The multiple extremes of temperature, salt and pH define narrower limits to microbial growth in <i>Halomonas hydrothermalis</i> than individual extremes

Abstract Although the limits of life under individual extremes have been extensively studied, systematic experiments to quantify how combined extremes set the limits to life are lacking. We investigated the combined effects of extremes in temperature, salinity (NaCl) and pH on the growth limits of the marine bacterium Halomonas hydrothermalis , to test the hypothesis that limits to growth under combinations of the extremes establish a more restricted niche than the individual extremes. We show that the combination of supra-optimal temperature, pH and NaCl act synergistically in defining the limits of growth under multiple extremes. Although at optimal growth temperatures (30°C) maximum growth was achieved at pH 7, the maximum temperature limit of 43°C was achieved at pH 8. Under these conditions, the maximum NaCl concentration limit was 6.58% (wt/vol). Decreasing the temperature to 42 and 41/40°C increased the salinity limit to 7.01 % and 8.24 %, respectively. These data show that multiple extremes restrict the limits to growth of this organism to a greater extent than individual extremes and show how natural environments with extremes of temperature, pH and salinity could have restricted microbial diversity, or be uninhabitable, even when each individual extreme lies within the bounds of known microbial growth. These data imply that ‘maps’ of the limits to the biosphere based on laboratory-derived individual extremes may over-exaggerate growth limits in natural environments, which are rarely subject to single extremes, highlighting the need for multi-parameter analyses.