Growth, physiology, and metabolism of Halomonas meridiana in aqueous ammonium sulfate with implications for icy moon astrobiology

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mtext>NH</mml:mtext></mml:mrow> <mml:mrow><mml:mn>4</mml:mn></mml:mrow> <mml:mrow><mml:mo>+</mml:mo></mml:mrow> </mml:msubsup> </mml:math> 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 <i>Halomonas meridiana</i> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mtext>NH</mml:mtext></mml:mrow> <mml:mrow><mml:mn>4</mml:mn></mml:mrow> <mml:mrow><mml:mo>+</mml:mo></mml:mrow> </mml:msubsup> </mml:math> nor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mtext>SO</mml:mtext></mml:mrow> <mml:mrow><mml:mn>4</mml:mn></mml:mrow> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> 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.