Abstract:

The current conventional food production systems are neither ecologically sound, nor do
they address the issues of the circular economy of decreasing waste. Aquaponics, a technology
that integrates aquatic organisms and plants, provides part of the solution. As microbes are vital
possible strategy for promoting sustainable agriculture.
Prior to the commencement of this research, there was no published data on the effects
of addition of probiotics on microbial diversity in aquaponics. The aim of this research was to
investigate the effects of the addition of a commercial probiotic mixture of Bacillus subtilis
and B. licheniformis to a coupled aquaponic system on plant and fish growth, water quality,
and bacterial diversity. Results from plant growth trials indicated that vegetative growth of
lettuce was significantly higher in the Z?acz7/zM'-treated systems than the untreated control
systems. The concentrations of phosphorus, potassium, and zinc in the lettuce leaves were
significantly higher in Bacillus-treated than control systems. There was a significant increase
in the concentration of nitrate and phosphate in the 2tacz7/iw-treated systems which may have
contributed to improved lettuce growth. Although Proteobacteria was the most dominant
bacterial phylum associated with the lettuce root and leaf surfaces, 16S rRNA gene sequence
analysis showed that the addition of Bacillus species significantly increased species richness
and species evenness of the lettuce roots as compared to leaves. The alpha diversity indices
showed that root-associated bacterial communities were significantly more diverse than those
associated with leaves when a probiotic was added to aquaponic water. Analysis of lettuce
epiphytic bacterial communities revealed that fresh lettuce leaf surfaces were inhabited by
food-borne pathogens belonging to Shigella and Aeromonas.
Trials related to fish growth, digestive enzyme activities, and bacterial gut communities
found that the addition of Bacillus stimulated the activities of intestinal amylase, protease, and
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to the functioning of aquaponic systems, the integration of probiotics in aquaponics is a
alkaline phosphatase and showed a significantly higher percentage increase in weight gain,
Specific growth rate, and improved feed conversion ratio than the control. The use of the
commercial Bacillus preparation did not affect whole-body composition of O. mossambicus
suggesting that optimum conditions for keeping this species were maintained. Water quality
remained within recommended ranges for tilapia culture.
Illumina MiSeq sequencing of 16S rRNA gene amplicons showed that bacterial gut
communities of O. mossambicus were dominated by the phylum Fusobacteria, specifically the
species Cetobacterium somerae. Members from the Enterobacteriaceae family were part of
lhe O. mossambicus gut microbiota, which was mostly represented by Plesiomonas
fhigelloides. Other OTUs assigned to Aeromonas diversa, Edwardsiella anguillarum, and
Klebsiella pneumoniae were also observed, at low abundances.
A further study investigated the bacterial communities associated with the dry, root, and
nineralised zones of a flood-and-drain media bed system. Bacteroidetes and Proteobacteria
vere the most dominant phyla across the three zones and in the effluent water. At genus level,
n the effluent water samples of -SaczY/us-treated systems, Spirosoma was the most dominant,
vith a relative abundance of 71.44% of the total bacterial community compared to 1.68% of
he total bacterial community from the control systems. All three Spirosoma OTUs had
:equences closely related to the bacterium Spirosoma aerdphilum.
In conclusion, the results from this research suggest that integrating Bacillus species into
¦quaponic systems may be considered beneficial, having positive effects on plant and fish
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our understanding of the
growth, as well as enhancement of plant nutrients and the modulation of the composition of
>acterial communities. Therefore, this research has contributed to
bacterial community changes as a result of adding probiotics.