Growing Ulva (Chlorophyta) in integrated systems as a commercial crop for abalone feed in South Africa: a SWOT analysis

Over 1,000t fresh weight of Ulva was cultivated on South African abalone farms in 2007, primarily for feed, but in one case to allow partial re-circulation by nutrient removal. The potential of this new commercial crop is analysed. Material is collected from natural free-floating populations and at least four species are commonly grown, with different ecophysiological characteristics. A brown epiphyte, Myrionema strangulans, causes a disease of the Ulva, which is currently managed by farmers by re-stocking. The main potential threat is that some farmers are wary of integrated systems, fearing they may promote disease in abalone, although Ulva has been grown in abalone effluent and fed back to the abalone on one farm for 6 years without adverse effects. Opportunities exist for the expansion of Ulva cultivation via further spread of the abalone industry, the inclusion of seaweed raceways in proposed fish-farming activities, or the potential for the inclusion of high quality cultivated Ulva as a constituent in aquafeed. A conservative estimate of production over a full year from these raceway systems was 26.1 g dw m⁻² day⁻¹ (2006) and 19.7 g dw m⁻² day⁻¹ (2007), similar to maximum figures for total annual microalgal biomass production in outdoor systems.     

Resolving the identity of commercially cultivated Ulva (Ulvaceae,Chlorophyta) in integrated seaweed-abalone aquaculture farms in South Africa

Species of Ulva have a wide range of commercial applications and are increasingly being recognized as promising candidates for integrated aquaculture. In South Africa, Ulva has been commercially cultivated in integrated seaweed-abalone aquaculture farms since 2002, with more than 2000 tonnes of biomass cultivated per annum in land-based paddle raceways. However, the identity of the species of Ulva grown on these farms remains uncertain. We therefore characterized samples of Ulva cultivated in five integrated multi-trophic aquaculture farms (IMTA) across a wide geographical range and compared them with foliose Ulva specimens from neighboring seashores. The molecular markers employed for this study were the chloroplast-encoded Ribulose-1,5-bisphosphate carboxylase oxygenase (rbcL), the Internal Transcribed Spacer (ITS) of the nuclear, and the chloroplast elongation factor tufA. All currently cultivated specimens of Ulva were molecularly resolved as a single species, U. lacinulata. The same species has been cultivated for over a decade, although a few specimens of two other species were also present in early South African IMTA systems. The name Ulva uncialis is adopted for the Ulva “Species A” by Fort et al. (2021), Molecular Ecology Resources, 22, 86) significantly extending the distribution range for this species. A comparison with wild Ulva on seashores close to the farms resulted in five new distribution records for South Africa (U. lacinulata, U. ohnoi, U. australis, U. stenophylloides, and U. aragoënsis), the first report of a foliose form of U. compressa in the region, and one new distribution record for Namibia (U. australis). This study reiterates the need for DNA confirmation, especially when identifying morphologically simple macroalgae with potential commercial applications.