A taxonomically informed DNA reference library to facilitate future biodiversity assessments and monitoring: a case study using seaweeds along a tropical-temperate transition zone in South Africa

The role of seaweeds in the blue bioeconomy has stimulated research efforts around the world but proper species identification and biodiversity assessments, remain a challenge. The South African coast hosts the confluence of the Indian Ocean and Atlantic Ocean, creating a dynamic evolutionary environment that has over time yielded a rich diversity of seaweeds with the highest seaweed diversity occurring along the Agulhas Marine Province. Although South Africa harbours one of the richer seaweed floras in the world, only 24% of the known species are represented by DNA barcodes. We therefore initiated the construction of a taxonomically guided DNA reference library for seaweeds in South Africa with the aim of continuously adding to it in the future. To do this, a seaweed biodiversity survey of the Rhodophyta occurring along a temperate-tropical biogeographic transition zone situated within the Agulhas Marine Province (AMP) in South Africa was carried out. Seaweeds were identified in the field using available field or taxonomic guides and herbarium vouchers were prepared. Subsamples were preserved for DNA analyses and three DNA barcodes (LSU D2-D3; rbcL-3P; COI 5P) were amplified. Sequences were verified on BLAST and preliminary phylogenetic analyses or comparison with the literature were carried out where necessary. A total of 220 barcodes was generated for 88 species and one species variety, including 17 species from or near their type localities and eight generitypes. Novel barcodes were generated for 73 species, nearly half of which were species endemic to Southern Africa. In addition, 21 taxa representing new, potentially new, or reinstated species and at least two new genera were identified as well as one new distribution recorded, all of which require further study. This study significantly adds to the foundational biodiversity knowledge of the South African seaweed flora and highlights new avenues for further research.