Sporulation, Pilot-Scale Farming, Agar Quality and Ecotypic Variation of the Agar Seaweed Gracilaria Sordida.

Abstract

The purpose of this study was to investigate the sporulation behaviour, the feasibility of farming in an open-water system, the quality and quantity of agar from a range of populations, and the genetic variation of the important agarophyte Gracilaria sordida W.A. Nelson (Gracilariales, Rhodophyta), which is widely distributed around New Zealand. The mean, total output of carpospores and tetraspores, and the periodicity of their release from G. sordida plants collected in the Wellington area, were measured under different levels of salinity, temperature, light intensity and daily exposure time to the air. The conditions that gave the greatest carpospore release were found to be approximately 15-35%. NaCl, l5-20° C, 50-200 uE.m-2 s-1 and 1-3 h daily exposure time. The conditions that gave the greatest tetraspore release were found to be approximately 15%. NaCl, l5-20° C, 150-200 uE.m-2 s-1 and 2-4 h daily exposure time. The diurnal periodicity of carpospore and tetraspore discharge based on hourly recordings was also measured. The peak rate of spore output occurred in the morning (7:00-8:00 hours) and in the late afternoon (16:00-17:00 hours). The carpospores and tetraspores not only had the same size but also showed the same germination pattern. A year-long pilot-scale farming project involving seeding the spores of sexually mature plants of G. sordida onto nets and ropes in Pauatahanui Inlet, Porirua Harbour, proved that this seaweed can be grown from spores on artificial substrates in field conditions. Of the artificial substrates tested, the best one for spore attachment was found to be polypropylene rope, and the relative growth rate on this substrate was maximal in spring (3.4%.d-1). The estimated production rate was 18.2 tonnes of dry weight per hectare per year. Problems encountered during field culture included epiphytes, sedimentation, storm damage and theft of floats and ropes. Yield and gel strength of agar extracts were studied from G. sordida collected in summer from 23 sites around the country. Native agar yield ranged from 17% to 32% (dry weight). The yield of native agar from cultured samples of these populations ranged from l0% to 29%. The yield of alkali-pretreated agar ranged from 9% to 24%. The gel strength of native agar ranged from 30 to 307 g.cm-2. Agar gel strength after alkali-pretreatment ranged from 230 to 625 g.cm-2. Native agar gel strength from cultured samples ranged from 177 to 342 g.cm-2. The gelling temperature of agar from wild populations ranged from 39 to 47° C for native agar and from 38 to 45° C for alkali-pretreated agar. The melting temperature of native agar ranged from 79 to 98° C. The melting temperature of alkali-pretreated agar ranged from 85 to 98° C. The maximum relative growth rate obtained from samples of G. sordida populations cultured in plastic bags was 6.2%.d-1, which was obtained from the Aramoana population. Starch gel electrophoresis of proteins was used to measure genetic variation in G. sordida. Protein extracts were prepared from 17 wild populations around New Zealand and from samples of these populations cultured in plastic bags. 20 isozyme loci were examined in G. sordida samples. Results indicated that G. sordida has low levels of genetic variation. Only two loci (Gd-1 and Pgm-1) of the 20 loci investigated were polymorphic (10%). Estimated heterozygosity of G. sordida was 0.011. There was no genetic variation between a native population and its cultured sample. The genetic distances between all populations were small. From the cluster analysis, all populations could be divided into two groups. Results indicated that populations were independent of each other, in which the effects of selection and genetic drift prevail. The buffer systems which gave the best protein resolution were Ridgeway (RW), Tris-EDTA-Borate (TEB) and Tris-Glycine (TG) and the enzyme which gave the best result in all buffer systems tested was Glucose-6-phosphate dehydrogenase (GD).