GRACILARIA CONFERTA AND ITS EPIPHYTES .3. ALLELOPATHIC INHIBITION OF THE RED SEAWEED BY ULVA CF LACTUCA

In a previous study (Svirski et al., 1993), it was found that growth i nhibition of Gracilaria spp., when cultured in the presence of Ulva cf . lactuca, was not due to shading or nutrient depletion, but seemed to be caused by competition for inorganic carbon or some type of allelop athy. In the present study, we attempted to differentiate between thes e two possible influences by (1) growing the two algae in biculture un der various conditions, but keeping inorganic carbon levels constant a nd measuring net photosynthesis, respiration and growth rates, and by (2) measuring growth rates of Gracilaria spp. in the presence of extra cts derived from media previously used to grow Ulva cf. lactuca. Both net photosynthesis and growth rates of Gracilaria spp. in biculture we re inhibited, despite CO2 (and also HCO3-) levels being kept constantl y high in the culture media. It is likely that these responses were du e to markedly enhanced rates of dark respiration in Gracilaria spp. wh en grown together with Ulva cf. lactuca. Growth of Gracilaria spp. was also inhibited by extracts derived from seawater in which Ulva cf. la ctuca had previously been grown. The strong inhibition by ethyl acetat e and chloroform extracts indicate an allelopathic effect on Gracilari a spp.