Functional effects of Polysiphonia sp. epiphytism on the farmed Kappaphycus alvarezii (Doty) Liao: Competition for the resource, parasitism or both?

Seaweed farming for the production of carrageenan is a growing economic activity. Like everywhere in the marine environment, farmed algae such as Kappaphycus alvarezii can host algal organisms as epiphytes. Epiphytes ensure important functions in natural ecosystems, but these organisms can have negative impacts on their hosts and, in aquaculture be considered a plague responsible for significant economic losses. The mechanisms by which epiphytes act functionally on their hosts are multiple: shading effects, competition for nutrients or parasitism. Parasitism is characterised by the epiphyte diverting a proportion of the host's resources. The objective of our work was to assess the impact of the epiphyte Polysiphonia sp. on the N and C acquisition of its farmed host K. alvarezii using two isotopic experiments with C-13 and N-15 as tracers. Our results demonstrated a double cumulative action: epiphytes could be capable of quickly outcompeting their hosts in terms of nutrient acquisition because of their better efficiency in C and N uptake, while also functionally qualifying as true parasites, as they divert some of the N resources acquired by their host. In terms of biocontrol, we suggest that the choice of nutrient-rich areas to practice Kappaphycus farming is likely to favour the epiphytes rather than their hosts, considering their relative needs and abilities to incorporate nutrients.

Automated summary

Seaweed farming for carrageenan production is becoming more economically important. However, farmed algae can host epiphytes, which can be detrimental and cause economic losses. In this study, we investigated the impact of the epiphyte Polysiphonia sp. on the nutrient acquisition of the farmed host K. alvarezii. Our results showed that the epiphytes could outcompete their hosts in nutrient uptake and act as true parasites by diverting some of the host's resources. This has implications for biocontrol and highlights the importance of choosing nutrient-rich areas for seaweed farming.