Convection‐Diffusion Competition Within Mixed Layers of Stratified Natural Waters

Steiner, Oscar Sepúlveda; Bouffard, Damien; Wüest, Alfred

doi:10.1029/2019gl085361

Cited by 10 publications

(28 citation statements)

References 34 publications

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“…Once a minimum number of cells is reached, C. okenii ’s phototactic but aerophobic movement (Pfennig 1962 ) produces a local remixing of water, triggering a process called bioconvection (Sommer et al . 2017 ; Sepúlveda Steiner, Bouffard and Wüest 2019 ). This phenomenon is strongly linked to the number of C. okenii cells and to the presence of oxygen, which, by stopping the cells, increases the density of the water that precipitates downwards.…”

Section: Introductionmentioning

confidence: 99%

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

Nezio

Beney

Roman

et al. 2021

FEMS Microbiology Ecology

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Meromictic lakes are interesting ecosystems to study anaerobic microorganisms due their permanent stratification allowing the formation of a stable anoxic environment. The crenogenic meromictic Lake Cadagno harbors an important community of anoxygenic phototrophic sulfur bacteria responsible for almost half of its total productivity. Besides their ability to fix CO2 through photosynthesis, these microorganisms also showed high rates of dark carbon fixation via chemosyntesis. Here, we grew in pure cultures three populations of anoxygenic phototrophic sulfur bacteria previously isolated from the lake, accounting for 72.8% of the total microbial community, and exibiting different phenotypes: 1) the motile, large-celled purple sulfur bacterium (PSB) Chromatium okenii, 2) the small-celled PSB Thiodictyon syntrophicum, and 3) the green sulfur bacterium (GSB) Chlorobium phaeobacteroides. We measured their ability to fix CO2 through photo- and chemo-synthesis, both in situ in the lake and in laboratory under different incubation conditions. We also evaluated the efficiency and velocity of H2S photo-oxidation, an important reaction in the anoxygenic photosynthesis process. Our results confirm that phototrophic sulfur bacteria strongly fix CO2 in the presence of light and that oxygen increases chemosynthesis at night, in laboratory conditions. Moreover, substancial differences were displayed between the three selected populations in terms of activity and abundance.

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Section: Introductionmentioning

confidence: 99%

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

Nezio

Beney

Roman

et al. 2021

FEMS Microbiology Ecology

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“…Weather conditions during the field campaign were warm with mild winds not exceeding 5 m s −1 . However, the observed ML was less prominent compared to previous years (Sommer et al 2017; Sepúlveda Steiner et al 2019).…”

Section: Measurements and Methodsmentioning

confidence: 62%

“…In this paper, this limiting effect is referred to as convection‐diffusion competition. We used the symmetric diffusive‐shape model of Sepúlveda Steiner et al (2019) to interpret convection‐diffusion competition within the measured ML temperature profiles. The model ( Φ T ) was deducted from the 1D vertical diffusion equation and reads as follows:

Φ_{T} ((), z) = normalΔ T ([], \erf ((), \frac{\frac{h}{2} - (z - z_{o})}{δ}) - \erf ((), \frac{\frac{h}{2} + (z - z_{o})}{δ})) + T_{o} - G_{T} ((), z - z_{o})

…”

Section: Measurements and Methodsmentioning

confidence: 99%

“…Development of such MLs has been previously described using direct numerical simulations (DNS; Sommer et al 2017). Once the ML is formed, the continuous bacterial upward migration sustain convection‐diffusion competition (Sepúlveda Steiner et al 2019) between the ML and its adjacent stratified waters above and below. This determines the ML vertical structure (Fig.…”

Section: Bioconvection In Lake Cadagnomentioning

confidence: 99%

“…This process explains the presence of a ML during daytime hours. Nevertheless, Sepúlveda Steiner et al (2019) describe MLs that also occur at night. Three distinct hypotheses might explain the persistence of such a “dark ML”.…”

Section: Bioconvection In Lake Cadagnomentioning

confidence: 99%

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Persistence of bioconvection‐induced mixed layers in a stratified lake

Steiner

Bouffard

Wüest

2021

Limnology & Oceanography

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In situ observations of biophysical interactions in natural waters typically focus on physical mechanisms influencing biological activity. Yet, biological activity can also drive physical processes in aquatic environments. A community of photoautotrophic, motile and heavy bacteria—Chromatium okenii, which requires light, sulfide, and anoxic conditions to perform anoxygenic photosynthesis, accumulates below the chemocline of the meromictic Lake Cadagno (Switzerland). Upward vertical migration drives bioconvection, which modifies the physical environment of the bacteria‐populated water to create a deep and homogeneous mixed layer of up to 1 m thickness. Continuous convection within the mixed layer and diapycnal diffusivity from its adjacent stratified surroundings determine ecologically relevant gradients. The daytime vertical migration that induce convective instabilities is well‐established. It consists in bacteria swimming upward towards light and accumulating at the upper part of the anoxic layer, leading to a locally‐unstable density excess. However, nocturnal activity has not yet been analyzed. An intensive 48‐h survey was conducted in August 2018 using standard and microstructure profilers, as well as a moored high‐resolution current meter coupled with temperature and turbidity sensors deployed across the mixed layer depth. This survey revealed a persistent mixed layer also during nighttime hours. Using a mixed layer shape model, vertical velocity observations and turbulent dissipation estimates, we conclude that photoautotrophic bacteria continue their vertical migration at night. This nocturnal activity thereby drives “dark bioconvection” and maintains the subsurface mixed bacterial layer in Lake Cadagno throughout the diel cycle.

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Detritus‐hosted methanogenesis sustains the methane paradox in an alpine lake

Bartosiewicz

Venetz

Läubli

et al. 2022

Limnology & Oceanography

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Accumulation of methane in oxic waters of lakes and the ocean has been widely reported. Despite the importance for the greenhouse gas budget, mechanistic controls of such "methane paradox" remain elusive. Here, we use a combination of CH 4 concentration and isotopic (δ 13 C CH4 , δD H2O and δ 18 O H2O ) measurements, plankton incubations and microbial community assessments to demonstrate the existence of the methane paradox in oxygenated waters of a meromictic lake (Lake Cadagno, Switzerland). Following mass dynamics using water isotopes, we exclude the possibility that the accumulation of CH 4 at the thermocline results solely from lateral transport. Interannual variability in the magnitude of the methane paradox (between 0.5 and 5 μmol L À1 ) is associated to stratification patterns, changes in zooplankton biomass and planktonic detritus accumulation along density gradients, as well as fluctuating microbial cell numbers. The links between hydrodynamic conditions, aggregation of planktonic detritus and its microbiome, as well as the accumulation of CH 4 in the water column are further supported by high-resolution echosounder revealing backscatter maxima at the top of the thermocline, where detritus is effectively trapped, and by oxic incubations showing that CH 4 is produced in zooplankton detritus (0.046 nmol L À1 to 0.095 CH 4 mg dry mass L À1 d À1 ). Our results also show that detritushosted methanogenesis is stimulated through the addition of methylphosphonate, suggesting that zooplankton-associated microbiomes exploit organic phosphorus compounds to release CH 4 . Understanding the variability of the methane paradox in relation to changing hydrodynamics and plankton communities will be crucial to predict the future role of lakes in the global methane budget.

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Convection‐Diffusion Competition Within Mixed Layers of Stratified Natural Waters

Cited by 10 publications

References 34 publications

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

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

Persistence of bioconvection‐induced mixed layers in a stratified lake

Detritus‐hosted methanogenesis sustains the methane paradox in an alpine lake

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