Abiotic triggers for maximising germling numbers in Asparagopsis taxiformis (Rhodophyta, Bonnemaisoniales) via tetrasporogenesis

Abstract

There is global interest in cultivating the red alga Asparagopsis taxiformis due to its efficacy as a potent anti-methanogenic feed supplement and as a biofilter for the bioremediation of nutrient-enriched waters. However, the development of A. taxiformis cultivation is currently hindered by a lack of information about the conditions required to maximise tetraspore release and thus secure a reliable source of germlings for out-planting. In this study, we examined the effects of temperature, irradiance, and standard nutrient supplementation (F/8, potassium iodide (KI) and arsenic trioxide (As₂O₃)) on the number of germlings produced per tetrasporophyte, using a strain of A. taxiformis widespread within the Great Barrier Reef, Australia. Temperature, irradiance and nutrient supplementation played a pivotal role in germling numbers, which was optimised at 22 °C under 7 µmol photons m⁻² s⁻¹ and with supplementation of F/8 nutrient media, arsenic trioxide (As₂O₃; 98 µg L⁻¹) and potassium iodide (KI; 166 µg L⁻¹). Once tetrasporophytes were removed from these inducing conditions, tetrasporogenesis ceased within 12 days. In a further five-week experiment investigating the effect of separate supplementation of As₂O₃ and KI, germling numbers were maximised under supplementation with either As₂O₃ or As₂O₃ + KI, with the relative growth rate of tetrasporophytes maximised under supplementation with F/8 + As₂O₃ + KI. Under optimum conditions, an average of 3,261 ± 826 (SD) germlings were produced per tetrasporophyte over a five-week period. Our results provide a strong starting point for developing hatchery protocols for generating a reliable supply of germlings for nursery cultivation in tropical settings.