Bacterial diversity, ecosystem functioning and ecosystem services: Evidence from two dilution-to-extinction experiments
By now, there is convincing evidence that diversity–ecosystem functioning relationships are positive but often saturate at low richness levels. This raises the question if diversity plays a role in mega-diverse bacterial communities. Given the tremendously important role that bacteria play in all ecosystems and the crucial services they provide to mankind, this question is relevant both from a theoretical and very practical point of view. Here I present two experiments in which we established diversity gradients from natural bacterial lake communities, resulting in richness levels spanning two orders of magnitude.
In the first experiment we incubated dilution series of 4 Swedish lake communities for 6 weeks outdoors, and measured biomass, temporal stability and the carbon source utilization potential. In the second experiment, a diversity gradient was crossed with a gradient of ciprofloxacin, a widespread and common antibiotic in human wastewaters. In addition, the tolerance to a range of other antibiotics was tested with a novel well-plate based assay. In both experiments diversity was measured using illumina amplicon sequencing, which enabled us to 1) accurately measure realized diversity 2) determine the community composition and 3) calculate phylogenetic diversity of the samples.
Neither species richness nor the effective number of species nor phylogenetic diversity was significantly correlated with either of the measured ecosystem functions - including maximum biomass, temporal stability of community biomass and the carbon source utilization potential of the communities. The levels of functioning where nevertheless highly variable, suggesting that species identity may have been more important. In contrast, the second experiment showed that although ciprofloxacin did not reduce the overall biomass, the lowest diversity treatment was significantly more sensitive than the undiluted community to a range of environmental toxins.
In conclusion our findings suggest that diversity is uncorrelated to traditional metrics of ecosystem functioning in the investigated mega-diverse bacterial communities, suggesting high functional redundancy. However, there is evidence that reduced diversity significantly impacts the community tolerance to a range of antibiotics.