Microbial response to climate-induced nutrient alterations in high Arctic freshwaters

In the Arctic, climate change leads to increased nutrient levels and organic carbon in freshwaters, caused by factors like permafrost thaw and growing populations of geese. Such alterations significantly impact freshwater ecosystems, potentially influencing community composition and diversity across various levels, including general microbial metabolism. We tested the hypothesis that a transition from autotrophy to heterotrophy occurs across a chronosequence of lakes in the high Arctic as a result of glacier retreat, influenced by distinct nutrient supplies and varying ecological succession statuses. To do so, we studied 5 lakes in the vicinity of Ny-Ålesund (Svalbard) following a chronosequence. The older lakes, closer to the fjord, were strongly impacted by birds, notably geese. For each lake, we tested the response to nutrients by adding an artificial nutrient solution with N and P, and the response to light or dark conditions. We incubated unfiltered water samples (80 mL) at 4 ºC in 120 mL flasks with atmospheric air as headspace. After 24h, samples for gases (O₂, CO₂, CH₄ and N₂O), nutrients (organic C, P and N) and eDNA (16S metabarcoding) were collected. Ar-corrected gas saturation of each GHG was used as a proxy of net metabolic changes. Regardless of the treatment applied, our results showed an increase in N₂O saturation coupled with a decrease in O₂ saturation after 24h in bird-impacted lakes, likely related to heterotrophic microbial activity. In such lakes, dark conditions promoted P accumulation, while N accumulated equally in light and dark incubations. In younger lakes (i.e., not impacted by birds), increased O₂ saturation after 24h of incubation suggested that phototrophic metabolism was dominant. For nutrients, no significant pattern was observed for both light and dark incubations in younger lakes. Bacterial community composition differed between locations after 24h of incubation with a greater uniformity of species in younger lakes. This research advances our understanding of how nutrient enrichment affects biodiversity in the Arctic and metabolism in freshwater ecosystems.