of microbial communities, together with ecological and metabolic effects. Next, we analyzed the ephemeral lakes from a metacommunity perspective to investigate the role of the different assembly processes along the hydrological cycle and their interactions to shape the observed dynamics.
As expected, we noticed that increasing salinity gradients strongly determined microbial communities compositions. While a taxonomic replacement was observed in the bacterial communities driven by mass extinctions followed by colonization by better-adapted halophilic groups, eukaryal communities suffered a severe environmental filtering which produced a simplified assembly dominated by a few halophilic groups, mainly the photosynthetic algae Dunaliella and Navicula.
Conversely, the rich archaeal assemblage of Monegros become more abundant as lakes reached hypersaline concentrations. Through a novel genomic trait-based approach we set up the transition from stochasticity to deterministic assembly describing a consistent threshold at salinities c.a. 5%. Beyond this threshold, increasing selective pressure reduced the potential colonization success among lakes, but also the potential colonization by allochthonous microbes from other biomes as sediments and soils. Potential interspecies interactions along the salinity gradient were addressed by a network analysis that showed that increasing selective pressure, and the associated transition from generalists to specialists groups, reduced competition and did not promote facilitative associations. Overall, the multiscale assessment of the deterministic and stochastic assembly processes in the Monegros ephemeral lacustrine system, together with the previous vast knowledge on both the ecology and microbiology of saline system and the physiological effects of salinity gradients, allowed us to describe the biological assembly from an unusual combination of mechanistic and integrative points of view.
