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An interdisciplinary approach to determine biodiversity and ecosystem function relationships

Final Activity Report Summary - DIVECOFUN (An interdisciplinary approach to determine biodiversity and ecosystem function relationships)

A major challenge in ecological and ecosystems research is the need to understand the relationships between biodiversity and ecosystem function. The extent of environmental change, including climate change, makes this challenge particularly important and urgent. Bacteria are important for ecosystem functioning because of their essential role in carbon and nutrient cycling, in particular as degraders of both natural organic substrates and pollutants. Microbiologists are therefore increasingly contributing to the knowledge within this research area that investigates biodiversity-ecosystem function relationships, because it is important to increase the knowledge about the regulating factors of bacterial diversity and about consequences of changes in bacterial diversity for ecosystem functions, such as substrate degradation. On the other hand, ecologists have realised that microbial model systems are particularly valuable, and underutilised, tools to address general ecological questions. This is mostly a result of the small size and fast generation times of bacteria, which makes them ideal organisms for the construction of experiments covering a range of manipulations and sampling over time that would be impossible to achieve through the use of higher organisms.

The major objective of this project was therefore to integrate previously separated microbiological and ecological approaches to study the biodiversity-ecosystem function relationship. We therefore explored a bacterial model system to address the following questions: 1) how is diversity affected by resource availability?; 2) how is diversity related to substrate utilisation?; and 3) how are diversity-substrate utilisation relationships influenced by environmental fluctuations?

To answer the first question, i.e. how resource availability influences bacterial diversity, we performed an experiment involving stable isotope probing, a method that enables direct identification of bacterial populations able to degrade a specific substrate (in our case, benzoate) in the environment. This was achieved by adding a heavy isotope, i.e. 13C-labelled benzoate, to soil samples and 13C-labelled nucleic acids that were derived from bacteria that had utilised the substrate were then separated from the bulk bacterial communities and further analysed by a range of molecular biology methods. This study showed that that there were different assemblages active at different substrate concentrations. In addition, differences in the diversity of the active bacterial community were found and there was a decrease in both species richness and evenness with increasing substrate concentration. This result is analogous to studies of plant communities, which show that fertilisation promotes species dominance.

To answer the second question, i.e. how bacterial species richness is related to resource utilisation, we performed an experiment with so-called designer communities, i.e. highly controlled, simple communities, that were constructed from bacterial strains that were isolated from soil at an early stage of the project. We constructed communities that differed in species richness and studied the effect on substrate utilisation over time and with substrate complexity. Using a modelling approach during data analysis we demonstrated that species richness had a beneficial effect on substrate utilisation, irrespective of time or the complexity of the available substrate.

To answer the third question, i.e. how biodiversity ecosystem function relationships differ in dependence of environmental fluctuations, we used the designer-community approach described above. We set up an experiment to study the effect of bacterial species richness on substrate utilisation under different temperature fluctuation regimes. The results show that species richness and temperature fluctuations had an effect on substrate utilisation, indicating that not only species richness but richness in combination with an environmental factor or the strength of a disturbance event, affects resource utilisation. Our results also suggest that species dominance is most pronounced in systems with high temperature fluctuations.