Reduction of the temperature sensitivity of Halomonas hydrothermalis by iron starvation combined with microaerobic conditions

Abstract

The limits to biological processes on Earth are determined by physicochemical parameters, such as extremes of temperature and low water availability. Research into microbial extremophiles has enhanced our understanding of the biophysical boundaries which define the biosphere. However, there remains a paucity of information on the degree to which rates of microbial multiplication within extreme environments are determined by the availability of specific chemical elements. Here, we show that iron availability and the composition of the gaseous phase (aerobic versus microaerobic) determine the susceptibility of a marine bacterium, Halomonas hydrothermalis, to suboptimal and elevated temperature and salinity by impacting rates of cell division (but not viability). In particular, iron starvation combined with microaerobic conditions (5% [vol/vol] O2, 10% [vol/vol] CO2, reduced pH) reduced sensitivity to temperature across the 13°C range tested. These data demonstrate that nutrient limitation interacts with physicochemical parameters to determine biological permissiveness for extreme environments. The interplay between resource availability and stress tolerance, therefore, may shape the distribution and ecology of microorganisms within Earth's biosphere.

FIG 1

Growth rates of Halomonas hydrothermalis under different culture conditions. (A) Cells grown under conditions of plentiful iron and oxygen. (B) Cells grown in iron-deprived media under aerobic conditions. (C) Cells grown in the presence of freely available iron under microaerobic conditions. (D) Cells grown in iron-deprived media under microaerobic conditions. Low, Mid, and High, 1%, 3.5%, and 11.8% (wt/vol) NaCl, respectively. Growth rates were calculated from growth curves as described in Materials and Methods. Data are presented as untransformed means ± standard errors of the means (SE) (n = 5). Different letters above the bars indicate significant differences between samples within a given combination of conditions of iron availability and type of atmosphere (aerobic or microaerobic) (P < 0.05 [Tukey's HSD test following 2-way ANOVA; see Table 1 for ANOVA results]).

FIG 2

Viability of stationary-phase Halomonas hydrothermalis cells under different culture conditions. Cultures were incubated under iron-rich (Fe+) or iron-starved (Fe−) conditions, in the presence of an aerobic atmosphere (Aer+) or a microaerobic atmosphere (Aer−). Data are presented as untransformed means ± SE (n = 54). Different letters above the bars indicate significant differences between samples within a given combination of conditions of iron availability and type of atmosphere (aerobic or microaerobic) (P < 0.05 [Tukey's HSD test]), determined following a one-way ANOVA using Box-Cox-transformed data (P < 0.001) (F_3,212 = 6.13). The measurements are based on LIVE/DEAD staining of cells followed by flow cytometry, with data pooled from two consecutive days. For details on the experimental protocol and instrumental calibration, see Materials and Methods and Fig. S1 in the supplemental material.