Selecting optimal substrates for ocean-based cultivation of the red seaweed Asparagopsis taxiformis (Rhodophyta, Bonnemaisoniales)

Abstract The red seaweed Asparagopsis taxiformis has the potential to reduce methane emissions from livestock, but key challenges remain in developing scalable, ocean-based cultivation protocols using a closed-lifecycle approach. A critical milestone in this effort is developing seeding methodologies and identifying effective substrates for the attachment of gametophyte germlings. This study evaluates the suitability of synthetic and natural twine substrates and a sodium alginate binder for A. taxiformis germling cultivation under laboratory conditions. Polypropylene, nylon, cotton and sisal twines and a smooth polypropylene control surface were seeded with a germling solution, either directly or within a sodium alginate binder. Germling cover (%) was assessed after six weeks of tank-based cultivation and again following unravelling and exposure to moderate-velocity flow to simulate ocean deployment conditions. Polypropylene twine retained the highest germling cover (104% cover post-exposure), while cotton twine (70% cover) showed promise as a biodegradable alternative. Reduced germling cover was observed on the sodium alginate binder treatments across the majority of substrates, with germling loss particularly high (81% reduction in cover) on the smooth surface of the polypropylene control. By demonstrating a scalable seeding methodology and identifying two effective, widely available substrates for cultivation; polypropylene and cotton twine, this study provides a foundation for developing seeded twine for A. taxiformis mariculture. Field trials will be essential to validate this nursery-stage performance at-sea. Ultimately, optimising seeding techniques and cultivation substrates will be key to scaling A. taxiformis mariculture and unlocking its potential for sustainable bioproduct applications.