Introduction
Soilless food production systems are increasingly considered as sustainable alternatives to conventional agriculture, which faces mounting pressures from climate change, ecosystem degradation, and disrupted biogeochemical cycles (Richardson et al., 2023). Systems such as hydroponics and aquaponics offer improved resource-use efficiency, reduced reliance on pesticides, and precise control over plant growth conditions, making them well suited for high-yield production in constrained environments (Rodríguez et al., 2022; Gonnella et Renna, 2021). Aquaponics, in particular, integrates aquaculture and plant production through microbial nutrient cycling, reducing dependency on synthetic fertilizers (Lennard et Goddek, 2019). However, nutrient availability in aquaponic systems can be variable and often suboptimal for plant growth due to complex mineralization processes and imbalanced fish-derived inputs (Kasozi et al., 2019). Biostimulants, especially plant growth-promoting bacteria (PGPB), offer a promising strategy to enhance nutrient use efficiency and plant development. While well studied in soil systems, their application in soilless agriculture remains limited despite the rich and potentially beneficial microbiota present in aquaponic environments (Eck et al., 2019). This study evaluates whether aquaponics-derived bacterial strains can promote lettuce growth and influence root system architecture across different soilless systems and growing cycles, comparing single-strain and consortium inoculations.
Material and Methods
Plant growth–promoting bacteria (PGPB) were isolated from an aquaponic system and characterized in vitro for key functional traits, including indole-3-acetic acid production, siderophore production, nutrient solubilization, biofilm production, metabolic activity and antagonism. Selected strains were then evaluated in vivo on lettuce grown in controlled, small-scale, soilless systems over the course of two growth cycles. Seedlings were inoculated with either a single strain or a three-strain consortium and cultivated under hydroponic and aquaponic conditions. Aquaponic waters were sourced from trout (Oncorhynchus mykiss) and carp (Cyprinus carpio carpio) systems. Plant performance was assessed through measurements of above and below-ground parameters after four weeks of growth. This experimental design allowed the comparison of treatment effects across contrasting systems and the evaluation of the consistency of plant growth responses across trials.
Results
Out of the 31 retrieved bacterial strains, three of them (strains A, H, and T) were selected based on their performance during in vitro characterization. Biofilm formation and metabolism depended on strain and medium, with strain T showing the greatest consistency and versatility. Antagonism was limited, though strain A inhibited others in nutrient-rich carp water, reflecting environmental influence. Bacterial inoculation significantly enhanced lettuce growth across soilless systems, with consistent increases in shoot biomass and leaf development relative to non-inoculated controls. These positive effects were observed across water types and trials, although their magnitude varied depending on environmental conditions. Co-inoculation did not consistently outperform single-strain treatments. Root responses were more variable, with treatment effects depending on both water source and trial. Inoculation influenced root system architecture, affecting total root length, proportion of fine roots and specific root length. While root and shoot biomass were positively correlated, root allocation remained unchanged, indicating coordinated growth without consistent shifts in biomass partitioning.
Discussion
Bacterial inoculation improved lettuce growth across contrasting soilless systems, confirming the potential of aquaponic-derived strains as plant growth-promoting bacteria. Aboveground responses were generally robust, with increased shoot biomass and leaf development observed across treatments. However, the magnitude of these effects varied between trials and water sources, highlighting strong context dependency. Co-inoculation did not consistently outperform single-strain application, suggesting that functional redundancy, competition, or environmental filtering may limit consortium benefits. In contrast to shoots, root responses were more variable and revealed substantial plasticity. Changes in root system architecture involved both overall development and shifts in structural traits, indicating adaptive responses to environmental conditions and bacterial treatments. Notably, shoot and root biomass were positively correlated, while root allocation remained independent of shoot growth, suggesting coordinated plant vigor without consistent shifts in biomass partitioning. Together, these results emphasize that while PGPB can enhance plant performance in soilless systems, their effectiveness depends on environmental conditions and inoculation strategies, calling for context-aware optimization of microbial applications without compromising water quality for fish.
References
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Gonnella, M. et Renna, M. (2021) « The Evolution of Soilless Systems towards Ecological Sustainability in the Perspective of a Circular Economy. Is It Really the Opposite of Organic Agriculture? », Agronomy, 11(5), p. 950. https://doi.org/10.3390/agronomy11050950
Kasozi, N., Kaiser, H. et Wilhelmi, B. (2021) « Effect of Bacillus spp. on Lettuce Growth and Root Associated Bacterial Community in a Small-Scale Aquaponics System », Agronomy, 11(5), p. 947. https://doi.org/10.3390/agronomy11050947
Lennard, W. et Goddek, S. (2019) « Aquaponics: The Basics », in S. Goddek et al. (éd.) Aquaponics Food Production Systems: Combined Aquaculture and Hydroponic Production Technologies for the Future. Cham: Springer International Publishing, p. 113‑143. https://doi.org/10.1007/978-3-030-15943-6_5
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