Microbial communities responses in fluvial biofilms under metal stressed scenarios
- Autores: María Argudo
- Directores de la Tesis: Frederic Gich Batlle (dir. tes.), Helena Guasch i Padró (codir. tes.)
- Lectura: En la Universitat de Girona ( España ) en 2021
- Idioma: español
- Tribunal Calificador de la Tesis: Eugènia Martí Roca (presid.), Lluís Bañeras (secret.), Chloé Bonnineau (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología del Agua por la Universidad de Girona
- Materias:
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- Resumen
SUMMARY Fluvial ecosystems are one of the most complex and diverse systems on the planet. Biological communities that live in these ecosystems depend on environmental factors and biotic interactions. Nowadays, freshwater ecosystems are subjected to multiple stress conditions, which can include natural and anthropic stressors. A stressor, which is still under concern today, is metal pollution due to its high biotoxicity, perdurability and bioaccumulation across the food chain, which causes adverse effects on biota and contributes to the deterioration of fluvial ecosystem integrity. A good indicator of metal stress is the response of fluvial biofilm, especially prokaryotes living in it. Prokaryotes respond quickly to environmental changes due to their abundance, high diversity and fast growth rate. Moreover, they support important ecosystem functions and ensure the stability and recovery of fluvial ecosystems, so any stressor affecting microorganisms would cause serious consequences to the ecosystems. Consequently, microbial ecotoxicology, with the help of metagenomics, provides a good approach to understand and evaluate the impact of metal pollution on the structure and function of microbial communities. However, there is still some uncertainty in the assessment and prediction of the effects of metals on a large scale (ecosystem scale). Field studies help us get closer to this ecological reality over a holistic approach.
This thesis aims to investigate the effects of metals from natural and anthropogenic sources on the composition and function of the prokaryotic communities living in epilithic fluvial biofilms, based on molecular analyses and field studies. The thesis includes three field studies carried out at different temporal and spatial scales as a multi-scale way of approaching the complexity of fluvial ecosystems. First, a passive biomonitoring study with biofilms was conducted along different points of the Osor, Llémena and Ter Rivers to understand the procedures and interest of performing amplicon sequencing of the DNA and RNA fractions of the 16 rRNA gene analysis of the prokaryotic component of biofilms, in a eutrophic environment with low metal pollution. Second, an experiment with mesocosms filled with natural colonized cobbles was carried out over the Osor River, where the presence of fish was controlled, to determine their impact on the structure and function of biofilm microbial communities in a multiple stressed environment. Finally, the third study was performed in an iron (Fe) groundwater spring (Can Verdaguer) at the Llémena watershed, where water, biofilm and leaf samples were collected to examine the environmental and biotic drivers of the structure and function of microbial community.
The results obtained in this thesis show that the analysis of DNA and RNA to determine α and β-diversity of prokaryotic communities provided different and complementary information about the ecological integrity of the ecosystem. The DNA fraction (resident community) was a poor indicator of metal pollution, although it detected a change in the composition of the bacteria over upstream-downstream gradient of mineralization and nutrient contents. However, the RNA fraction (active community) detected community responses to low metal pollution with similar bacterial communities in sites affected by metals. In addition, the high content of RNA in the most polluted samples indicated the presence of a very active microbial community, which suggested a specific response to metal exposure, such as detoxification processes.
With respect to the presence or absence of fish in a multiple-stressed scenario, the results obtained show that the combined effects of pollution, water stress and fish had a pronounced effect on the structure of microbial communities. Biomass, nutrient uptake of biofilm and α-diversity of prokaryotes did not follow the gradient of metal pollution. However, the differences in the composition of prokaryotes (β-diversity) were very clear over the metal pollution gradient and therefore, some indicator families of each site could be classified. Interestingly, endosymbiotic bacteria appeared in the site most affected by nutrient enrichment, metal pollution and hydrological alteration. Moreover, the effects of the fish bioturbation on biofilm reduced Chl-a and AFDW and increased the toxicity of metals, thus confirming the importance of these macroconsumers in the biofilm architecture, and consequently, in the functioning of fluvial ecosystems.
In relation to the Fe spring, the main drivers of microbial community were water chemistry and biological competition. As reported for mining metals, α-diversity of prokaryotes was not affected by chemical stress in the Fe spring. In fact, a large number of species passed the extreme environmental filter of high Fe concentration creating a unique and very rich prokaryotic community sustained by chemiolitotrophic species. However, β-diversity of prokaryotes, primary production and leaf litter decomposition rates followed the chemical stress gradient. The extreme conditions were deleterious for algae and organisms responsible for leaf decomposition leading to a very low primary production and decomposition rates. As the concentration of Fe decreased downstream, the primary production and breakdown increased generating competitive exclusion, in such a way that a different prokaryotic community less diverse and with other functions was found.
Overall, this thesis shows how the metals of natural and anthropic origin change significantly the composition of the prokaryotic communities (mainly the composition of bacteria). The β-diversity is the most sensitive variable to the effects of metals, unlike α-diversity, which is hardly affected or even benefited. In scenarios with high and chronic metal pollution, the resident community (DNA fraction) suffers changes in its composition that can be detected at phylum or class taxonomic level. However, in fluvial ecosystems, subjected to lower levels of metal pollution, only the RNA fraction is affected by selecting more active OTUs/ASVs. In addition, the knowledge about the bacterial composition of communities and, consequently, the selection of specific taxa is useful to find some potential prokaryotes functions important in stress response caused by metals.
