Mass cultivation of economically important red alga Gracilariopsis lemaneiformis (Gracilariaceae, Rhodophyta) from tetraspores and carpospores

• The spore culture method was successfully applied in G. lemaneiformis.
• Both tetraspores and carpospores were found to be the ideal seed materials.
• Optimal conditions for inoculation density and culture depth were also obtained.
• Spore coalescence of G. lemaneiformis was also observed in this study.

Gracilariopsis lemaneiformis is an economically important red alga worldwide, and is currently cultured by vegetative propagation. However, information related to the practice of spore culturing in red alga, especially for this species, is limited. Here, we describe a successful pilot study that used tetraspores and carpospores for the industrial cultivation of G. lemaneiformis in Jiaozhou Bay, Qingdao, China. Morphological observation and culture studies of G. lemaneiformis spores were carried out in the laboratory and field. Related spore parameters of daily output, germination rate, survival number, and growth rate were determined. The results showed that the maximum daily output occurred on the 6th day for tetraspores, with a value of 5.71 ± 0.68 × 105 g− 1 fresh weight, and on the 4th day for carpospores, with the value of 1876 ± 153 carpospores per cystocarp. During early culturing, the development and growth of tetraspores and carpospores were very similar. Inoculation density variation had significant effects on germination rate, survival number, and growth rate, the highest values of which were recorded at medium density, high density, and medium density inoculation, respectively. After 3 weeks in the field, the tetrasporelings and carposporelings were ~ 3 cm in length. Different heights below sea level also had a notable effect on sporeling growth, and the optimal condition was 0.5 m below sea level. After further culturing, the sporelings showed fast adaptation to the sea with average growth rates of 6.21 ± 0.48% day− 1 for tetrasporophytes and 4.21 ± 0.36% day− 1 for gametophytes. This study has demonstrated that the mass cultivation of G. lemaneiformis using tetraspores and carpospores under controlled culture conditions is feasible, and has also provided valuable information for improving the only available culture method (vegetative propagation) for G. lemaneiformis in the field.