Long-term stability of soil microbiome structure and function in liquid soil extracts

Soil microbial communities (SMCs) play an important role in various ecological processes, including plant growth, carbon cycling, and greenhouse gas production and consumption. There have been many prior studies of soil microbiome function and structure. However, soil is a complex environment in which to conduct biological studies. Therefore, simplified SMC models, often adapted to liquid culture, have been employed in the laboratory to study specific microbial interactions and individual microbial functions. Specific advantages of these laboratory liquid SMC models include the ability to modulate community membership, control environmental conditions, and employ high-throughput assay techniques. The disadvantages of current laboratory liquid SMC models include long cycles for growing bacteria in vitro, the obligatory use of strains that are culturable in isolation, intricate media requirements, and complex community assembly protocols. To address some limitations of current liquid SMC models, we sought to create a streamlined process for extracting and maintaining a liquid culture of an existing SMC. Soil-Extracted Solubilized Organic Matter (SESOM) was made from four different soil types, including rich organic potting soils and environmental samples, and filtered to maintain the SMC. These SESOM liquid SMC models were cultured for 28 days, and SMC composition was measured by 16S rDNA sequencing. The SESOM SMCs maintain high alpha and beta diversity over time, including strains that are not culturable in isolation, with the greatest stability correlated with higher soil organic carbon. Further, the SESOM SMCs maintain unique signatures of their starting solid soils, suggesting that drift in SMC composition over extended time in liquid culture does not eliminate the defining microbial relationships of a given soil type. Network analysis of SESOM SMCs relative to solid soils suggests the functional roles of bacterial taxa were maintained in the liquid models over time. We further demonstrate that the platform can be applied to monitor the survival and persistence of a model engineered microbe - the common synthetic biology chassis Pseudomonas putida - within a native SMC. We conclude that the SESOM model is a valuable tool for facilitating the study of SMCs in the laboratory.