COMPARISON OF GROWTH-RATE IN CULTURE, DRY-MATTER CONTENT, AGAR CONTENT AND AGAR QUALITY OF 2 NEW-ZEALAND RED SEAWEEDS, GRACILARIA-CHILENSIS BIRD, MCLACHLAN ET OLIVEIRA AND GRACILARIA-TRUNCATA KRAFT

Plant growth rates and agar characteristics were compared for two agarophytes, Gracilaria chilensis (Gracilariales, Rhodophyta) and G. truncata, so that the suitability of G. truncata for mariculture could be assessed. G. chilensis plants grew steadily in the laboratory culture system at rates of 5-8% day-1 (mean Relative Growth Rate) throughout the 6 week experimental period, with no decline in health. G. truncata plants grew at rates of 2-4 % day-1 for 5 weeks, but during the sixth week their apical tips became necrotic and growth rates fell to zero. There was no significant difference in the dry matter content (as a percentage of their wet weight) between the two species of Gracilaria, with values falling in the range of 16-19%. Slightly higher agar yields were obtained from alkali-treated G. chilensis (17-20% dry matter) than from untreated G. truncata (16-18%) although the agar from G. truncata formed softer gels from which it was more difficult to recover. The gel strength of untreated agar extracted from G. chilensis was very low (ca. 100 g cm-2 for a 1% gel) but when an alkali treatment step was included in the extraction process, it increased to 520 g cm-2 for a 1% gel. Contrary to an earlier report, untreated G. truncata agar also had a very low gel strength (ca. 100 g cm-2 for a 1.5% gel), but it rose to only 167 g cm-2 after alkaline treatment.