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Microbial loop of Proterozoic ocean analogue

Meromictic Lake Cadagno, an ancient ocean analogue is known for its permanent stratification and persistent anoxygenic microbial bloom within the chemocline. Although the alpine lake has been extensively studied for at least 25 years, a comprehensive picture of the microbial food web relating phytoplankton, zooplankton, bacteria, and viruses is still missing. This study sought to better understand the abundances and productivity of microbes in the context of nutrient biogeochemical cycling across the stratified zones of Lake Cadagno. Photosynthetic pigments and chloroplast 16S rRNA gene phylogenies suggested the presence of eukaryotic microbial guilds in chemocline and oxic mixolimnion where oxygenic-primary production peaked. Anoxygenic phototroph, Chromatium okenii, was the most abundant bacteria along with the high abundances of Lentimicrobium whose members are known to perform fermentation. Secondary production peaked in the chemocline suggesting that primary producers rely on heterotrophs for nutrients remineralization. Virus-to-microbe ratios (VMR) peaked in the zone of phytoplankton and were minimum at the peak Chromatium okenii hinting viruses modulating oxygenic and anoxygenic photo-and-chemosynthesis. Significance This study provides a window to the past on how microbial guilds of the chemocline may have functioned towards the production and recycling of organic matter in vertical water-columns of sulfur- and ammonia-containing ancient oceans, which slowly evolved to modern oxygenated systems. New observations of this work suggest that eukaryotic algae were persistent in the low oxygen upper-chemocline where they were found to be associated with purple sulfur and green sulfur bacteria in the lower half of the chemocline. This study provides the first insights into Lake Cadagno viral ecology. High viral abundances suggest viruses were essential components of the chemocline where they may cause the release and recycling of organic matter. The framework developed in this study through the integration of a multitude of geochemical and biological data lays the foundation for future studies to further resolve the microbial loop in this early anoxic ocean analogue.