Biodiversity is declining worldwide with detrimental effects on ecosystems functions and services that it sustains. The relationship between biodiversity and freshwater purification remains unclear. Freshwater purification is of paramount importance for humankind as eighty percent of the worlds population is exposed to high levels of threat in terms of water security. Bacteria are the most diverse and abundant organisms on Earth and they play, directly or indirectly, a key role in the majority of ecosystem services including water purification. The current work aimed, in freshwater systems, to unravel the relationships between microbial diversity and: (a) biodegradation of toxic compounds (i.e. specialised function); (b) respiration (i.e. broad function) and; (c) stability of broad functioning. Firstly, preliminary experiments were carried out to establish freshwater sample size to representatively evaluate bacterial communities diversity and also suitable natural and man-made toxic compounds for freshwater incubation experiments. Then, the microbial communities ability to degrade microcystin-LR was explored in the context of previous exposures and nutrient availability. Finally, we focused on the relationships between diversity and functioning. A decrease in microbial diversity caused a decrease in both broad and specialised ecosystem functions tested. Stability of broad functioning was also negatively affected by a decrease in microbial diversity. Both lakes (Scotland) and rivers (Australia) microcosms experiments resulted in comparable findings suggesting consistent relationships across different freshwater systems. These results highlight that, similarly to macro-organisms (plant and animals), declining diversity of the microbial communities has direct consequences for important ecosystem functioning and services and therefore, microbial diversity should be explicitly considered in all biodiversity conservation debates
