Use of NanoSIMS to Identify the Lower Limits of Metabolic Activity and Growth by Serratia liquefaciens Exposed to Sub-Zero Temperatures.

Petra Schwendner,Ann N. Nguyen,Andrew C. Schuerger

doi:10.3390/life11050459

Abstract

Serratia liquefaciens is a cold-adapted facultative anaerobic astrobiology model organism with the ability to grow at a Martian atmospheric pressure of 7 hPa. Currently there is a lack of data on its limits of growth and metabolic activity at sub-zero temperatures found in potential habitable regions on Mars. Growth curves and nano-scale secondary ion mass spectrometry (NanoSIMS) were used to characterize the growth and metabolic threshold for S. liquefaciens ATCC 27,592 grown at and below 0 °C. Cells were incubated in Spizizen medium containing three stable isotopes substituting their unlabeled counterparts; i.e., 13C-glucose, (15NH4)2SO4, and H218O; at 0, −1.5, −3, −5, −10, or −15 °C. The isotopic ratios of 13C/12C, 15N/14N, and 18O/16O and their corresponding fractions were determined for 240 cells. NanoSIMS results revealed that with decreasing temperature the cellular amounts of labeled ions decreased indicating slower metabolic rates for isotope uptake and incorporation. Metabolism was significantly reduced at −1.5 and −3 °C, almost halted at −5 °C, and shut-down completely at or below −10 °C. While growth was observed at 0 °C after 5 days, samples incubated at −1.5 and −3 °C exhibited significantly slower growth rates until growth was detected at 70 days. In contrast, cell densities decreased by at least half an order of magnitude over 70 days in cultures incubated at ≤ −5 °C. Results suggest that S. liquefaciens, if transported to Mars, might be able to metabolize and grow in shallow sub-surface niches at temperatures above −5 °C and might survive—but not grow—at temperatures below −5 °C.

Highlights

The understanding of microbial–niche interactions, the geological and thermodynamical contexts of frozen habitats, the microbiome, and the geochemical physical state of habitable niches are fundamental to describing the potential for the presence of microorganisms, their activity, detritus, and biosignatures on both Earth and in extraterrestrial environments
After 35 days, growth was observed in cultures only for samples incubated at 0 ◦ C (Figure 1A)—in which growth curves followed typical sigmoidal patterns with relatively long lag phases of about 14 days, followed by exponential growth for 14–21 days
Astrobiological model organisms are frequently used as proxies to study how to Astrobiological model frequently used as proxieslife tocan study how search for life on Mars

Summary

Introduction

The understanding of microbial–niche interactions, the geological and thermodynamical contexts of frozen habitats, the microbiome, and the geochemical physical state of habitable niches are fundamental to describing the potential for the presence of microorganisms, their activity, detritus, and biosignatures on both Earth and in extraterrestrial environments. On Earth, microbial communities have been found in frozen habitats such as glaciers, permafrost, and in ice at both polar regions where liquid water is only available intermittently, seasonally, or in insulated microenvironments [1,2,3,4,5,6]. Impurities in the ice can depress the freezing point of water and create thin networks of unfrozen water in icy environments which form microhabitats for microorganisms to survive and thrive [7]. These niches are characterized by low thermal energy, limited availability of nutrients, low water availability and water activity, and slower diffusion of metabolic waste products which can lead to increased toxicity. Environmental conditions in cold and/or frozen niches on other planetary bodies should be studied to determine whether they can provide potential habitats for microbial life to metabolize and grow in general, and for potential astrobiology model organisms such as Serratia liquefaciens, in particular

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