Radiotolerance of N-cycle bacteria and their transcriptomic response to low-dose space-analogue ionizing irradiation

Abstract

The advancement of regenerative life support systems (RLSS) is crucial to allow long-distance space travel. Within the Micro-Ecological Life Support System Alternative (MELiSSA), efficient nitrogen recovery from urine and other waste streams is vital to produce liquid fertilizer to feed food and oxygen production in subsequent photoautotrophic processes. This study explores the effects of ionizing radiation on nitrogen cycle bacteria that transform urea to nitrate. In particular, we assess the radiotolerance of Comamonas testosteroni, Nitrosomonas europaea, and Nitrobacter winogradskyi after exposure to acute γ-irradiation. Moreover, a comprehensive whole transcriptome analysis elucidates the effects of spaceflight-analogue low-dose ionizing radiation on the individual axenic strains and on their synthetic community o. This research sheds light on how the spaceflight environment could affect ureolysis and nitrification processes from a transcriptomic perspective.

Keywords: Microbiology; Space sciences.

Graphical abstract

Figure 1

Growth kinetic parameters of Comamonas testosteroni, Nitrosomonas europaea and Nitrobacter winogradskyi after exposure to acute γ-radiation Data is represented as mean ± SD (n = 4). Unpaired t-tests were performed between the control and the irradiated samples to identify significant differences: ∗p < 0.05.

Figure 2

Survival of Comamonas testosteroni after acute γ-irradiation (A) Log of colony-forming units (CFUs) per mL of C. testosteroni after exposure to acute gamma radiation. (B) Log of the survival percentage of bacteria after exposure to acute γ-radiation. The dotted line represents 10% survival and D₁₀ is situated at the intersection between the data and this threshold. Data is represented as mean ± SD (n = 4). Unpaired t-tests were performed between the control and the irradiated samples to identify significant differences: ∗p < 0.05.

Figure 3

Effect of low-dose ISS-like ionizing radiation exposure on growth and relative abundance of N-cycle bacteria (A) Endpoint OD₆₀₀ measurements of the tripartite culture and (B) relative abundance of the three constituents of the tripartite community. Data is represented as mean ± SD (n = 4). Unpaired t-tests were performed between the control and the irradiated samples to identify significant differences: ∗p < 0.05.

Figure 4

Comparative transcriptomic analysis of N-cycle bacteria in axenic and tripartite culture after exposure to low-dose ionizing radiation COG analysis and Venn diagrams of overlapping DEGs for axenic (purple) and tripartite (blue) (A) Comamonas testosteroni, (B) Nitrosomonas europaea and (C) Nitrobacter winogradskyi. COG classes ‘Function unknown’ and ‘General function prediction only’ were excluded.

Figure 5

Overview of transcriptomic changes in N-cycle bacteria exposed to low-dose ionizing radiation (IR) (1) Increased reactive oxygen species (ROS) load can be diminished by regulating the export of ROS-generating heavy metal ions. (2) The import of Fe²⁺ ions can increase ROS load through the Fenton-reaction. (3) Import of sulfate, which can be used for Fe-S cluster assembly and for oxidative stress response mechanisms. (4) The repair of DNA damaged by ROS. (5) The repair of proteins damaged by ROS. (6) ROS detoxification mechanisms. (7) Chaperone proteins that prevent misfolding of polypeptides damaged by ROS. (8) Adaptations to the cell envelope. Green arrows indicate upregulation of process-related genes while red arrows indicate downregulation. ND means no differential expression has been observed for that category in the bacterial strain.

Figure 6

Schematic overview of the experimental setup during low-dose chronic irradiation with a pure neutron source (Cf-252)