Response of the tropical red seaweed Gracilaria cornea to temperature, salinity and irradiance

The agarophyte Gracilaria cornea, collected over 2.5 y in the Florida Keys, shows adaptations to oceanic salinities and subtropical to tropical water temperatures in its photosynthetic and respiratory responses as measured wi th a respirometer. No seasonal pattern in responses to irradiance, temperat ure, and salinity were evident between five collections over a 20-month per iod, indicating the tropical nature of the populations from Bahia Honda and Pigeon Keys. Concentrations of chlorophyll a (0.09 to 0.41 mg g d wt(-1)) and phycoerythrin (0.06 to 0.36 mg g d wt(-1)) were low and reflect the low nutrient regime of the habitats, especially when compared to laboratory cu ltured plants. Compensation and saturation irradiances were also low (11-38 and 90-127 mu mol photon m(-2) s(-1)), indicating acclimation to lower irr adiances in their shallow (1-2 m depth) habitats where turbidity can be hig h. In comparison with other subtropical and warm temperate species of Graci laria, G. cornea had lower levels of pigment, but similarly high photosynth etic efficiency, demonstrating shade adaptation; it had only limited tolera nce to salinities below 20 parts per thousand and temperatures below 15 deg rees C. Thus, G. cornea from the Florida Keys in mariculture would require subtropical to tropical temperatures and stable oceanic salinities.