Effects of temperature, photoperiod, irradiance, and nutrient concentration on <i>Asparagopsis taxiformis</i> (Bonnemaisoniaceae, Rhodophyta) tetrasporophytes

Asparagopsis taxiformis has high potential to mitigate enteric methane emissions from livestock due to its high bromoform content. However, the current supply of gametophytes is limited due to insufficient biomass. Year-round production of gametophyte can address insufficient biomass through a continuous supply of initial biomass from tetrasporophyte. In this study, we evaluated the effects of temperature (10, 20, and 30°C) and photoperiod (8:16, 12:12, and 16:8 h light:dark cycles), as well as irradiance (10, 20, 40, 80, 160 μmol photons · m^-2 · s^-1) and nutrient conditions (high nutrient: 500 μM nitrate and 30 μM phosphate; low nutrient: 50 μM nitrate and 3 μM phosphate) on the growth and reproduction of tetrasporophyte of A. taxiformis. Temperature was a key factor in both growth and reproduction, whereas photoperiod was a key factor in reproduction. Growth of tetrasporophyte was inhibited by 10°C regardless of photoperiod. The development of tetrasporangia was only observed at 20°C with an 8:16 h light:dark cycle. At 20°C with an 8:16 cycle, irradiance affected the development of tetrasporangia at high nutrient concentration. The development of tetrasporangia was observed at 20 and 40 μmol photons · m^-2 · s^-1, with the highest growth rate observed at 160 μmol photons · m^-2 · s^-1 without the development of tetrasporangia. These results suggest that controlling irradiance at 20°C on an 8:16 h light:dark cycle under high nutrient concentration can regulate the growth and reproduction of A. taxiformis tetrasporophyte during cultivation.