The biogeochemical vertical structure renders a meromictic volcanic lake a trap for geogenic CO₂ (Lake Averno, Italy) - Fig 2

<p>Vertical profiles along the Lake Averno water column of (a) water temperature (°C), (b) pH, (c) electrical conductivity (EC, in mS/cm), and (d) dissolved O₂ (mg/L).</p

Schematic map of the Campi Flegrei caldera with the location of Lake Averno.

<p>This Figure is similar but not identical to the original image, and is therefore for illustrative purposes only.</p

The biogeochemical vertical structure renders a meromictic volcanic lake a trap for geogenic CO₂ (Lake Averno, Italy) - Fig 8

<p>(a): Vertical profile of water density (in g/cm³), calculated using data measured in 2015 in Eq (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193914#pone.0193914.e002" target="_blank">2</a>) (black line), compared to that computed assuming T = 7°C ([<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193914#pone.0193914.ref026" target="_blank">26</a>]; red line) and T = 10°C (blue line) in the epilimnion. At the latter temperature, the density of epilimnetic water is higher than that measured at >8 m depth. (b): Vertical profile of water density (in g/cm³), calculated using data measured in 2015 in Eq (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193914#pone.0193914.e002" target="_blank">2</a>) (black line), compared to that resulting from changes in the concentrations of dissolved CH₄ and CO₂ (considering CH₄×20 and CO₂ = 0; green line). The dashed arrows show the decrease of the water density at ≥20 m depth. (c): Vertical profiles of <i>P</i>tot of dissolved gases (in atm) in February and October 2005 ([<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193914#pone.0193914.ref026" target="_blank">26</a>]; magenta and light blue lines, respectively), September 2010 ([<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193914#pone.0193914.ref031" target="_blank">31</a>]; green line) and June 2015 (this paper, black line), compared with that of the hydrostatic pressure (black straight line). The vertical profile of <i>P</i>tot of dissolved gases in June 2015 assuming <i>P</i>CH₄×20 (see Fig 8B) is also reported (grey line).</p

Sampling depth (m), temperature (°C), pH, salinity (expressed as TDS, in mg/L) alkalinity (alk, in mg/L) and chemical composition (in mg/L) of the main solutes, reduced sulfur species and trace elements in water samples from Lake Averno.

<p>The δ¹³C-TDIC (in ‰ vs. V-PDB), δD-H₂O (in ‰ vs. V-SMOW) and δ¹⁸O-H₂O (in ‰ vs. V-SMOW) values are also reported.</p

Sampling depth (m) and chemical composition (in atm) of the main dissolved gases in samples from Lake Averno.

<p>The isotopic composition of dissolved CO₂ (δ¹³C-CO_2, in ‰ vs. V-PDB) and CH₄ (δ¹³C-CH₄ and δD-CH₄, in ‰ vs. V-PDB and ‰ vs. V-SMOW, respectively), and the <i>P</i>CO₂ values expected at equilibrium with alkalinity (<i>P</i>CO_2calc, in atm) are also reported.</p

The biogeochemical vertical structure renders a meromictic volcanic lake a trap for geogenic CO₂ (Lake Averno, Italy)

<div><p>Volcanic lakes are characterized by physicochemical favorable conditions for the development of reservoirs of C-bearing greenhouse gases that can be dispersed to air during occasional rollover events. By combining a microbiological and geochemical approach, we showed that the chemistry of the CO₂- and CH₄-rich gas reservoir hosted within the meromictic Lake Averno (Campi Flegrei, southern Italy) are related to the microbial niche differentiation along the vertical water column. The simultaneous occurrence of diverse functional groups of microbes operating under different conditions suggests that these habitats harbor complex microbial consortia that impact on the production and consumption of greenhouse gases. In the epilimnion, the activity of aerobic methanotrophic bacteria and photosynthetic biota, together with CO₂ dissolution at relatively high pH, enhanced CO₂- and CH₄ consumption, which also occurred in the hypolimnion. Moreover, results from computations carried out to evaluate the dependence of the lake stability on the CO₂/CH₄ ratios, suggested that the water density vertical gradient was mainly controlled by salinity and temperature, whereas the effect of dissolved gases was minor, excepting if extremely high increases of CH₄ are admitted. Therefore, biological processes, controlling the composition of CO₂ and CH₄, contributed to stabilize the lake stratification of the lake. Overall, Lake Averno, and supposedly the numerous worldwide distributed volcanic lakes having similar features (namely bio-activity lakes), acts as a sink for the CO₂ supplied from the hydrothermal/magmatic system, displaying a significant influence on the local carbon budget.</p></div

Relationships between environmental variables (chemical parameters and dissolved gases) and taxonomic composition, considering the 40 most abundant bacterial genera in the samples across sites.

<p>Nonmetric MultiDimensional Scaling (NMDS) ordination plot represents the typifying microbial composition in the transition from the surface to deep waters. Stress value indicates the significant concordance between the distance among samples in the NMDS plot and the actual Bray-Curtis distance among samples. Each dot represents the microbial community at a specific depth. Distance between the sample dots signifies similarity; the closer the samples are, the more similar microbial composition they have. The chemical parameters were incorporated in the NMDS analysis with a vector-fitting procedure.</p

Schematic conceptual model of the interactions between microbial populations and geochemical parameters at different depth in Lake Averno.

<p>Schematic conceptual model of the interactions between microbial populations and geochemical parameters at different depth in Lake Averno.</p

The biogeochemical vertical structure renders a meromictic volcanic lake a trap for geogenic CO₂ (Lake Averno, Italy) - Fig 3

<p>Vertical profiles along the Lake Averno water column of (a) P, Fe and Mn concentrations, (b) ΣS^2-/SO₄^2- and NH₄⁺/NO₃^- ratios, and (c) δ¹³C-CH₄ values (‰ vs. V-PDB) and CO₂ and CH₄ partial pressures (atm).</p

Vertical microbiological profiles along the Lake Averno water column estimated by CARD-FISH.

<p>(a) archaea and bacteria (cell ml^-1); (b) Proteobacteria (Alpha, Beta, Gamma, Delta and Epsilonproteobacteria), <i>Bacteroidetes</i>-Flavobacteria (cf) and autofluorescent cells (expressed as % of total bacteria).</p