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.

Anoxygenic photo- and chemo-synthesis of phototrophic sulfur bacteria from an alpine meromictic lake

“…In both studies, measurements were taken in September, when C. okenii population normally declines, making way for the development of small-celled PSB populations (Danza et al . 2018 ), known to be able to fix CO 2 in the dark (Storelli et al . 2013 ).…”

“… 2017 ; Danza et al . 2018 ). A BD Accuri C6 cytometer (Becton Dickinson, San José, CA) equipped with two lasers (488 and 680 nm), two scatter detectors and four fluorescence detectors (laser 488 nm: FL1 = 533/30, FL2 = 585/40, FL3 = 670; laser 640 nm: FL4 = 675/25) was used for samples analysis.…”

“… 2012 ; Danza et al . 2018 ). Moreover, Lake Cadagno is considered an open living archive system to study microorganisms that have been key for the development of early life on Earth (Crowe et al .…”

Mixotrophic Growth Under Micro-Oxic Conditions in the Purple Sulfur Bacterium “Thiodictyon syntrophicum”

“…Whereas the monimolimnion has been described as anoxic (Del Don et al, 2001 ), recurrent blooms of oxygenic plankton (Camacho et al, 2001 ; Danza et al, 2018 ) and optode-based measurements of dissolved oxygen have revealed micro-oxic conditions below 20 nM in the chemocline (Milucka et al, 2015 ). CTD measurements (this study) and FCM counts of the chemocline throughout the summer 2017 (Danza et al, 2018 ) revealed a cyanobacterial bloom down to the monimolimnion from July 2017 onward that resulted in a partly oxygenated chemocline with around 0.6 mg O 2 L −1 (19 μM) below a depth of 12 m. The chemocline also retained some of the produced oxygen through the night ( Figure 2 ). Consequently, the O 2 present at the chemocline could possibly be used by str.…”

“…Flow-cytometry (FCM) based cell counting was performed as in Danza et al ( 2018 ). In short, phototrophic bacteria were identified using 50 μl sub-samples in triplicates with a BD Accuri C6 cytometer (Becton Dickinson, San José, CA, USA).…”