Microbial community characterization of multi-crop growouts in the XROOTS aeroponic-hydroponic system on the International Space Station.

Plant growth systems tested on the International Space Station (ISS) are small-area growth units that mostly use solid media. With NASA's plan to send astronauts on long-duration exploration missions, there is a need to produce larger amounts of fresh food with limited upmass and resources. The eXposed Root On-Orbit Test System (XROOTS) is an aeroponic-hydroponic nutrient delivery system designed for exploration missions and was tested on the ISS. Post-harvest samples were returned for microbiological analyses of the plant leaves, roots, and fruit from lettuce, mizuna mustard, wheat, radish, tomato, and pea plants grown in the XROOTS. The microbiological food safety of crops was evaluated through culture-based microbial enumeration and identification. The microbial communities were compared between different plants and plant tissues by sequencing the prokaryotic V4 variable region of the 16S ribosomal RNA (rRNA) gene amplicons and fungal internal transcribed spacer (ITS) region. The microbial counts from the root module surface samples demonstrated a reduction after cleansing. The bacterial counts in the nutrient solution ranged from 65 to 3,800 CFU/ml. The bacterial counts in the distal leaf sections were lower than those in the leaf proximal, wick, and roots in all plant samples. The tomato fruit and the pea pod samples had the lowest average counts. The microbial counts from the leaves and wicks harvested from XROOTS were similar to the ranges found on previous Veggie (Vegetable Production System)-grown leafy greens. All screening tests for potential foodborne pathogenic bacteria were negative. Sequencing analyses showed that diversity was low in the leaves and higher in the roots, and the microbial community was more diversified in the XROOTS samples compared with previous Veggie experiments. Pseudomonas had the highest relative abundance in the majority of samples. Although some microbes were shared in the majority of plant tissues, unique microbes were identified for each plant type grown in XROOTS and when compared with previous Veggie demonstrations. Microbial surveys of ISS-grown plants and the associated hardware provide valuable data that can reveal potential challenges in deep-space crop production operations and ensure the quality of crops intended for crew consumption.