Experimental and molecular observations associated with sub-lethal autotoxicity in cultivated Asparagopsis taxiformis following exudate accumulation

The farming of the red seaweed Asparagopsis taxiformis is expanding due to its ability to mitigate methane emissions from livestock when supplemented into cattle feed, with increasing focus on developing land-based, recirculating aquaculture systems to enhance production. The exudates of Asparagopsis are allelopathic towards a range of organisms; however, there remains a lack of data on the potential autotoxicity of exudate-derived metabolites from the A. taxiformis holobiont, which may have implications on the outcomes of land-based A. taxiformis aquaculture. To address this research gap, we conducted an omics-wide examination of conspecific allelopathy in the holobiont of A. taxiformis tetrasporophytes. Following a 5-day cultivation in seawater pre-conditioned with its own exudates, the growth rate of tetrasporophytes was dramatically reduced at some stocking densities compared to those in control seawater without pre-existing A. taxiformis exudates. An untargeted exometabolomic analysis identified that various peptide and amino acid-based compounds, alkaloids, terpenoids, and fatty acids were at least three to four times more abundant in cultures that experienced reduced growth, suggesting these compounds may be associated with autotoxicity-like effects. Furthermore, in cultures experiencing reduced growth, we detected increased expression of stress-related genes involved in the production and scavenging of reactive oxygen species (e.g. vanadium-dependent haloperoxidases, animal heme peroxidases and manganese superoxide dismutase). Altogether, our study provides the first characterisation of A. taxiformis secreted molecules, some of which may be considered autotoxicity candidates and further emphasises that regular water exchange or ultrafiltration is vital for successful cultivation of A. taxiformis in closed or low-exchange systems. • Seaweed growth at low stocking densities was reduced upon exposure to conspecific exudates. • 57 putative autotoxicity candidates from the A. taxiformis holobiont were identified. • Expression of A. taxiformis stress-related genes increases in response to conspecific exudates.