Bacterial diversity in the water column of meromictic Lake Cadagno and evidence for seasonal dynamics

Abstract

The meromictic Lake Cadagno is characterized by a compact chemocline with high concentrations of anoxygenic phototrophic purple and green sulfur bacteria. However, a complete picture of the bacterial diversity, and in particular of effects of seasonality and compartmentalization is missing. To characterize bacterial communities and elucidate relationships between them and their surrounding environment high-throughput 16S rRNA gene pyrosequencing was conducted. Proteobacteria, Chlorobi, Verrucomicrobia, and Actinobacteria were the dominant groups in Lake Cadagno water column. Moreover, bacterial interaction within the chemocline and between oxic and anoxic lake compartments were investigated through fluorescence in situ hybridization (FISH) and flow cytometry (FCM). The different populations of purple sulfur bacteria (PSB) and green sulfur bacteria (GSB) in the chemocline indicate seasonal dynamics of phototrophic sulfur bacteria composition. Interestingly, an exceptional bloom of a cyanobacteria population in the oxic-anoxic transition zone affected the common spatial distribution of phototrophic sulfur bacteria with consequence on chemocline location and water column stability. Our study suggests that both bacterial interactions between different lake compartments and within the chemocline can be a dynamic process influencing the stratification structure of Lake Cadagno water column.

Fig 1. Physico-chemical and turbidity profiles of Lake Cadagno water column.

Oxygen [mg L^-1, blue line], H₂S [mg L^-1], conductivity [mS cm^-1] (left), and turbidity profile [NTU] (right) (12 July 2016). The pink shadow highlight the population of anoxygenic phototrophic sulfur bacteria.

Fig 2. Percentage of relative abundance of bacterial communities at the phylum level according to mixolimnion, chemocline and monimolimnion water layers in Lake Cadagno (October 2012).

Fig 3. Percentage of relative abundance of bacterial communities at the family and genus level according to mixolimnion, chemocline and monimolimnion water layers in Lake Cadagno (October 2012).

Fig 4. PSB and GSB FISH quantification in July and October 2016.

Number of cells (cells mL^-1) with standard error bar for (A) PSB C. okenii, L. roseopersicina, “T. syntrophicum”, L. purpurea and (B) GSB C. clathratiforme, C. phaeobacteroides. Cell concentrations were determined at maximal turbidity, corresponding to 12.2 meters depth in July and 14 meters depth in October.

Fig 5. Flow cytometry detection of phototrophic populations in the oxic-anoxic transition of Lake Cadagno scatter plot SSC versus FSC for chlorophyll-pigmented cells (upper panel) and phycocyanin/phycobillin pigmented cells (lower panel).

Chlorophyll and phycocyanin were used as hallmarks for phototrophic microorganisms and cyanobacteria, respectively. Threshold for pigmentation determination was set to FL3 > 1’100 and FL4 > 1’100 for chlorophyll/bacteriochlorophyll and phycocyanin/phycobillin, respectively. P: C. okenii, G: Chlorobium spp., phycobilin: Cyanobacteria (A) July 2016, (B) July 2017, (C) August 2017.

Fig 6. FCM determined quantification of C. okenii (P), Chlorobium spp. (G), and cyanobacteria (phycobilin) are reported at four depths (± 5%, maximal machine error).