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.

In this study, we demonstrate how stable carbon (C) and nitrogen (N) isotope analysis can be used to estimate the allocation of specific dietary ingredients, incorporated into four artificially formulated feeds supplemented with varying amounts of dried seaweed Ulva armoricana (0, 50, 150 and 200 g/kg), to the production of gonads by Tripneustes gratilla over a 20‐week growth trial. Results indicate that U. armoricana is an important dietary ingredient for gonad production, accounting for an average of 37.33 ± 1.03% of the isotopic signal recorded for gonads at the end of the growth trial. This is significantly more than fish meal and soya, which only contributed an average of 11.40 ± 1.81 and 3.80 ± 0.09%, respectively, of the isotopic signal of the gonads. Maize and wheat each account for almost 25% of all the dietary ingredients used to formulate the feeds, but their relative contribution to gonad production only averaged 8.55 ± 0.58 and 3.80 ± 0.05%, respectively, among the diets. This indicates that, at least for gonad development, these ingredients may not be required at such high levels. These findings support previous data on the importance of Ulva in abalone and urchin diets, and demonstrate the value of stable isotope analysis for assessing the contribution of specific dietary ingredients in new feed formulations, particularly where growth of specific tissues is being investigated.