Biological nitrogen cycling within terrestrial hot springs: a Mars analogue system

Noachian-age (4.1–3.7Ga) hot spring deposits have been identified on Mars, in addition to fixed nitrogen compounds within Martian surface materials, yet the planet’s nitrogen cycle remains enigmatic and implicates the plausibility of a Martian biosphere. On Earth’s surface, nitrogen is cycled almost exclusively by biological processes which create distinctive isotopic fractionations. We combine geochemical and metagenomic analysis to investigate biological nitrogen cycling within four Mars analogue geothermal systems in Iceland ranging in temperature from 37.8 to 57.1 °C, and propose the geochemical parameters that control biological nitrogen fixation as the primary source of nitrogen into the microbial communities present. We find complete nitrogen fixation and ammonium assimilation gene clusters at all sites, which are also the most abundant nitrogen-cycling genes present. Isotopic fractionations (Δ¹⁵ N) of ∼6 ‰ between locally dissolved N₂ gas and biomass are most parsimoniously explained by organisms relying on Mo-independent nitrogenases for fixation of dinitrogen (N₂). This hypothesis is supported by the presence of genes encoding these enzymes in three out of four sites included in this study, in addition to the more commonly used MoFe nitrogenase. Finally, we find that molybdenum availability is low in the Icelandic hot springs investigated, and potentially therefore in Martian hot springs. This may inhibit conventional Mo-dependent nitrogen fixation in such settings and highlights the influence of wider geological conditions on local biochemistry.