BENEFITS AND RISKS ASSOCIATED WITH THE INTEGRATION OF ULVA IN LAND-BASED AQUACULTURE SYSTEMS: IMPLICATIONS FOR SEA URCHIN AQUACULTURE

Inclusion of Ulva  in  sea urchin and other aquafeeds has  been shown  to enhance feed consumption, growth, product quality and  animal health. In South Africa,  large-scale  production of Ulva  in land-based  aquaculture systems has occurred successfully since 2002. However, despite the obvious benefits of growing Ulva in integrated systems f or  water  bioremediation  and recirculation  as well as  its  reported benefits as an aquafeed component, production is still  limited to a small number of farms. Biosecurity of integrated systems remains one of the main constraints preventing wider adoption of integrated technologies , since Ulva  grown in effluent water is often considered to be a disease risk when fed back to animals on the farm.

To better understand potential disease risks , we studied the microbial communities associated with  seawater and  Ulva  obtained from raceway systems  supplied with  effluent sea water (ES W) or natural seawater that had been fertilized (FSW) . Water samples were collected at the inlet and outlet of each system. A culture (traditional plate count) and non-culture (NextGen sequencing of the 16S rRNA gene) based approach was adopted. The culture based approach utilised three selective media for the isolation and quanti fication of bacteria; namely tryptic toy agar (TSA), thiosulfate-citrate-bile-sucrose (TCBS) agar (V ibrio selective), and Ulvan agar (primary polysaccharide of Ulva  used as the sole carbohydrate source) .

In both raceway systems Ulva had a strong inhibitory effect on the total number of Vibrio species within the water column (Fig.1). The inhibitory effect appeared to be nutrient dependent, with a more substantial decrease in Vibrio species observed in ESW. NGS revealed high microbial diversity in all samples. A PCoA showed a clear separation between Ulva and seawater samples, while a linear discriminate analysis (LEfSe) showed that FSW had the largest number of genus level OTUs associated with it. Bacteria assigned to Hellea, Saprospiraceae, Gammaproteobacteria and Rhodobacteriaceae were shown to be abundant on Ulva. These organisms are often isolated from marine environments and seaweeds (Incl. Ulva), and bacterial species within some of these taxa have been shown to play important roles in the morphogenesis of Ulva. Conversely, OTUs assigned to the family Vibrionaceae had low abundance on Ulva. Our data supports the potential health and environmental benefits associated with the inclusion of Ulva in integrated systems.