Microbial Community Characterization in Semi-Hydroponic Systems of Starbor Kale (Brassica oleracea L.) Grown Under Normal Gravity and Simulated Microgravity

• Bacteria were more abundant in coco coir, particularly in stationary clinostats • Pseudomonadota and Actinomycetota phyla were abundant under simulated gravity • Biomarkers were highest in horizontal clinostats' coco coir under simulated gravity • The GT2 and GT4 classes of glycosyl transferases were abundant in coco-coir samples • The top four antibiotic resistance genes were adeF, vanY, vanT , and qacG Kale is a member of the Brassicaceae family and contains a range of beneficial compounds. Given the global context of climate change, various vegetable production systems using advanced technologies, such as hydroponics, are being explored to alleviate food insecurity. Herein, we characterized the comprehensive microbial community associated with Starbor kale cultivation systems under normal gravity and simulated microgravity in coco coir, representing an innovative approach compared to previous studies. The kale seedlings were planted in growth vessels set into custom 2D clinostats and placed in a CONVIRON growth chamber for 43 days. The microbial DNA from coco-coir and root samples of grown kale was extracted and subjected to shotgun metagenomic sequencing. Comparisons between components revealed a higher abundance of bacteria in the soilless, while the kale roots were dominated by Eukaryota and a rchaea . The phyla Pseudomonadota and Actinomycetota were highly prevalent across all samples, with relatively high abundance in the coco coir samples from horizontal clinostats (HCR) under simulated gravity and from rotating vertical clinostats (VCR). The HCR group was associated with the highest number of biomarkers (28). Both CAZymes, glycoside hydrolases and carbohydrate esterases, exhibited higher relative abundances in the coco coir samples under normal gravity, whereas carbohydrate-binding modules were more abundant in HCR and VCR. The root samples showed much higher abundances of polysaccharide lyases (ranging from 0.00088 to 0.00097) and carbohydrate esterases (ranging from 0.030 to 0.033). The top four prevalent antibiotic resistance genes were adeF, vanY, vanT , and qacG . The findings of this investigation are crucial for the cultivation of kale and leafy green agriculture in hydroponic systems.