Bacterial community composition of recycled irrigation water of a NFT-type experimental system, with and without a slow sand filter

Abstract

The bacterial community composition (BCC) of recycled irrigation freshwater was monitored on a Nutrient Film Technique (NFT)-type experimental hydroponic system. Two identical NFT systems in a greenhouse were used to grow tomato plants. One was connected to a slow sand filter (SSF), and one was used as control. DNA was isolated from irrigation freshwater, and molecular methods were carried out to characterise the BCC. These included terminal-restriction fragment length polymorphism (T-RFLP), cloning and 454 pyrosequencing. A total number of 291 442 trimmed sequences, of 211 bp average length. A strong differentiation of the bacterial community composition in recycled irrigation water was triggered by the activity of the SSF. Phylogenetic affiliation revealed that Bacilli, Alpha- and Gammaproteobacteria, and Nitrospira were differentially abundant during filtration with the SSF. This study showed that SSF modified the relative amount of a set of bacterial genera. These included Pseudomonas, Bacillus, Flavobacterium, Burkholderia and Azospirillum. These bacteria have been previously described as plant growth promoting rhizobacteria (PGPR). The presence, increased or exclusive, in Water ssf of bacteria such as Bacteroidetes, Gemmatimonadetes, Nitrospira, Firmicutes, Alpha-, Beta-, Gamma- and Deltaproteobacteria, was associated with an increased biomass in plants. Such findings are promising for future applications of a combined system NFT-SSF: NFT guarantees a controlled closed environment for the growth of plants, while the SSF secures the microbiological balance of recycled irrigation water.