THE INFLUENCE OF PHOTON IRRADIANCE, DAY LENGTH, DARK TREATMENT, TEMPERATURE, AND GROWTH-RATE ON THE AGAR COMPOSITION OF GRACILARIA-SORDIDA NELSON,W. AND GRACILARIA-VERRUCOSA (HUDSON) PAPENFUSS (GIGARTINALES, RHODOPHYTA)

The results from the present study clearly show that the agar composition of Gracilaria sordida and Gracilaria verrucosa is highly variable and to a large extent influenced by the prevailing culture conditions. An increased weekly photon irradiance at 26 °C resulted in an increased yield of the agar fraction soluble in water at 25 °C. Furthermore, the increased yield of this agar fraction appeared to be more related to the increased light availability than to the average age of the algal tissue. The gel strength of the different agar fractions from Gracilaria sordida appeared to be one of the most fluctuating characters tested, together with the agar solubility and the 3,6-anhydrogalactose concentration. The cultivation temperature and the post-treatment in darkness showed a clear influence on the fractional agar gel strengths. About one third of the extracted agar from Gracilaria sordida, cultivated at 18 °C, showed a gel strength comparable to that of a commercial Difco Bacto-Agar. This commercially interesting part of the total agar yield showed a 100% increase after a dark-treatment of 3 days. The observed changes in agar properties are interpreted as reflecting adaptation of the cell-wall to growth conditions. © 1990, Walter de Gruyter. All rights reserved.