Hydroponic systems alter the microbiota of the nutrient solution

The design of the soilless cultivation system influences the bacterial load and composition of the microbiota present in the nutrient solution. Drip irrigation recorded the highest counts of mesophilic aerobic bacteria among five models evaluated by researchers at Pennsylvania State University in the United States. The production cycle also modified the microbial communities. In bok choy (Brassica rapa) leaves, the counts did not show differences between the systems at harvest time (doi 10.1128/aem.01167-26).

The experiment compared deep water cultivation, the Kratky method, the nutrient film technique, flood and drain irrigation, and drip irrigation. The team conducted two cycles of bok choy and analyzed bacteria in the nutrient solution and on the leaves. The work also used 16S ribosomal RNA gene sequencing to characterize the bacterial composition.

Highest averages

The drip irrigation showed the highest averages in both cycles. In the fall, the count reached 5,9 base-ten logarithms of colony-forming units per milliliter. In the spring, it reached 5,44 base-ten logarithms of colony-forming units per milliliter.

The researchers linked this result to the solution's contact with the peat-based substrate. In drip irrigation, water enters through the surface and passes through the substrate by gravity. This flow can transport microorganisms associated with the organic material. In flood and drain irrigation, the solution rises from the base and reaches the substrate by capillary action.

Differentiating factor

The type of system represented the main factor associated with differences in bacterial composition. Cultivation cycle, sampling day, and temperature also influenced the community profile. pH showed a negative association with bacterial load. For each increase of one unit, the count decreased by approximately 0,36 base-ten logarithms of colony-forming units per milliliter. Temperature did not alter the concentration but influenced the composition of the microbiota.

The genus Acidovorax appeared in all samples, across all five systems and both cycles. The study does not specify which Acidovorax species were detected. The plants did not show any symptoms of disease. Other recurring genera included Devosia, Bdellovibrio, Legionella, Rhodanobacter, Sphingobium, Sediminibacterium, Massilia, and Caulobacter.

Bacterial loads

The leaves maintained similar bacterial loads. According to the researchers, the limited contact of the aerial parts with the nutrient solution may explain the result. The study does not allow us to conclude whether systems with a higher microbial load favor the survival of foodborne pathogens.

The data indicate the need for specific microbial management strategies for each production model. Solution flow, substrate contact, oxygenation, and water retention create distinct environments throughout the cycle.