Ecological dynamics of anoxygenic phototrophs in stably redox-stratified waters: Intra and inter-seasonal variability of Lake Cadagno

Abstract

Lake Cadagno differs from typical alpine lakes as it is stratified into two water layers that never vertically mix. This stratification creates a niche for the development of primordial anoxygenic phototrophs, which thrive in the chemocline of the lake, forming a characteristic bacterial layer (BL). Yet, the relationship between the temporal variation of meteorological factors that regulate stratification and the development of the BL remains unclear. Here, we explored the intra- and inter-seasonal stability of the water column stratification and ecological dynamics of the anoxygenic phototroph community of the BL over three years. Our continuous monitoring showed that the meromixis of the lake is highly stable, with density stratification seemingly unaffected by external meteorological factors. Further reanalysis of the lake’s recent history substantiated this remarkable stability. In contrast, the community of anoxygenic phototrophs showed significant intra- and inter-seasonal variability, modulated by weather events that primarily impacted light penetration. In fact, an exceptional intra-seasonal light increases in September 2020 led to an overgrowth of purple sulfur bacteria compared to commonly dominant, green ones. At the inter-seasonal level, there is a difference in BL development in July 2021, which was characterized by much precipitation and less light, compared with that in 2019/2020.