Data Sheet 1_Growth, physiology, and metabolism of Halomonas meridiana in aqueous ammonium sulfate with implications for icy moon astrobiology.pdf

The discovery of extraterrestrial reservoirs of liquid water has motivated missions to icy moons Europa and Titan. Tentative evidence of ammonium sulfate ((NH4)2SO4) has been detected on the surface of Europa, and (NH4)2SO4 could be a prominent constituent of the Titan subsurface ocean. While NH4+ acts as a nitrogen source for many organisms, detrimental impacts of (NH4)2SO4 fertilizer have been documented in bacteria. Consequently, the presence of (NH4)2SO4 within icy moon environments may constrain the capacity of these environments to support life. In this study, the bacterial survival limits and physiological response to aqueous (NH4)2SO4 were assessed using the extremophile Halomonas meridiana Slthf1. Growth assays demonstrated concentrations exceeding 0.25 M (NH4)2SO4 led to a measurable slowing of the growth rate. Cell density remained comparable to control conditions up to 0.75 M (NH4)2SO4 at which a decline was observed. Contrary to existing hypotheses, alterations to cell density were not determined by pH, osmolarity, salinity, ionic strength, or water activity of the aqueous (NH4)2SO4 solution. Furthermore, neither NH4+ nor SO42- alone accounted for these alterations. Metabolite profiling revealed that exposure to (NH4)2SO4 reduced the abundance of glutamine compared to control, indicating an alteration to nitrogen, carbon, and energy metabolism. Active catabolism was suggested by reduced levels of purine metabolites and amino acids. Metabolites within the methylaspartate cycle were detected. We discuss these results with regards to the potential for habitability in aqueous extraterrestrial (NH4)2SO4 environments as well as terrestrial environments in which (NH4)2SO4 fertilizer is applied.

地外液态水储层的发现，推动了针对冰卫星木卫二（Europa）与土卫六（Titan）的探测任务。木卫二表面已探测到硫酸铵（ammonium sulfate，(NH₄)₂SO₄）的初步证据，而硫酸铵或为土卫六次表层海洋的重要组成成分。尽管NH₄⁺是诸多生物的氮源，但已有文献记录硫酸铵肥料对细菌产生的不利影响。因此，冰卫星环境中硫酸铵的存在，或会限制该环境维持生命的能力。

本研究以嗜极菌盐单胞菌（Halomonas meridiana）Slthf1为实验对象，探究了细菌在含水硫酸铵((NH₄)₂SO₄)环境中的生存极限与生理响应。生长实验结果显示，当硫酸铵浓度超过0.25 mol/L时，细菌生长速率出现可观测的减缓；当浓度升至0.75 mol/L时，细胞密度开始出现下降，此前直至0.75 mol/L，细胞密度均与对照组无显著差异。

与现有假说相悖的是，细胞密度的变化并非由含水硫酸铵溶液的pH值、渗透压、盐度、离子强度或水分活度所决定。此外，单独的NH₄⁺或SO₄²⁻均无法解释上述变化。代谢组分析结果显示，与对照组相比，硫酸铵暴露会导致谷氨酰胺的丰度下降，表明细菌的氮、碳及能量代谢发生了改变。嘌呤代谢物与氨基酸水平的下调，提示细菌处于活跃的分解代谢状态。本研究还检测到了甲基天冬氨酸循环中的代谢物。

最后，我们结合本研究结果，讨论了地外含水硫酸铵环境以及施用硫酸铵肥料的陆地环境的宜居性潜力。