Bacterial microbiome dynamics in commercial integrated aquaculture systems growing Ulva in abalone effluent water

In South Africa, the green seaweed Ulva lacinulata is grown in land-based integrated multi-trophic aquaculture (IMTA) farms with the abalone Haliotis midae. The Ulva serves as a biofilter and the co-produced Ulva is often used as feed for the abalone. To better understand the potential benefits and risks associated with this practice, this study characterised the bacterial microbiome associated with the seawater and Ulva raceways receiving abalone effluent (IMTA system) and compared this to Ulva tanks supplied with fertilised seawater (non-IMTA; control). Ulva samples were collected from each Ulva system, and water samples were collected at the inlet and outlet of each system. Bacterial communities were assessed using a culture-based approach and next-generation sequencing (NGS) of the V3-V4 16S rDNA region. It was observed that Ulva has the potential to reduce the bacterial load of abalone effluent, with the total number of potential culturable Vibrio species declining from 150×10³ cells mL^-1 in the inlet to 37×10³ cells mL^-1 in the outlet of the Ulva system. The NGS dataset supported these findings, with a reduction observed in Vibrio and Pseudoalteromonas from the inlet to outlet samples. A lower number of genera (p < 0.05) were observed on Ulva when compared with water samples, indicating that Ulva has a beneficial, modulatory effect on bacteria. These findings contribute towards the growing body of evidence for the benefits of seaweeds in IMTA and addresses the biosecurity concerns of abalone farmers wishing to improve the circularity of their farming activities by incorporating seaweeds.