Growth and biochemical responses of tropical and subtropical strains of Gracilaria domingensis (Gracilariales, Rhodophyta) to temperature and irradiance variations

Abstract

Gracilaria domingensis (Kutzing) Sonder ex Dickie is widely distributed in the Brazilian coast and is one of a few Gracilaria species with occurrence in the southern region of the country. Its commercial importance is in the use in human food (in natura) and knowledge of the physiology is fundamental to make its cultivation successful. The objectives of the present work were to determine the limits of tolerance to temperature and irradiance variations and to evaluate comparatively their effects on growth rates and contents of total soluble proteins and pigments in female gametophytes of G. domingensis from Brazilian tropical region (ES strain) and from subtropical region (SC strain). Variations in temperature (from 15 to 35 °C) and irradiance (from 20 to 250 μmol photons m−2 s−1) were tested. Both strains tolerated temperature variation from 15 to 30 °C, but died at 35 °C. The ES strain had higher growth rates (from 3.1 to 6.1% day−1) in relation to the SC strain (from 1.3 to 4.1% day−1), but the two strains had the highest growth rate at 25 °C. The ES strain had higher concentrations of total soluble proteins at low temperature (15 °C), while the SC strain showed higher protein concentrations at high temperatures (25 and 30 °C). In addition, the chlorophyll a concentration of the ES strain was higher at 20 °C and lower at 30 °C. The irradiance variation influenced the growth rates of the two strains (from 2.7 to 6.0% day−1) and the lowest growth rates were observed at irradiance of 20 μmol photons m−2 s−1 and the highest growth rates were observed at a wide range of irradiance variation (from 40 to 250, and from 60 to 250 μmol photons m−2 s−1 for SC and ES strains, respectively). The studied strains of G. domingensis showed higher concentrations of total soluble protein at low irradiances (20, 40, and 80 μmol photons m−2 s−1). The highest concentrations of phycoerythrin (423.0 μg g−1 FW) and phycocyanin (215.3 μg g−1 FW) were observed in the ES strain grown at low irradiance (40 μmol photons m−2 s−1). The chlorophyll a concentration of the two strains decreased with increased irradiance, indicating a photoacclimation. Our results showed that the tropical strain of G. domingensis has higher growth rates over a broader temperature range than the subtropical strain, which could be an advantageous feature for maricultural purposes.