Modeled energetics of bacterial communities in ancient subzero brines

Abstract

Cryopeg brines are isolated volumes of hypersaline water in subzero permafrost. The cryopeg system at Utqiaġvik, Alaska, is estimated to date back to 40 ka BP or earlier, a remnant of a late Pleistocene Ocean. Surprisingly, the cryopeg brines contain high concentrations of organic carbon, including extracellular polysaccharides, and high densities of bacteria. How can these physiologically extreme, old, and geologically isolated systems support such an ecosystem? This study addresses this question by examining the energetics of the Utqiaġvik cryopeg brine ecosystem. Using literature-derived assumptions and new measurements on archived borehole materials, we first estimated the quantity of organic carbon when the system formed. We then considered two bacterial growth trajectories to calculate the lower and upper bounds of the cell-specific metabolic rate of these communities. These bounds represent the first community estimates of metabolic rate in a subzero hypersaline environment. To assess the plausibility of the different growth trajectories, we developed a model of the organic carbon cycle and applied it to three borehole scenarios. We also used dissolved inorganic carbon and nitrogen measurements to independently estimate the metabolic rate. The model reconstructs the growth trajectory of the microbial community and predicts the present-day cell density and organic carbon content. Model input included measured rates of the in-situ enzymatic conversion of particulate to dissolved organic carbon under subzero brine conditions. A sensitivity analysis of model parameters was performed, revealing an interplay between growth rate, cell-specific metabolic rate, and extracellular enzyme activity. This approach allowed us to identify plausible growth trajectories consistent with the observed bacterial densities in the cryopeg brines. We found that the cell-specific metabolic rate in this system is relatively high compared to marine sediments. We attribute this finding to the need to invest energy in the production of extracellular enzymes, for generating bioavailable carbon from particulate organic carbon, and the production of extracellular polysaccharides for cryoprotection and osmoprotection. These results may be relevant to other isolated systems in the polar regions of Earth and to possible ice-bound brines on worlds such as Europa, Enceladus, and Mars.

Keywords: Arctic; cryopeg; extremophiles; maintenance energy; permafrost.

Figure 1

NNW-facing cross-sectional diagram of the Barrow Permafrost Tunnel providing access to cryopeg brines near Utqiaġvik, Alaska. Depicted are the cryopeg boreholes CB1, CBIW and CB4 considered in this study. CB1 and CB4 accessed intra-sediment brines 7–8 m below the massive ice; CBIW accessed intra-ice brine thought to have migrated upwards into the massive ice 11,000 years BP (Iwahana et al., 2021). Units I-III refer to permafrost regions (Meyer et al., 2010). Figure adapted from Iwahana et al. (2021).

Figure 2

Hypothetical growth trajectories to account for a given endpoint cell density. Four possible cases for growth are illustrated here to describe why endpoint measurements leave open many possible trajectories. The case for rapid initial growth followed by stasis is presented in blue. The case for slow growth leading to the observed cell density is presented in green. In red, an intermediate growth rate, followed by decline due to insufficient energy and then an input of organic carbon (OC), depicts recovery of the community to the given endpoint. The no growth scenario, requiring the starting community to be as dense as the endpoint, is indicated by the dashed purple line. Growth lines are intended to be logarithmic, but all lines are conceptual (not drawn to scale).

Figure 3

Graphical representation of processes and quantities modeled for permafrost-enclosed cryopeg brine. Particulate organic carbon (POC) is shown as dark brown circles; dissolved organic carbon (DOC), as yellow semi-circles; and dissolved inorganic carbon (DIC), as black triangles. Extracellular enzymes (teal-colored shapes) produced and released by bacteria hydrolyze POC to DOC. Bacteria take up DOC, respiring it to dissolved DIC or assimilating it into biomass to grow and reproduce. They release DOC back to the brine upon death, attributed explicitly to starvation in the model.

Figure 4

Sensitivity analysis of microbial parameters used in the organic carbon model. Numerical values represent the first-order (in orange) and total-effect (in blue) Sobol indices of the selected parameters: maximum growth rate ( ${\mu }_{\max }$ ), cell-specific metabolic rate ( $m$ ), cell carbon content ( ${\alpha }_D$ ), carrying capacity ( $N_{\max }$ ), cell specific extracellular enzyme activity ( ${\gamma }_{cell}$ ), half-velocity constant for carbon uptake ( $K_D$ ), and starting cell density ( $N_0$ ). First-order indices show the sensitivity of the model when varying only the parameter in question. Total-effect indices show the sensitivity of the model when varying the selected parameter in conjunction with the other parameters selected in this analysis. Values shown are rounded at 10⁻².

Figure 5

Model predictions of cell density over the lifetime of the system for each cryopeg brine scenario. Measured cell densities at 40,000 years are recreated by the model only under conditions shown in panel A: lower bound net growth rate ( ${\mu }_{\max }$ ), lower bound cell-specific metabolic rate ( $m)$ , and the calculated cell-specific extracellular enzyme activity ( ${\gamma }_{cell}$ ); and for CBIW in panel B: lower bound net growth rate ( ${\mu }_{\max }$ ), lower bound cell-specific metabolic rate ( $m)$ , and the measured cell-specific extracellular enzyme activity ( ${\gamma }_{cell}$ ). Panels C through H depict cases where the predicted end cell density did not match the observed cell density, regardless of the combination of bounds applied.