Biodiversity of poly-extremophilic Bacteria: does combining the extremes of high salt, alkaline pH and elevated temperature approach a physico-chemical boundary for life?

Abstract

Bacterial microorganisms that grow optimally at Na+ concentrations of 1.7 M, or the equivalent of 10% (w/v) NaCl, and greater are considered to be extreme halophiles. This review focuses on the correlation between the extent of alkaline pH and elevated temperature optima and the extent of salt tolerance of extremely halophilic eubacteria; the focus is on those with alkaline pH optima, above 8.5, and elevated temperature optima, above 50 degrees C. If all three conditions are required for optimal growth, these microorganisms are termed "poly-extremophiles". However, only a very few extreme halophiles able to grow optimally under alkaline conditions as well as at elevated temperatures have been isolated so far. Therefore the question is: do the combined extreme growth conditions of the recently isolated poly-extremophiles, i.e., anaerobic halophilic alkalithermophiles, approach a physico-chemical boundary for life? These poly-extremophiles are of interest, as their adaptive mechanisms give insight into organisms' abilities to survive in environments which were previously considered prohibitive to life, as well as to possible properties of early evolutionary and extraterrestrial life forms.

Figure 1

Correlation of [Na⁺] optimum and pH or temperature optima of extreme halophiles. Figure 1a. Correlation between [Na⁺] optimum and pH optimum. [Na⁺] optimum (M) is plotted against pH optimum; number of microorganisms at each locus is plotted on z-axis as indicated by color coding; no representation (zero) is indicated in the darkest shade. Figure 1b. Correlation between [Na⁺] vs. temperature optima. [Na⁺] optimum (M) is plotted against temperature optimum (°C); number of microorganisms at each locus is plotted on z-axis as indicated by color coding; no representation (zero) is indicated in the darkest shade.

Figure 2

Clustering of poly-extremophiles relative to other extreme halophiles. Representation of extremely halophilic Bacteria for which both additionally-considered growth conditions (pH and temperature) approach or exceed thermophilic and alkaliphilic levels. The optima of the discussed recently isolated poly-extremophiles cluster in the upper range for each criterion, well-separated from other representative microorganisms. The z-axis color coding depicts the temperature optimum of each bacterium. Species represented: 1. Natranaerobius 'jonesii', 2. Natronovirga wadinatrunensis, 3. Natranaerobius thermophilus, 4. Natranaerobius 'grantii', 5. Natranaerobius truperi, 6. Halorhodospira halochloris, 7. Dichotomicrobium thermohalophilum, 8. Halanaerobacter salinarius, 9. Salinibacter ruber