Divergent microbial functional pathways and declining redundancy in lake sediments across eutrophication gradients.

Microbial communities underpin nutrient cycling across aquatic ecosystems and are strongly influenced by human disturbance, yet the implication for functional redundancy remains poorly understood. Lake eutrophication is a global threat to freshwater systems, primarily driven by human activities within lakes and surrounding catchments. We hypothesize that eutrophication alters functional redundancy of key metabolic pathways in lake sediments. To test this, we conducted a national scale metagenomic study of surface sediments from 144 New Zealand lakes spanning a broad nutrient gradient. Increasing eutrophication is associated with reduced taxon-based functional redundancy, the potential of multiple species to perform the same function, indicating a narrower taxa pool supporting core functions. In contrast, abundance-based functional redundancy, characterized by the abundance of organisms that can perform the function, showed pathway-specific responses across trophic gradients. For example, nitrification and denitrification have greater abundance-based functional redundancy under higher nutrient conditions, whereas phosphorus transport shows reduced redundancy. By eroding microbial functional redundancy, eutrophication may increase susceptibility of microbial communities to environmental perturbations, potentially compromising their ability to sustain key metabolic processes. As lakes play key roles in biogeochemical cycles, the lower stability of vital biogeochemical pathways in lake sediments is likely to have an impact across ecosystems globally.